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| author | nfenwick <nfenwick@pglaf.org> | 2025-01-17 13:20:57 -0800 |
|---|---|---|
| committer | nfenwick <nfenwick@pglaf.org> | 2025-01-17 13:20:57 -0800 |
| commit | 1158b95271d331644711e8308a9f968e1515945c (patch) | |
| tree | 1f98d03d9f6db4cba7aa219aef740ce29b2dec3c /72062-h | |
| parent | 460ca254a38482b81c4366e84e798e4ed827eea1 (diff) | |
As captured January 17, 2025
Diffstat (limited to '72062-h')
| -rw-r--r-- | 72062-h/72062-h.htm | 22068 |
1 files changed, 11034 insertions, 11034 deletions
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-<body>
-<div style='text-align:center'>*** START OF THE PROJECT GUTENBERG EBOOK ELECTRICITY ***</div>
-
-<div class="transnote section">
-<p class="center larger">Transcriber’s Note</p>
-
-<p>Larger versions of most illustrations may be seen by right-clicking them
-and selecting an option to view them separately, or by double-tapping and/or
-stretching them.</p>
-
-<p><a href="#Transcribers_Notes">Additional notes</a> will be found near the end of this ebook.</p>
-<div> </div>
-</div>
-
-<div class="section">
-<figure id="coversmall" class="figcenter" style="max-width: 30em;">
- <img src="images/coversmall.jpg" width="746" height="1024" alt="(cover)"></figure>
-<div> </div>
-</div>
-
-<div class="section p4">
-<p class="right l4 b2">“ROMANCE OF REALITY” SERIES<br>
-<span style="padding-right: 2.5em;">Edited by <span class="smcap">Ellison Hawks</span></span></p>
-
-<h1>ELECTRICITY</h1>
-<div> </div>
-</div>
-
-<hr class="chap x-ebookmaker-drop">
-
-<div class="chapter section">
-<p class="p1 in0 in4 b1 gesperrt"><span class="in2 wspace"><i>VOLUMES ALREADY ISSUED</i></span></p>
-</div>
-
-<div class="hang">
-<p>1. THE AEROPLANE. By <span class="smcap">Grahame White</span> and <span class="smcap">Harry Harper</span>.</p>
-
-<p>2. THE MAN-OF-WAR. By Commander <span class="smcap">E. H. Currey</span>, R.N.<br></p>
-
-<p>3. MODERN INVENTIONS. By <span class="smcap">V. E. Johnson</span>, M.A.<br></p>
-
-<p>4. ELECTRICITY. By <span class="smcap">W. H. McCormick</span>.<br></p>
-
-<p>5. ENGINEERING. By <span class="smcap">Gordon D. Knox</span>.</p>
-<div> </div>
-</div>
-
-<div class="section p4">
-<figure id="plate_0" class="figcenter" style="max-width: 25em;">
- <img src="images/i_004.jpg" width="631" height="994" alt=" ">
- <figcaption class="caption"><p class="floatr">(<a href="images/i_004large.jpg"><i>Larger</i></a>)</p><p class="clear">THE MARCONI TRANSATLANTIC WIRELESS STATION
-AT GLACE BAY, NOVA SCOTIA</p>
-
-<p>Drawing by Irene Sutcliffe</p>
-</figcaption></figure>
-<div> </div>
-</div>
-
-<hr class="chap x-ebookmaker-drop">
-
-<div class="chapter center vspace">
-<p>
-<i class="bb">“ROMANCE OF REALITY” SERIES</i></p>
-
-<p class="p1 xxlarge gesperrt bold">ELECTRICITY</p>
-
-<p class="p2">BY<br>
-<span class="large">W. H. McCORMICK</span></p>
-
-<figure id="i_5" class="figcenter" style="max-width: 20em;">
- <img src="images/i_005.png" width="1531" height="1072" alt="X-ray tube"></figure>
-
-<p class="p2">NEW YORK<br>
-<span class="larger">FREDERICK A. STOKES COMPANY</span><br>
-PUBLISHERS
-</p>
-</div>
-
-<hr class="chap x-ebookmaker-drop">
-
-<div class="chapter section p4">
-<p class="xsmall wspace center"><i>Printed in Great Britain</i></p>
-<div> </div>
-</div>
-
-<hr class="chap x-ebookmaker-drop">
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_v">v</span></p>
-<h2 class="nobreak" id="PREFACE">PREFACE</h2>
-</div>
-
-<p class="in0"><span class="firstword">I gladly</span> take this opportunity of acknowledging the generous
-assistance I have received in the preparation of this book.</p>
-
-<p>I am indebted to the following firms for much useful information
-regarding their various <span class="locked">specialities:—</span></p>
-
-<p>Chloride Electrical Storage Co. Ltd.; General Electric Co. Ltd.;
-Union Electric Co. Ltd.; Automatic Electric Co., Chicago; Westinghouse
-Cooper-Hewitt Co. Ltd.; Creed, Bille & Co. Ltd.; India
-Rubber, Gutta Percha, and Telegraph Works Co. Ltd.; W. Canning
-& Co.; C. H. F. Muller; Ozonair Ltd.; Universal Electric Supply
-Co., Manchester; and the Agricultural Electric Discharge Co. Ltd.</p>
-
-<p>For illustrations my thanks are due <span class="locked">to:—</span></p>
-
-<p>Marconi’s Wireless Telegraph Co. Ltd.; Chloride Electrical
-Storage Co. Ltd.; Harry W. Cox & Co. Ltd.; C. H. F. Muller;
-W. Canning & Co.; Union Electric Co. Ltd.; Creed, Bille & Co.
-Ltd.; Ozonair Ltd.; Kodak Ltd.; C. A. Parsons & Co.; Lancashire
-Dynamo and Motor Co. Ltd.; Dick, Kerr & Co. Ltd.;
-Siemens Brothers Dynamo Works Ltd.; Vickers Ltd.; and
-Craven Brothers Ltd.</p>
-
-<p>Mr. Edward Maude and Mr. J. A. Robson have most kindly
-prepared for me a number of the diagrams, and I am indebted
-to Dr. Myer Coplans for particulars and a diagram of the heat-compensated
-salinometer.</p>
-
-<p>I acknowledge also many important suggestions from Miss
-E. C. Dudgeon on Electro-Culture, and from Mr. R. Baxter and
-Mr. G. Clark on Telegraphy and Telephony.</p>
-
-<p>Amongst the many books I have consulted I am indebted<span class="pagenum" id="Page_vi">vi</span>
-specially to <cite>Electricity in Modern Medicine</cite>, by Alfred C. Norman,
-M.D.; <cite>Growing Crops and Plants by Electricity</cite>, by Miss E. C.
-Dudgeon; and <cite>Wireless Telegraphy</cite> (Cambridge Manuals), by
-Prof. C. L. Fortescue. I have derived great assistance also from
-the <cite>Wireless World</cite>.</p>
-
-<p>Finally, I have to thank Mr. Albert Innes, A.I.E.E., of Leeds,
-for a number of most valuable suggestions, and for his kindness in
-reading through the proofs.</p>
-
-<p class="right">
-<span style="margin-right: 2em;">W. H. McC.</span>
-</p>
-
-<p><span class="smcap">Leeds, 1915<span class="pagenum" id="Page_vii">vii</span></span></p>
-
-<hr class="chap x-ebookmaker-drop">
-
-<div class="chapter">
-<h2 class="nobreak" id="CONTENTS">CONTENTS</h2>
-</div>
-
-<table id="toc">
-<tr class="small">
- <td class="tdr">CHAPTER</td>
- <td></td>
- <td class="tdr">PAGE</td>
-</tr>
-<tr>
- <td class="tdr top">I.</td>
- <td class="tdl"><span class="smcap">The Birth of the Science of Electricity</span></td>
- <td class="tdr"><a href="#toclink_1">1</a></td>
-</tr>
-<tr>
- <td class="tdr top">II.</td>
- <td class="tdl"><span class="smcap">Electrical Machines and the Leyden Jar</span></td>
- <td class="tdr"><a href="#toclink_9">9</a></td>
-</tr>
-<tr>
- <td class="tdr top">III.</td>
- <td class="tdl"><span class="smcap">Electricity in the Atmosphere</span></td>
- <td class="tdr"><a href="#toclink_18">18</a></td>
-</tr>
-<tr>
- <td class="tdr top">IV.</td>
- <td class="tdl"><span class="smcap">The Electric Current</span></td>
- <td class="tdr"><a href="#toclink_27">27</a></td>
-</tr>
-<tr>
- <td class="tdr top">V.</td>
- <td class="tdl"><span class="smcap">The Accumulator</span></td>
- <td class="tdr"><a href="#toclink_38">38</a></td>
-</tr>
-<tr>
- <td class="tdr top">VI.</td>
- <td class="tdl"><span class="smcap">Magnets and Magnetism</span></td>
- <td class="tdr"><a href="#toclink_44">44</a></td>
-</tr>
-<tr>
- <td class="tdr top">VII.</td>
- <td class="tdl"><span class="smcap">The Production of Magnetism by Electricity</span></td>
- <td class="tdr"><a href="#toclink_56">56</a></td>
-</tr>
-<tr>
- <td class="tdr top">VIII.</td>
- <td class="tdl"><span class="smcap">The Induction Coil</span></td>
- <td class="tdr"><a href="#toclink_61">61</a></td>
-</tr>
-<tr>
- <td class="tdr top">IX.</td>
- <td class="tdl"><span class="smcap">The Dynamo and the Electric Motor</span></td>
- <td class="tdr"><a href="#toclink_66">66</a></td>
-</tr>
-<tr>
- <td class="tdr top">X.</td>
- <td class="tdl"><span class="smcap">Electric Power Stations</span></td>
- <td class="tdr"><a href="#toclink_75">75</a></td>
-</tr>
-<tr>
- <td class="tdr top">XI.</td>
- <td class="tdl"><span class="smcap">Electricity in Locomotion</span></td>
- <td class="tdr"><a href="#toclink_83">83</a></td>
-</tr>
-<tr>
- <td class="tdr top">XII.</td>
- <td class="tdl"><span class="smcap">Electric Lighting</span></td>
- <td class="tdr"><a href="#toclink_93">93</a></td>
-</tr>
-<tr>
- <td class="tdr top">XIII.</td>
- <td class="tdl"><span class="smcap">Electric Heating</span></td>
- <td class="tdr"><a href="#toclink_109">109</a></td>
-</tr>
-<tr>
- <td class="tdr top">XIV.</td>
- <td class="tdl"><span class="smcap">Electric Bells and Alarms</span></td>
- <td class="tdr"><a href="#toclink_116">116</a></td>
-</tr>
-<tr>
- <td class="tdr top">XV.</td>
- <td class="tdl"><span class="smcap">Electric Clocks</span></td>
- <td class="tdr"><a href="#toclink_124">124</a></td>
-</tr>
-<tr>
- <td class="tdr top">XVI.</td>
- <td class="tdl"><span class="smcap">The Telegraph</span></td>
- <td class="tdr"><a href="#toclink_128">128</a></td>
-</tr>
-<tr>
- <td class="tdr top">XVII.</td>
- <td class="tdl"><span class="smcap">Submarine Telegraphy</span></td>
- <td class="tdr"><a href="#toclink_144">144</a></td>
-</tr>
-<tr>
- <td class="tdr top">XVIII.</td>
- <td class="tdl"><span class="smcap">The Telephone</span></td>
- <td class="tdr"><a href="#toclink_154">154</a></td>
-</tr>
-<tr>
- <td class="tdr top">XIX.</td>
- <td class="tdl"><span class="smcap">Some Telegraphic and Telephonic Inventions</span></td>
- <td class="tdr"><a href="#toclink_171">171</a></td>
-</tr>
-<tr>
- <td class="tdr top">XX.</td>
- <td class="tdl"><span class="smcap">Wireless Telegraphy and Telephony—Principles and Apparatus</span></td>
- <td class="tdr"><a href="#toclink_179">179</a></td>
-</tr>
-<tr>
- <td class="tdr top">XXI.</td>
- <td class="tdl"><span class="smcap">Wireless Telegraphy—Practical Applications</span></td>
- <td class="tdr"><a href="#toclink_203">203</a></td>
-</tr>
-<tr>
- <td class="tdr top">XXII.</td>
- <td class="tdl"><span class="smcap">Electroplating and Electrotyping</span></td>
- <td class="tdr"><a href="#toclink_213">213</a></td>
-</tr>
-<tr>
- <td class="tdr top">XXIII.</td>
- <td class="tdl"><span class="smcap">Industrial Electrolysis</span></td>
- <td class="tdr"><a href="#toclink_224">224</a></td>
-</tr>
-<tr>
- <td class="tdr top">XXIV.</td>
- <td class="tdl"><span class="smcap">The Röntgen Rays</span></td>
- <td class="tdr"><a href="#toclink_228">228</a></td>
-</tr>
-<tr>
- <td class="tdr top">XXV.</td>
- <td class="tdl"><span class="smcap">Electricity in Medicine</span></td>
- <td class="tdr"><a href="#toclink_241">241</a></td>
-</tr>
-<tr>
- <td class="tdr top">XXVI.</td>
- <td class="tdl"><span class="smcap">Ozone</span></td>
- <td class="tdr"><a href="#toclink_247">247</a></td>
-</tr>
-<tr>
- <td class="tdr top">XXVII.</td>
- <td class="tdl"><span class="smcap">Electric Ignition</span></td>
- <td class="tdr"><a href="#toclink_253">253</a></td>
-</tr>
-<tr>
- <td class="tdr top">XXVIII.</td>
- <td class="tdl"><span class="smcap">Electro-Culture</span></td>
- <td class="tdr"><a href="#toclink_258">258</a></td>
-</tr>
-<tr>
- <td class="tdr top">XXIX.</td>
- <td class="tdl"><span class="smcap">Some Recent Applications of Electricity—An Electric Pipe Locator, etc.</span></td>
- <td class="tdr"><a href="#toclink_266">266</a></td>
-</tr>
-<tr>
- <td class="tdr top">XXX.</td>
- <td class="tdl"><span class="smcap">Electricity in War</span></td>
- <td class="tdr"><a href="#toclink_274">274</a></td>
-</tr>
-<tr>
- <td class="tdr top">XXXI.</td>
- <td class="tdl"><span class="smcap">What is Electricity?</span></td>
- <td class="tdr"><a href="#toclink_287">287</a></td>
-</tr>
-<tr>
- <td></td>
- <td class="tdl"><span class="smcap">Index</span></td>
- <td class="tdr"><a href="#toclink_295">295</a></td>
-</tr>
-</table>
-
-<hr class="chap x-ebookmaker-drop">
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_viii">viii</span></p>
-
-<h2 class="nobreak" id="LIST_OF_PLATES">LIST OF PLATES</h2>
-</div>
-
-<table id="loi">
-<tr>
- <td class="tdl norpad" colspan="2"><span class="smcap">Plate in Colour:
-The Marconi Transatlantic Wireless Station at Glace Bay, Nova Scotia</span>
- <span class="fright"><a href="#plate_0"><i>Frontispiece</i></a></span></td>
-</tr>
-<tr class="small">
- <td class="tdr" colspan="2">FACING PAGE</td>
-</tr>
-<tr>
- <td class="tdl"><span class="smcap">Hydro-Electric Power Station</span></td>
- <td class="tdr"><a href="#plate_I">30</a></td>
-</tr>
-<tr>
- <td class="tdl">(<i>a</i>) <span class="smcap">Experiment to show Magnetic Induction</span></td>
- <td class="tdr"><a href="#plate_IIa">48</a></td>
-</tr>
-<tr>
- <td class="tdl">(<i>b</i>) <span class="smcap">Experiment to show the Production of Magnetism by an Electric Current</span></td>
- <td class="tdr"><a href="#plate_IIb">48</a></td>
-</tr>
-<tr>
- <td class="tdl">(<i>a</i>) <span class="smcap">Lines of Magnetic Force of Two Opposite Poles</span></td>
- <td class="tdr"><a href="#plate_III">50</a></td>
-</tr>
-<tr>
- <td class="tdl">(<i>b</i>) <span class="smcap">Lines of Magnetic Force of Two Similar Poles</span></td>
- <td class="tdr"><a href="#plate_III">50</a></td>
-</tr>
-<tr>
- <td class="tdl"><span class="smcap">A Typical Dynamo and its Parts</span></td>
- <td class="tdr"><a href="#plate_IV">70</a></td>
-</tr>
-<tr>
- <td class="tdl"><span class="smcap">Lots Road Electric Power Station, Chelsea</span></td>
- <td class="tdr"><a href="#plate_V">76</a></td>
-</tr>
-<tr>
- <td class="tdl"><span class="smcap">Power Station Battery of Accumulators</span></td>
- <td class="tdr"><a href="#plate_VI">80</a></td>
-</tr>
-<tr>
- <td class="tdl"><span class="smcap">Electric Colliery Railway</span></td>
- <td class="tdr"><a href="#plate_VII">86</a></td>
-</tr>
-<tr>
- <td class="tdl"><span class="smcap">Typical Electric Locomotives</span></td>
- <td class="tdr"><a href="#plate_VIII">90</a></td>
-</tr>
-<tr>
- <td class="tdl"><span class="smcap">Night Photographs, taken by the Light of the Arc Lamps</span></td>
- <td class="tdr"><a href="#plate_IXa">96</a></td>
-</tr>
-<tr>
- <td class="tdl"><span class="smcap">Where Electrical Machinery is made</span></td>
- <td class="tdr"><a href="#plate_X">120</a></td>
-</tr>
-<tr>
- <td class="tdl"><span class="smcap">Specimen of the Work of the Creed High-Speed Printing Telegraph</span></td>
- <td class="tdr"><a href="#plate_XI">140</a></td>
-</tr>
-<tr>
- <td class="tdl"><span class="smcap">Large Electric Travelling Crane at a Railway Works</span></td>
- <td class="tdr"><a href="#plate_XII">164</a></td>
-</tr>
-<tr>
- <td class="tdl">(<i>a</i>) <span class="smcap">Marconi Operator Receiving a Message</span></td>
- <td class="tdr"><a href="#plate_XIIIa">188</a></td>
-</tr>
-<tr>
- <td class="tdl">(<i>b</i>) <span class="smcap">Marconi Magnetic Detector</span></td>
- <td class="tdr"><a href="#plate_XIIIb">188</a></td>
-</tr>
-<tr>
- <td class="tdl"><span class="smcap">Röntgen Ray Photograph of British and Foreign Fountain Pens</span></td>
- <td class="tdr"><a href="#plate_XIV">240</a></td>
-</tr>
-<tr>
- <td class="tdl"><span class="smcap">Bachelet “Flying Train” and its Inventor</span></td>
- <td class="tdr"><a href="#plate_XV">272</a></td>
-</tr>
-<tr>
- <td class="tdl">(<i>a</i>) <span class="smcap">Cavalry Portable Wireless Cart Set</span></td>
- <td class="tdr"><a href="#plate_XVIa">280</a></td>
-</tr>
-<tr>
- <td class="tdl">(<i>b</i>) <span class="smcap">Aeroplane fitted with Wireless Telegraphy</span></td>
- <td class="tdr"><a href="#plate_XVIb">280</a></td>
-</tr>
-</table>
-
-<hr class="chap x-ebookmaker-drop">
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_1">1</span></p>
-
-<h2 class="nobreak" id="toclink_1"><span class="larger">ELECTRICITY</span></h2>
-
-<h2 class="nobreak" id="chapter_I">CHAPTER I<br>
-
-<span class="subhead">THE BIRTH OF THE SCIENCE OF ELECTRICITY</span></h2>
-</div>
-
-<p class="in0"><span class="firstword">Although</span> the science of electricity is of comparatively
-recent date, electricity itself has existed from the beginning
-of the world. There can be no doubt that man’s introduction
-to electricity was brought about through the
-medium of the thunderstorm, and from very early times
-come down to us records of the terror inspired by thunder
-and lightning, and of the ways in which the ancients tried
-to account for the phenomena. Even to-day, although we
-know what lightning is and how it is produced, a severe
-thunderstorm fills us with a certain amount of awe, if not
-fear; and we can understand what a terrifying experience
-it must have been to the ancients, who had none of our
-knowledge.</p>
-
-<p>These early people had simple minds, and from our
-point of view they had little intelligence; but they possessed
-a great deal of curiosity. They were just as anxious to
-explain things as we are, and so they were not content
-until they had invented an explanation of lightning and
-thunder. Their favourite way of accounting for anything
-they did not understand was to make up a sort of romance
-about it. They believed that the heavens were inhabited
-by various gods, who showed their pleasure or anger by<span class="pagenum" id="Page_2">2</span>
-signs, and so they naturally concluded that thunder was
-the voice of angry gods, and lightning the weapon with
-which they struck down those who had displeased them.
-Prayers and sacrifices were therefore offered to the gods, in
-the hope of appeasing their wrath.</p>
-
-<p>Greek and Roman mythology contains many references
-to thunder and lightning. For instance, we read about
-the great god Zeus, who wielded thunder-bolts which had
-been forged in underground furnaces by the giant Cyclops.
-There was no doubt that the thunder-bolts were made in
-this way, because one only had to visit a volcano in order
-to see the smoke from the furnace, and hear the rumbling
-echo of the far-off hammering. Then we are told the
-tragic story of Phaeton, son of the Sun-god. This youth,
-like many others since his time, was daring and venturesome,
-and imagined that he could do things quite as well
-as his father. On one occasion he tried to drive his father’s
-chariot, and, as might have been expected, it got beyond
-his control, and came dangerously near the Earth. The
-land was scorched, the oceans were dried up, and the whole
-Earth was threatened with utter destruction. In order to
-prevent such a frightful catastrophe, Jupiter, the mighty
-lord of the heavens, hurled a thunder-bolt at Phaeton, and
-struck him from the chariot into the river Po. A whole
-book could be written about these ancient legends concerning
-the thunderstorm, but, interesting as they are, they
-have no scientific value, and many centuries were to elapse
-before the real nature of lightning was understood.</p>
-
-<p>In order to trace the first glimmerings of electrical
-knowledge we must leave the thunderstorm and pass on to
-more trivial matters. On certain sea-coasts the ancients
-found a transparent yellow substance capable of taking a
-high polish, and much to be desired as an ornament; and
-about 600 years <span class="allsmcap">B.C.</span> it was discovered that this substance,<span class="pagenum" id="Page_3">3</span>
-when rubbed, gained the power of drawing to it bits of
-straw, feathers, and other light bodies. This discovery is
-generally credited to a Greek philosopher named Thales,
-941–563 <span class="allsmcap">B.C.</span>, and it must be regarded as the first step
-towards the foundation of electrical science. The yellow
-substance was amber. We now know it to be simply a
-sort of fossilized resin, but the Greeks gave it a much more
-romantic origin. When Phaeton’s rashness brought him
-to an untimely end, his sorrowing sisters, the Heliades,
-were changed into poplar trees, and their tears into amber.
-Amongst the names given to the Sun-god was Alector,
-which means the shining one, and so the tears of the
-Heliades came to have the name Electron, or the shining
-thing. Unlike most of the old legends, this story of the
-fate of the Sun-maidens is of great importance to us, for
-from the word “electron” we get the name Electricity.</p>
-
-<p>Thales and his contemporaries seem to have made no
-serious attempts to explain the attraction of the rubbed
-amber, and indeed so little importance was attached to the
-discovery that it was completely forgotten. About 321 <span class="allsmcap">B.C.</span>
-one Theophrastus found that a certain mineral called
-“lyncurium” gained attractive powers when rubbed, but
-again little attention was paid to the matter, and astonishing
-as it may seem, no further progress worth mention was
-made until towards the close of the sixteenth century, when
-Doctor Gilbert of Colchester began to experiment seriously.
-This man was born about 1543, and took his degree of
-doctor of medicine at Cambridge in 1569. He was very
-successful in his medical work, and became President of the
-College of Physicians, and later on physician to Queen
-Elizabeth. He had a true instinct for scientific research,
-and was not content to accept statements on the authority
-of others, but tested everything for himself. He found
-that sulphur, resin, sealing-wax, and many other substances<span class="pagenum" id="Page_4">4</span>
-behaved like amber when rubbed, but he failed to get any
-results from certain other substances, such as the metals.
-He therefore called the former substances “electrics,” and
-the latter “anelectrics,” or non-electrics. His researches
-were continued by other investigators, and from him dates
-the science of electricity.</p>
-
-<figure id="fig_1" class="figleft" style="max-width: 8em;">
- <img src="images/i_014.png" width="609" height="1021" alt=" ">
- <figcaption class="caption hang"><span class="smcap">Fig. 1.</span>—Suspended
-pith ball for showing electric attraction.
-</figcaption></figure>
-
-<p>Leaving historical matters for the present, we will
-examine the curious power which is gained by substances
-as the result of rubbing. Amber is not always obtainable,
-and so we will use in its place a glass
-rod and a stick of sealing-wax. If the
-glass rod is rubbed briskly with a dry
-silk handkerchief, and then held close
-to a number of very small bits of paper,
-the bits are immediately drawn to the
-rod, and the same thing occurs if the
-stick of sealing-wax is substituted for
-the glass. This power of attraction is
-due to the presence of a small charge
-of electricity on the rubbed glass and
-sealing-wax, or in other words, the two
-substances are said to be electrified.
-Bits of paper are unsatisfactory for careful
-experimenting, and instead of them
-we will use the simple piece of apparatus shown in <a href="#fig_1">Fig. 1</a>.
-This consists of a ball of elder pith, suspended from a glass
-support by means of a silk thread. If now we repeat our
-experiments with the electrified glass or sealing-wax we
-find that the little ball is attracted in the same way as the
-bits of paper. But if we look carefully we shall notice that
-attraction is not the only effect, for as soon as the ball
-touches the electrified body it is driven away or repelled.
-Now let us suspend, by means of a thread, a glass rod
-which has been electrified by rubbing it with silk, and bring<span class="pagenum" id="Page_5">5</span>
-near it in turn another silk-rubbed glass rod and a stick of
-sealing-wax rubbed with flannel. The two glass rods are
-found to repel one another, whereas the sealing-wax attracts
-the glass. If the experiment is repeated with a suspended
-stick of sealing-wax rubbed with flannel, the glass and the
-sealing-wax attract each other, but the two sticks of wax
-repel one another. Both glass and sealing-wax are
-electrified, as may be seen by bringing them near the pith
-ball, but there must be some difference between them as
-we get attraction in one case and repulsion in the other.</p>
-
-<p>The explanation is that the electric charges on the silk-rubbed
-glass and on the flannel-rubbed sealing-wax are of
-different kinds, the former being called positive, and the
-latter negative. Bodies with similar charges, such as the
-two glass rods, repel one another; while bodies with unlike
-charges, such as the glass and the sealing-wax, attract each
-other. We can now see why the pith ball was first
-attracted and then repelled. To start with, the ball was
-not electrified, and was attracted when the rubbed glass or
-sealing-wax was brought near it. When however the
-ball touched the electrified body it received a share of the
-latter’s electricity, and as similar charges repel one another,
-the ball was driven away.</p>
-
-<p>The kind of electricity produced depends not only on
-the substance rubbed, but also on the material used as the
-rubber. For instance, we can give glass a negative charge
-by rubbing it with flannel, and sealing-wax becomes
-positively charged when rubbed with silk. The important
-point to remember is that there are only two kinds of
-electricity, and that every substance electrified by rubbing
-is charged either positively, like the silk-rubbed glass, or
-negatively, like the flannel-rubbed sealing-wax.</p>
-
-<p>If we try to electrify a metal rod by holding it in the
-hand and rubbing it, we get no result, but if we fasten to<span class="pagenum" id="Page_6">6</span>
-the metal a handle of glass, and hold it by this while
-rubbing, we find that it becomes electrified in the same way
-as the glass rod or the sealing-wax. Substances such as
-glass do not allow electricity to pass along them, so that in
-rubbing a glass rod the part rubbed becomes charged, and
-the electricity stays there, being unable to spread to the
-other parts of the rod. Substances such as metals allow
-electricity to pass easily, so that when a metal rod is
-rubbed electricity is produced, but it immediately spreads
-over the whole rod, reaches the hand, and escapes. If we
-wish the metal to retain its charge we must provide it with
-a handle of glass or of some other material which does not
-allow electricity to pass. Dr. Gilbert did not know this,
-and so he came to the conclusion that metals were non-electrics,
-or could not be electrified.</p>
-
-<p>Substances which allow electricity to pass freely are
-called conductors, and those which do not are called non-conductors;
-while between the two extremes are many
-substances which are called partial conductors. It may be
-said here that no substance is quite perfect in either respect,
-for all conductors offer some resistance to the passage of
-electricity, while all non-conductors possess some conducting
-power. Amongst conductors are metals, acids, water,
-and the human body; cotton, linen, and paper are partial
-conductors; and air, resin, silk, glass, sealing-wax, and
-gutta-percha are non-conductors. When a conductor is
-guarded by a non-conductor so that its electricity cannot
-escape, it is said to be insulated, from Latin, <i lang="la">insula</i>, an
-island; and non-conductors are also called “insulators.”</p>
-
-<p>So far we have mentioned only the electric charge
-produced on the substance rubbed, but the material used as
-rubber also becomes electrified. The two charges, however,
-are not alike, but one is always positive and the other
-negative. For instance, if glass is rubbed with silk, the<span class="pagenum" id="Page_7">7</span>
-glass receives a positive, and the silk a negative charge.
-It also can be shown that the two opposite charges are
-always equal in quantity.</p>
-
-<p>The two kinds of electricity are generally represented
-by the signs + and -, the former standing for positive
-and the latter for negative electricity.</p>
-
-<p>The electricity produced by rubbing, or friction, is
-known as Static Electricity; that is, electricity in a state of
-rest, as distinguished from electricity in motion, or current
-electricity. The word static is derived from a Greek word
-meaning to stand. At the same time it must be understood
-that this electricity of friction is at rest only in the
-sense that it is a prisoner, unable to move. When we
-produce a charge of static electricity on a glass rod, by
-rubbing it, the electricity would escape fast enough if it
-could. It has only two possible ways of escape, along the
-rod and through the air, and as both glass and air are non-conductors
-it is obliged to remain at rest where it was
-produced. On the other hand, as we have seen, the
-electricity produced by rubbing a metal rod which is not
-protected by an insulating handle escapes instantly, because
-the metal is a good conductor.</p>
-
-<p>When static electricity collects in sufficient quantities
-it discharges itself in the form of a bright spark, and we
-shall speak of these sparks in <a href="#chapter_III">Chapter III</a>. Static electricity
-is of no use for doing useful work, such as ringing bells or
-driving motors, and in fact, except for scientific purposes,
-it is more of a nuisance than a help. It collects almost
-everywhere, and its power of attraction makes it very
-troublesome at times. In the processes of textile manufacture
-static electricity is produced in considerable
-quantities, and it makes its presence known by causing the
-threads to stick together in the most annoying fashion. In
-printing rooms too it plays pranks, making the sheets of<span class="pagenum" id="Page_8">8</span>
-paper stick together so that the printing presses have to be
-stopped.</p>
-
-<p>Curiously enough, static electricity has been detected in
-the act of interfering with the work of its twin brother,
-current electricity. A little while ago it was noticed that
-the electric incandescent lamps in a certain building were
-lasting only a very short time, the filaments being found
-broken after comparatively little use. Investigations
-showed that the boy was in the habit of dusting the lamp
-globes with a feather duster. The friction set up in this
-way produced charges of electricity on the glass, and this
-had the effect of breaking the filaments. When this
-method of dusting was discontinued the trouble ceased, and
-the lamps lasted their proper number of hours.</p>
-<hr class="chap x-ebookmaker-drop">
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_9">9</span></p>
-
-<h2 class="nobreak" id="toclink_9"><a id="chapter_II"></a>CHAPTER II<br>
-
-<span class="subhead">ELECTRICAL MACHINES AND THE LEYDEN JAR</span></h2>
-</div>
-
-<p class="in0"><span class="firstword">The</span> amount of electricity produced by the rubbing of glass
-or sealing-wax rods is very small, and experimenters soon
-felt the need of apparatus to produce larger quantities. In
-1675 the first electrical machine was made by Otto von
-Guericke, the inventor of the air-pump. His machine consisted
-of a globe of sulphur fixed on a spindle, and rotated
-while the hands were pressed against it to provide the
-necessary friction. Globes and cylinders of glass soon
-replaced the sulphur globe, and the friction was produced
-by cushions instead of by the hands. Still later, revolving
-plates of glass were employed. These machines worked
-well enough in a dry atmosphere, but were very troublesome
-in wet weather, and they are now almost entirely
-superseded by what are known as <em>influence</em> machines.</p>
-
-<p>In order to understand the working of influence
-machines, it is necessary to have a clear idea of what is
-meant by the word influence as used in an electrical sense.
-In the previous chapter we saw that a pith ball was
-attracted by an electrified body, and that when the ball
-touched that body it received a charge of electricity.
-We now have to learn that one body can receive a charge
-from another body without actual contact, by what is called
-“influence,” or electro-static induction. In <a href="#fig_2">Fig. 2</a> is seen a
-simple arrangement for showing this influence or induction.
-A is a glass ball, and BC a piece of metal, either solid or<span class="pagenum" id="Page_10">10</span>
-hollow, made somewhat in the shape of a sausage, and
-insulated by means of its glass support. Three pairs of
-pith balls are suspended from BC as shown. If A is
-electrified positively, and brought near BC, the pith balls
-at B and C repel one another, showing that the ends of
-BC are electrified. No repulsion takes place between the
-two pith balls at the middle, indicating that this part of
-BC is not electrified. If the charges at B and C are tested
-they are found to be of opposite kinds, that at B being
-negative, and that at C positive. Thus it appears that
-the positive charge on A has attracted a negative charge
-to B, and repelled a positive one to C. If A is taken
-away, the two opposite charges on BC unite and neutralise
-one another, and BC is left in its original uncharged condition,
-while A is found to have lost none of its own charge.
-If BC is made in two parts, and if these are separated while
-under the influence of A, the two charges cannot unite
-when A is removed, but remain each on its own half of
-BC. In this experiment A is said to have induced electrification
-on BC. Induction will take place across a considerable
-distance, and it is not stopped by the interposition
-of obstacles such as a sheet of glass.</p>
-
-<figure id="fig_2" class="figcenter" style="max-width: 26em;">
- <img src="images/i_020.png" width="2014" height="892" alt=" ">
- <figcaption class="caption"><span class="smcap">Fig. 2.</span>—Diagram to illustrate Electro-static Induction.
-</figcaption></figure>
-
-<p><span class="pagenum" id="Page_11">11</span></p>
-
-<p>We can now understand why an electrified body
-attracts an unelectrified body, as in our pith ball experiments.
-If we bring a positively charged glass rod near
-a pith ball, the latter becomes electrified by induction, the
-side nearer the rod receiving a negative, and the farther
-side a positive charge. One half of the ball is therefore
-attracted and the other half repelled, but as the attracted
-half is the nearer, the attraction is stronger than the repulsion,
-and so the ball moves towards the rod.</p>
-
-<figure id="fig_3" class="figright" style="max-width: 13em;">
- <img src="images/i_021.png" width="1007" height="754" alt=" ">
- <figcaption class="caption"><span class="smcap">Fig. 3.</span>—The Electrophorus.
-</figcaption></figure>
-
-<p><a href="#fig_3">Fig. 3</a> shows an appliance for obtaining strong charges
-of electricity by influence or induction. It is called the
-<em>electrophorus</em>, the name coming from two Greek words,
-<em>electron</em>, amber, and <em>phero</em>,
-I yield or bear; and it was
-devised in 1775 by Volta, an
-Italian professor of physics.
-The apparatus consists of a
-round cake, A, of some
-resinous material contained
-in a metal dish, and a round
-disc of metal, B, of slightly
-smaller diameter, fitted with
-an insulating handle. A simple electrophorus may be
-made by filling with melted sealing-wax the lid of a
-round tin, the disc being made of a circular piece of
-copper or brass, a little smaller than the lid, fastened to
-the end of a stick of sealing-wax. To use the electrophorus,
-the sealing-wax is electrified negatively by rubbing
-it with flannel. The metal disc is then placed on the
-sealing-wax, touched for an instant with the finger, and
-lifted away. The disc is now found to be electrified
-positively, and it may be discharged and the process repeated
-many times without recharging the sealing-wax.
-The charge on the latter is not used up in the process,<span class="pagenum" id="Page_12">12</span>
-but it gradually leaks away, and after a time it has to
-be renewed.</p>
-
-<p>The theory of the electrophorus is easy to understand
-from what we have already learnt about influence. When
-the disc B is placed on the charged cake A, the two surfaces
-are really in contact at only three or four points,
-because neither of them is a true plane; so that on the
-whole the disc and the cake are like A and BC in <a href="#fig_2">Fig. 2</a>,
-only much closer together. The negative charge on
-A acts by induction
-on the disc B, attracting
-a positive charge
-to the under side, and
-repelling a negative
-charge to the upper
-side. When the disc
-is touched, the negative
-charge on the
-upper side escapes, but
-the positive charge
-remains, being as it
-were held fast by the
-attraction of the negative
-charge on A. If
-the disc is now raised, the positive charge is no longer
-bound on the under side, and it therefore spreads over
-both surfaces, remaining there because its escape is cut
-off by the insulating handle.</p>
-
-<figure id="fig_4" class="figleft" style="max-width: 16em;">
- <img src="images/i_022.jpg" width="1229" height="1207" alt=" ">
- <figcaption class="caption"><span class="smcap">Fig. 4.</span>—Wimshurst Machine.
-</figcaption></figure>
-
-<p>We may now try to understand the working of influence
-machines, which are really mechanically worked electrophori.
-There are various types of such machines, but the
-one in most general use in this country is that known as
-the Wimshurst machine, <a href="#fig_4">Fig. 4</a>, and we will therefore
-confine ourselves to this. It consists of two circular plates<span class="pagenum" id="Page_13">13</span>
-of varnished glass or of ebonite, placed close together and
-so geared that they rotate in opposite directions. On the
-outer surfaces of the plates are cemented sectors of metal
-foil, at equal distances apart. Each plate has the same
-number of sectors, so that at any given moment the sectors
-on one plate are exactly opposite those on the other.
-Across the outer surface of each plate is fixed a rod of
-metal carrying at its ends light tinsel brushes, which are
-adjusted to touch the sectors as they pass when the plates
-are rotated. These rods are placed at an angle to each
-other of from sixty to ninety degrees, and the brushes are
-called neutralizing brushes. The machine is now complete
-for generating purposes, but in order to collect the electricity
-two pairs of insulated metal combs are provided, one pair
-at each end of the horizontal diameter, with the teeth
-pointing inward towards the plates, but not touching them.
-The collecting combs are fitted with adjustable discharging
-rods terminating in round knobs.</p>
-
-<p>The principle upon which the machine works will be
-best understood by reference to <a href="#fig_5">Fig. 5</a>. In this diagram
-the inner circle represents the front plate, with neutralizing
-brushes A and B, and the outer one represents the back
-plate, with brushes C and D. The sectors are shown
-heavily shaded. E and F are the pairs of combs, and the
-plates rotate in the direction of the arrows. Let us suppose
-one of the sectors at the top of the back plate to have a
-slight positive charge. As the plates rotate this sector will
-come opposite to a front plate sector touched by brush A,
-and it will induce a slight negative charge on the latter
-sector, at the same time repelling a positive charge along
-the rod to the sector touched by brush B. The two sectors
-carrying the induced charges now move on until opposite
-back plate sectors touched by brushes C and D, and these
-back sectors will receive by induction positive and negative<span class="pagenum" id="Page_14">14</span>
-charges respectively. This process continues as the plates
-rotate, and finally all the sectors moving towards comb E
-will be positively charged, while those approaching comb
-F will be negatively charged. The combs collect these
-charges, and the discharging rods K and L become highly
-electrified, K positively and L negatively, and if they are
-near enough together sparks will pass between them.</p>
-
-<figure id="fig_5" class="figcenter" style="max-width: 27em;">
- <img src="images/i_024.jpg" width="2086" height="1762" alt=" ">
- <figcaption class="caption"><span class="smcap">Fig. 5.</span>—Diagram to illustrate working of a Wimshurst Machine.
-</figcaption></figure>
-
-<p>At the commencement we supposed one of the sectors
-to have a positive charge, but it is not necessary to charge
-a sector specially, for the machine is self-starting. Why
-this is the case is not yet thoroughly understood, but probably
-the explanation is that at any particular moment no
-two places in the atmosphere are in exactly the same<span class="pagenum" id="Page_15">15</span>
-electro-static condition, so that an uneven state of charge
-exists permanently amongst the sectors.</p>
-
-<p>The Wimshurst machine provides us with a plentiful
-supply of electricity, and the question naturally arises,
-“Can this electricity be stored up in any way?” In 1745,
-long before the days of influence machines, a certain Bishop
-of Pomerania, Von Kleist by name, got the idea that if he
-could persuade a charge of electricity to go into a glass
-bottle he would be able to capture it, because glass was a
-non-conductor. So he partly filled a bottle with water, led
-a wire down into the water, and while holding the bottle in
-one hand connected the wire to a primitive form of electric
-machine. When he thought he had got enough electricity
-he tried to remove his bottle in order to examine the contents,
-and in so doing he received a shock which scared
-him considerably. He had succeeded in storing electricity
-in his bottle. Shortly afterwards the bishop’s experiment
-was repeated by Professor Muschenbrock of Leyden, and
-by his pupil Cuneus, the former being so startled by the
-shock that he wrote, “I would not take a second shock for
-the kingdom of France.” But in spite of shocks the end
-was achieved; it was proved that electricity could be collected
-and stored up, and the bottle became known as the
-Leyden jar. The original idea was soon improved upon,
-water being replaced by a coating of tinfoil, and it was
-found that better results were obtained by coating the
-outside of the bottle as well as the inside.</p>
-
-<p>As now used the Leyden jar consists of a glass jar
-covered inside and outside with tinfoil up to about two-thirds
-of its height. A wooden lid is fitted, through which passes
-a brass rod terminating above in a brass knob, and below
-in a piece of brass chain long enough to touch the foil
-lining. A Leyden jar is charged by holding it in one
-hand with its knob presented to the discharging ball of a<span class="pagenum" id="Page_16">16</span>
-Wimshurst machine, and even if the machine is small and
-feeble the jar will accumulate electricity until it is very
-highly charged. It may now be put down on the table,
-and if it is clean and quite dry it will hold its charge for
-some time. If the outer and inner coatings of the jar are
-connected by means of a piece of metal, the electricity will
-be discharged in the form of a bright spark. A Leyden
-jar is usually discharged by means of discharging tongs,
-consisting of a jointed brass rod with brass terminal
-knobs and glass handles. One knob is placed in contact
-with the outer coating of foil, and the other brought near
-to the knob of the jar, which of course is connected with
-the inner coating.</p>
-
-<p>The electrical capacity of even a small Leyden jar is
-surprisingly great, and this is due to the mutual attraction
-between opposite kinds of electricity. If we stick a piece
-of tinfoil on the centre of each face of a pane of glass, and
-charge one positively and the other negatively, the two
-charges attract each other through the glass; and in fact
-they hold on to each other so strongly that we can get very
-little electricity by touching either piece of foil. This
-mutual attraction enables us to charge the two pieces of
-foil much more strongly than if they were each on a
-separate pane, and this is the secret of the working of the
-Leyden jar. If the knob of the jar is held to the positive
-ball of a Wimshurst, the inside coating receives a positive
-charge, which acts inductively on the outside coating,
-attracting a negative charge to the inner face of the latter,
-and repelling a positive charge to its outer face, and thence
-away through the hand. The electricity is entirely confined
-to the sides of the jar, the interior having no charge
-whatever.</p>
-
-<p>Leyden jars are very often fitted to a Wimshurst
-machine as shown at A, A, <a href="#fig_4">Fig. 4</a>, and arranged so that they<span class="pagenum" id="Page_17">17</span>
-can be connected or disconnected to the collecting combs
-as desired. When the jars are disconnected the machine
-gives a rapid succession of thin sparks, but when the jars
-are connected to the combs they accumulate a number of
-charges before the discharge takes place, with the result
-that the sparks are thicker, but occur at less frequent
-intervals.</p>
-
-<p>It will have been noticed that the rod of a Leyden jar
-and the discharging rods of a Wimshurst machine are
-made to terminate not in points, but in rounded knobs or
-balls. The reason of this is that electricity rapidly leaks
-away from points. If we electrify a conductor shaped like
-a cone with a sharp point, the density of the electricity is
-greatest at that point, and when it becomes sufficiently
-great the particles of air near the point become electrified
-and repelled. Other particles take their place, and are
-electrified and repelled in the same way, and so a constant
-loss of electricity takes place. This may be shown in an
-interesting way by fastening with wax a needle to the knob
-of a Wimshurst. If a lighted taper is held to the point of
-the needle while the machine is in action, the flame is
-blown aside by the streams of repelled air, which form a
-sort of electric wind.</p>
-<hr class="chap x-ebookmaker-drop">
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_18">18</span></p>
-
-<h2 class="nobreak" id="toclink_18"><a id="chapter_III"></a>CHAPTER III<br>
-
-<span class="subhead">ELECTRICITY IN THE ATMOSPHERE</span></h2>
-</div>
-
-<p class="in0"><span class="firstword">If</span> the Leyden jars of a Wimshurst machine are connected
-up and the discharging balls placed at a suitable distance
-apart, the electricity produced by rotating the plates is
-discharged in the form of a brilliant zigzag spark between
-the balls, accompanied by a sharp crack. The resemblance
-between this spark and forked lightning is at once evident,
-and in fact it is lightning in miniature. The discharging
-balls are charged, as we have seen, with opposite kinds of
-electricity, and these charges are constantly trying to reach
-one another across the intervening air, which, being an
-insulator, vigorously opposes their passage. There is thus
-a kind of struggle going on between the air and the two
-charges of electricity, and this keeps the air in a state of
-constant strain. But the resisting power of the air is
-limited, and when the charges reach a certain strength the
-electricity violently forces its way across, literally rupturing
-or splitting the air. The particles of air along the path of
-the discharge are rendered incandescent by the heat produced
-by the passage of the electricity, and so the brilliant
-flash is produced. Just as a river winds about seeking the
-easiest course, so the electricity takes the path of least
-resistance, which probably is determined by the particles
-of dust in the air, and also by the density of the air, which
-becomes compressed in front, leaving less dense air and
-therefore an easier path on each side.</p>
-
-<p><span class="pagenum" id="Page_19">19</span></p>
-
-<p>The connexion between lightning and the sparks from
-electrified bodies and electrical machines was suspected by
-many early observers, but it remained for Benjamin
-Franklin to prove that lightning was simply a tremendous
-electric discharge, by actually obtaining electricity from a
-thunder-cloud. Franklin was an American, born at Boston
-in 1706. He was a remarkable man in every way, and
-quite apart from his investigations in electricity, will always
-be remembered for the great public services he rendered to
-his country in general and to Philadelphia in particular.
-He founded the Philadelphia Library, the American Philosophical
-Society, and the University of Pennsylvania.</p>
-
-<p>Franklin noticed many similarities between electricity
-and lightning. For instance, both produced zigzag sparks,
-both were conducted by metals, both set fire to inflammable
-materials, and both were capable of killing animals. These
-resemblances appeared to him so striking that he was
-convinced that the two were the same, and he resolved to
-put the matter to the test. For this purpose he hit upon
-the idea of using a kite, to the top of which was fixed a
-pointed wire. At the lower end of the flying string was
-tied a key, insulated by a piece of silk ribbon. In June
-1752, Franklin flew his kite, and after waiting a while he
-was rewarded by finding that when he brought his knuckle
-near to the key a little spark made its appearance. This
-spark was exactly like the sparks obtained from electrified
-bodies, but to make things quite certain a Leyden jar was
-charged from the key. Various experiments were then
-performed with the jar, and it was proved beyond all doubt
-that lightning and electricity were one and the same.</p>
-
-<p>Lightning is then an enormous electric spark between a
-cloud and the Earth, or between two clouds, produced when
-opposite charges become so strong that they are able to
-break down the intervening non-conducting layer of air.<span class="pagenum" id="Page_20">20</span>
-The surface of the Earth is negatively electrified, the
-electrification varying at different times and places; while
-the electricity of the air is usually positive, but frequently
-changes to negative in rainy weather and on other occasions.
-As the clouds float about they collect the electricity from
-the air, and thus they may be either positively or negatively
-electrified, so that a discharge may take place between one
-cloud and another, as well as between a cloud and the Earth.</p>
-
-<p>Lightning flashes take different forms, the commonest
-being forked or zigzag lightning, and sheet lightning.
-The zigzag form is due to the discharge taking the easiest
-path, as in the case of the spark from a Wimshurst machine.
-Sheet lightning is probably the reflection of a flash taking
-place at a distance. It may be unaccompanied by thunder,
-as in the so-called “summer lightning,” seen on the horizon
-at night, which is the reflection of a storm too far off for the
-thunder to be heard. A much rarer form is globular or
-ball lightning, in which the discharge takes the shape of a
-ball of light, which moves slowly along and finally disappears
-with a sudden explosion. The cause of this form
-of lightning is not yet understood, but it is possible that the
-ball of light consists of intensely heated and extremely
-minute fragments of ordinary matter, torn off by the
-violence of the lightning discharge. Another uncommon
-form is multiple lightning, which consists of a number of
-separate parallel discharges having the appearance of a
-ribbon.</p>
-
-<p>A lightning flash probably lasts from about 1/100,000 to
-1/1,000,000 of a second, and in the majority of cases the
-discharge is oscillatory; that is to say, it passes several times
-backwards and forwards between two clouds or between a
-cloud and the Earth. At times it appears as though we
-could see the lightning start downwards from the cloud or
-upwards from the Earth, but this is an optical illusion, and<span class="pagenum" id="Page_21">21</span>
-it is really quite impossible to tell at which end the flash
-starts.</p>
-
-<p>Death by lightning is instantaneous, and therefore
-quite painless. We are apt to think that pain is felt at the
-moment when a wound is inflicted. This is not the case
-however, for no pain is felt until the impression reaches the
-brain by way of the nerves, and this takes an appreciable
-time. The nerves transmit sensations at a speed of only
-about one hundred feet per second, so that in the case of a
-man killed by a bullet through the brain, no pain would be
-felt, because the brain would be deprived of sensibility
-before the sensation could reach it. Lightning is infinitely
-swifter than any bullet, so life would be destroyed by it
-before any pain could be felt.</p>
-
-<p>On one occasion Professor Tyndall, the famous
-physicist, received accidentally a very severe shock from
-a large battery of Leyden jars while giving a public lecture.
-His account of his sensations is very interesting. “Life
-was absolutely blotted out for a very sensible interval,
-without a trace of pain. In a second or so consciousness
-returned; I saw myself in the presence of the audience and
-apparatus, and, by the help of these external appearances,
-immediately concluded that I had received the battery discharge.
-The intellectual consciousness of my position was
-restored with exceeding rapidity, but not so the optical
-consciousness. To prevent the audience from being
-alarmed, I observed that it had often been my desire to
-receive accidentally such a shock, and that my wish had at
-length been fulfilled. But, while making this remark, the
-appearance which my body presented to myself was that of
-a number of separate pieces. The arms, for example, were
-detached from the trunk, and seemed suspended in the air.
-In fact, memory and the power of reasoning appeared to
-be complete long before the optic nerve was restored to<span class="pagenum" id="Page_22">22</span>
-healthy action. But what I wish chiefly to dwell upon
-here is, the absolute painlessness of the shock; and there
-cannot be a doubt that, to a person struck dead by lightning,
-the passage from life to death occurs without consciousness
-being in the least degree implicated. It is an abrupt
-stoppage of sensation, unaccompanied by a pang.”</p>
-
-<p>Occasionally branched markings are found on the
-bodies of those struck by lightning, and these are often
-taken to be photographic impressions of trees under which
-the persons may have been standing at the time of the
-flash. The markings however are nothing of the kind,
-but are merely physiological effects due to the passage of
-the discharge.</p>
-
-<p>During a thunderstorm it is safer to be in the house
-than out in the open. It is probable that draughts are a
-source of some danger, and the windows and doors of the
-room ought to be shut. Animals are more liable to be
-struck by lightning than men, and a shed containing
-horses or cows is a dangerous place in which to take
-shelter; in fact it is better to remain in the open. If one
-is caught in a storm while out of reach of a house or other
-building free from draughts and containing no animals,
-the safest plan is to lie down, not minding the rain.
-Umbrellas are distinctly dangerous, and never should be
-used during a storm. Wire fences, hedges, and still or
-running water should be given a wide berth, and it is
-safer to be alone than in company with a crowd of people.
-It is extremely foolish to take shelter under an isolated
-tree, for such trees are very liable to be struck. Isolated
-beech trees appear to have considerable immunity from
-lightning, but any tree standing alone should be avoided,
-the oak being particularly dangerous. On the other hand,
-a fairly thick wood is comparatively safe, and failing a
-house, should be chosen before all other places of refuge.<span class="pagenum" id="Page_23">23</span>
-Horses are liable to be struck, and if a storm comes on
-while one is out driving it is safer to keep quite clear of the
-animals.</p>
-
-<p>When a Wimshurst machine has been in action for a
-little time a peculiar odour is noticed. This is due to the
-formation of a modified and chemically more active form of
-oxygen, called <em>ozone</em>, the name being derived from the
-Greek <em>ozein</em>, “to smell.” Ozone has very invigorating effects
-when breathed, and it is also a powerful germicide, capable
-of killing the germs which give rise to contagious diseases.
-During a thunderstorm ozone is produced in large
-quantities by the electric discharges, and thus the air
-receives as it were a new lease of life, and we feel the
-refreshing effects when the storm is over. We shall speak
-again of ozone in <a href="#chapter_XXV">Chapter XXV</a>.</p>
-
-<p>Thunder probably is caused by the heating and sudden
-expansion of the air in the path of the discharge, which
-creates a partial vacuum into which the surrounding air
-rushes violently. Light travels at the rate of 186,000
-miles per second, and therefore the flash reaches us
-practically instantaneously; but sound travels at the rate of
-only about 1115 feet per second, so that the thunder takes
-an appreciable time to reach us, and the farther away the
-discharge the greater the interval between the flash and
-the thunder. Thus by multiplying the number of seconds
-which elapse between the flash and the thunder by 1115,
-we may calculate roughly the distance in feet of the
-discharge. A lightning flash may be several miles in
-length, the greatest recorded length being about ten miles.
-The sounds produced at different points along its path
-reach us at different times, producing the familiar sharp
-rattle, and the following rolling and rumbling is produced
-by the echoes from other clouds. The noise of a thunder-clap
-is so tremendous that it seems as though the sound<span class="pagenum" id="Page_24">24</span>
-would be heard far and wide, but the greatest distance at
-which thunder has been heard is about fifteen miles. In
-this respect it is interesting to compare the loudest
-thunder-clap we ever heard with the noise of the famous
-eruption of Krakatoa, in 1883, which was heard at the
-enormous distance of nearly three thousand miles.</p>
-
-<p>When Franklin had demonstrated the nature of
-lightning, he began to consider the possibility of protecting
-buildings from the disastrous effects of the lightning stroke.
-At that time the amount of damage caused by lightning
-was very great. Cathedrals, churches, public buildings,
-and in fact all tall edifices were in danger every time a
-severe thunderstorm took place in their neighbourhood, for
-there was absolutely nothing to prevent their destruction if
-the lightning chanced to strike them. Ships at sea, too,
-were damaged very frequently by lightning, and often
-some of the crew were killed or disabled. To-day, thanks
-to the lightning conductor, it is an unusual occurrence for
-ships or large buildings to be damaged by lightning. The
-lightning strikes them as before, but in the great majority
-of cases it is led away harmlessly to earth.</p>
-
-<p>Franklin was the first to suggest the possibility of
-protecting buildings by means of a rod of some conducting
-material terminating in a point at the highest part of the
-building, and leading down, outside the building, into the
-earth. Lightning conductors at the present day are
-similar to Franklin’s rod, but many improvements have
-been made from time to time as our knowledge of the
-nature and action of the lightning discharge has increased.
-A modern lightning conductor generally consists of one or
-more pointed rods fixed to the highest parts of the building,
-and connected to a cable running directly to earth. This
-cable is kept as straight as possible, because turns and
-bends offer a very high resistance to the rapidly oscillating<span class="pagenum" id="Page_25">25</span>
-discharge; and it is connected to large copper plates
-buried in permanently moist ground or in water, or to
-water or gas mains. Copper is generally used for the
-cable, but iron also may be employed. In any case, the
-cable must be of sufficient thickness to prevent the
-possibility of its being deflagrated by the discharge. In
-ships the arrangements are similar, except that the cable is
-connected to the copper sheathing of the bottom.</p>
-
-<p>The fixing of lightning conductors must be carried out
-with great care, for an improperly fixed conductor is not
-only useless, but may be a source of actual danger.
-Lightning flashes vary greatly in character, and while a
-carefully erected lightning conductor is capable of dealing
-with most of them, there are unfortunately certain kinds of
-discharge with which it now and then is unable to deal.
-The only absolutely certain way of protecting a building is
-to surround it completely by a sort of cage of metal, but
-except for buildings in which explosives are stored this
-plan is usually impracticable.</p>
-
-<p>The electricity of the atmosphere manifests itself in
-other forms beside the lightning. The most remarkable
-of these manifestations is the beautiful phenomenon known
-in the Northern Hemisphere as the Aurora Borealis, and
-in the Southern Hemisphere as the Aurora Australis.
-Aurora means the morning hour or dawn, and the phenomenon
-is so called from its resemblance to the dawn of
-day. The aurora is seen in its full glory only in high
-latitudes, and it is quite unknown at the equator. It
-assumes various forms, sometimes appearing as an arch of
-light with rapidly moving streamers of different colours,
-and sometimes taking the form of a luminous curtain
-extending across the sky. The light of the aurora is never
-very strong, and as a rule stars can be seen through it.
-Auroras are sometimes accompanied by rustling or crackling<span class="pagenum" id="Page_26">26</span>
-sounds, but the sounds are always extremely faint.
-Some authorities assert that these sounds do not exist, and
-that they are the result of imagination, but other equally
-reliable observers have heard the sounds quite plainly on
-several occasions. Probably the explanation of this confliction
-of evidence is that the great majority of auroras are
-silent, so that an observer might witness many of them
-without hearing any sounds. The height at which auroras
-occur is a disputed point, and one which it is difficult to
-determine accurately; but most observers agree that it
-is generally from 60 to 125 miles above the Earth’s
-surface.</p>
-
-<p>There is little doubt that the aurora is caused by the
-passage of electric discharges through the higher regions
-of the atmosphere, where the air is so rarefied as to act as
-a partial conductor; and its effects can be imitated in some
-degree by passing powerful discharges through tubes from
-which the air has been exhausted to a partial vacuum.
-Auroral displays are usually accompanied by magnetic
-disturbances, which sometimes completely upset telegraphic
-communication. Auroras and magnetic storms appear to
-be connected in some way with solar disturbances, for they
-are frequently simultaneous with an unusual number of
-sunspots, and all three run in cycles of about eleven and a
-half years.</p>
-<hr class="chap x-ebookmaker-drop">
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_27">27</span></p>
-
-<h2 class="nobreak" id="toclink_27"><a id="chapter_IV"></a>CHAPTER IV<br>
-
-<span class="subhead">THE ELECTRIC CURRENT</span></h2>
-</div>
-
-<p class="in0"><span class="firstword">In</span> the previous chapters we have dealt with electricity in
-charged bodies, or static electricity, and now we must turn
-to electricity in motion, or current electricity. In <a href="#chapter_I">Chapter I</a>.
-we saw that if a metal rod is held in the hand and rubbed,
-electricity is produced, but it immediately escapes along the
-rod to the hand, and so to the earth. In other words, the
-electricity flows away along the conducting path provided
-by the rod and the hand. When we see the word “flow” we
-at once think of a fluid of some kind, and we often hear
-people speak of the “electric fluid.” Now, whatever
-electricity may be it certainly is not a fluid, and we use
-the word “flow” in connexion with electricity simply because
-it is the most convenient word we can find for the purpose.
-Just in the same way we might say that when we hold a
-poker with its point in the fire, heat flows along it towards
-our hand, although we know quite well that heat is not a
-fluid. In the experiment with the metal rod referred to
-above, the electricity flows away instantly, leaving the rod
-unelectrified; but if we arrange matters so that the
-electricity is renewed as fast as it flows away, then we get
-a continuous flow, or current.</p>
-
-<p>Somewhere about the year 1780 an Italian anatomist,
-Luigi Galvani, was studying the effects of electricity upon
-animal organisms, using for the purpose the legs of freshly
-killed frogs. In the course of his experiments he happened
-to hang against an iron window rail a bundle of frogs’ legs<span class="pagenum" id="Page_28">28</span>
-fastened together with a piece of copper wire, and he
-noticed that the legs began to twitch in a peculiar manner.
-He knew that a frog’s leg would twitch when electricity
-was applied to it, and he concluded that the twitchings in
-this case were caused in the same way. So far he was
-quite right, but then came the problem of how any
-electricity could be produced in these circumstances, and
-here he went astray. It
-never occurred to him that
-the source of the electricity
-might be found in something
-quite apart from the legs,
-and so he came to the conclusion
-that the phenomenon
-was due to electricity produced
-in some mysterious
-way in the tissues of the
-animal itself. He therefore
-announced that he had discovered
-the existence of a
-kind of animal electricity,
-and it was left for his fellow-countryman,
-Alessandro
-Volta, to prove that the
-twitchings were due to electricity
-produced by the contact
-of the two metals, the iron of the window rail and the
-copper wire.</p>
-
-<figure id="fig_6" class="figleft" style="max-width: 14em;">
- <img src="images/i_038.jpg" width="1092" height="1523" alt=" ">
- <figcaption class="caption"><span class="smcap">Fig. 6.</span>—Voltaic Pile.
-</figcaption></figure>
-
-<p>Volta found that when two different metals were placed in
-contact in air, one became positively charged, and the other
-negatively. These charges however were extremely feeble,
-and in his endeavours to obtain stronger results he hit upon
-the idea of using a number of pairs of metals, and he constructed
-the apparatus known as the Voltaic pile, <a href="#fig_6">Fig. 6</a>.<span class="pagenum" id="Page_29">29</span>
-This consists of a number of pairs of zinc and copper
-discs, each pair being separated from the next pair by a
-disc of cloth moistened with salt water. These are piled
-up and placed in a frame, as shown in the figure. One
-end of the pile thus terminates in a zinc disc, and the other
-in a copper disc, and as soon as the two are connected by
-a wire or other conductor a continuous current of electricity
-is produced. The cause of the electricity produced by the
-voltaic pile was the subject of
-a long and heated controversy.
-There were two main theories;
-that of Volta himself, which
-attributed the electricity to the
-mere contact of unlike metals,
-and the chemical theory, which
-ascribed it to chemical action.
-The chemical theory is now
-generally accepted, but certain
-points, into which we need not
-enter, are still in dispute.</p>
-
-<p>There is a curious experiment
-which some of my readers
-may like to try. Place a copper
-coin on a sheet of zinc, and set an
-ordinary garden snail to crawl
-across the zinc towards the coin. As soon as the snail
-comes in contact with the copper it shrinks back, and shows
-every sign of having received a shock. One can well
-imagine that an enthusiastic gardener pestered with snails
-would watch this experiment with great glee.</p>
-
-<figure id="fig_7" class="figright" style="max-width: 11em;">
- <img src="images/i_039.jpg" width="830" height="1338" alt=" ">
- <figcaption class="caption"><span class="smcap">Fig. 7.</span>—Simple Voltaic Cell.
-</figcaption></figure>
-
-<p>Volta soon found that it was not necessary to have his
-pairs of metals in actual metallic contact, and that better
-results were got by placing them in a vessel filled with
-dilute acid. <a href="#fig_7">Fig. 7</a> is a diagram of a simple voltaic cell of<span class="pagenum" id="Page_30">30</span>
-this kind, and it shows the direction of the current when
-the zinc and the copper are connected by the wire. In
-order to get some idea of the reason why a current flows
-we must understand the meaning of electric potential. If
-water is poured into a vessel, a certain water pressure is
-produced. The amount of this pressure depends upon the
-level of the water, and this in turn depends upon the
-quantity of water and the capacity of the vessel, for a
-given quantity of water will reach a higher level in a small
-vessel than in a larger one. In the same way, if electricity
-is imparted to a conductor an electric pressure is
-produced, its amount depending upon the quantity of
-electricity and the electric capacity of the conductor, for
-conductors vary in capacity just as water vessels do.</p>
-
-<p>This electric pressure is called “potential,” and electricity
-tends to flow from a conductor of higher to one of lower
-potential. When we say that a place is so many feet
-above or below sea-level we are using the level of the sea
-as a zero level, and in estimating electric potential we take
-the potential of the earth’s surface as zero; and we regard
-a positively electrified body as one at a positive or relatively
-high potential, and a negatively electrified body as
-one at a negative or relatively low potential. This may be
-clearer if we think of temperature and the thermometer.
-Temperatures above zero are positive and represented by
-the sign +, and those below zero are negative and represented
-by the sign -. Thus we assume that an electric
-current flows from a positive to a negative conductor.</p>
-
-<figure id="plate_I" class="figcenter" style="max-width: 40em;">
- <p class="caption">PLATE I.</p>
- <img src="images/i_041.jpg" width="3173" height="2031" alt=" ">
- <figcaption class="caption">
-
-<p class="floatl"><i>By permission of</i></p>
-<p class="floatr"><i>Dick, Kerr & Co. Ltd.</i></p>
-
-<p class="floatc">HYDRO-ELECTRIC POWER STATION.</p>
-</figcaption></figure>
-
-<p>In a voltaic cell the plates are at different potentials, so
-that when they are connected by a wire a current flows,
-and we say that the current leaves the cell at the positive
-terminal, and enters it again at the negative terminal. As
-shown in <a href="#fig_7">Fig. 7</a>, the current moves in opposite directions
-inside and outside the cell, making a complete round called
-a <em>circuit</em>, and if the circuit is broken anywhere the current
-ceases to flow. If the circuit is complete the current keeps
-on flowing, trying to equalize the electric pressure or
-potential, but it is unable to do this because the chemical
-action between the acid and the zinc maintains the difference
-of potential between the plates. This chemical action
-results in wasting of the zinc and weakening of the acid,
-and as long as it continues the current keeps on flowing.
-When we wish to stop the current we break the circuit by
-disconnecting the wire joining the terminals, and the cell
-then should be at rest; but owing to the impurities in
-ordinary commercial zinc chemical action still continues.
-In order to prevent wasting when the current is not required
-the surface of the zinc is coated with a thin film of
-mercury. The zinc is then said to be amalgamated, and
-it is not acted upon by the acid so long as the circuit
-remains broken.</p>
-
-<p>The current from a simple voltaic cell does not remain
-at a constant strength, but after a short time it begins to
-weaken rapidly. The cell is then said to be polarized, and
-this polarization is caused by bubbles of hydrogen gas
-which accumulate on the surface of the copper plate during
-the chemical action. These bubbles of gas weaken the
-current partly by resisting its flow, for they are bad conductors,
-and still more by trying to set up another current
-in the opposite direction. For this reason the simple
-voltaic cell is unsuitable for long spells of work, and many
-cells have been devised to avoid the polarization trouble.
-One of the most successful of these is the Daniell cell. It
-consists of an outer vessel of copper, which serves as the
-copper plate, and an inner porous pot containing a zinc
-rod. Dilute sulphuric acid is put into the porous pot and
-a strong solution of copper sulphate into the outer jar.
-When the circuit is closed, the hydrogen liberated by the<span class="pagenum" id="Page_32">32</span>
-action of the zinc on the acid passes through the porous
-pot, and splits up the copper sulphate into copper and
-sulphuric acid. In this way pure copper, instead of
-hydrogen, is deposited on the copper plate, no polarization
-takes place, and the current is constant.</p>
-
-<p>Other cells have different combinations of metals, such
-as silver-zinc, or platinum-zinc, and carbon is also largely
-used in place of one metal, as in the familiar carbon-zinc
-Leclanché cell, used for ringing electric bells. This cell
-consists of an inner porous pot containing a carbon plate
-packed round with a mixture of crushed carbon and manganese
-dioxide, and an outer glass jar containing a zinc
-rod and a solution of sal-ammoniac. Polarization is
-checked by the oxygen in the manganese dioxide, which
-seizes the hydrogen on its way to the carbon plate, and
-combines with it. If the cell is used continuously however
-this action cannot keep pace with the rate at which the
-hydrogen is produced, and so the cell becomes polarized;
-but it soon recovers after a short rest.</p>
-
-<p>The so-called “dry” cells so much used at the present
-time are not really dry at all; if they were they would give
-no current. They are in fact Leclanché cells, in which
-the containing vessel is made of zinc to take the place of a
-zinc rod; and they are dry only in the sense that the liquid
-is taken up by an absorbent material, so as to form a moist
-paste. Dry cells are placed inside closely fitting cardboard
-tubes, and are sealed up at the top. Their chief advantage
-lies in their portability, for as there is no free liquid to
-spill they can be carried about and placed in any position.</p>
-
-<p>We have seen that the continuance of the current from
-a voltaic cell depends upon the keeping up of a difference
-of potential between the plates. The force which serves
-to maintain this difference is called the electro-motive force,
-and it is measured in volts. The actual flow of electricity<span class="pagenum" id="Page_33">33</span>
-is measured in amperes. Probably all my readers are
-familiar with the terms volt and ampere, but perhaps some
-may not be quite clear about the distinction between the
-two. When water flows along a pipe we know that it is
-being forced to do so by pressure resulting from a difference
-of level. That is to say, a difference of level produces
-a water-moving or water-motive force; and in a
-similar way a difference of potential produces an electricity-moving
-or electro-motive force, which is measured
-in volts. If we wish to describe the rate of flow of water
-we state it in gallons per second, and the rate of flow of
-electricity is stated in amperes. Volts thus represent the
-pressure at which a current is supplied, while the current
-itself is measured in amperes.</p>
-
-<p>We may take this opportunity of speaking of electric
-resistance. A current of water flowing through a pipe is
-resisted by friction against the inner surface of the pipe;
-and a current of electricity flowing through a circuit also
-meets with a resistance, though this is not due to friction.
-In a good conductor this resistance is small, but in a bad
-conductor or non-conductor it is very great. The resistance
-also depends upon length and area of cross-section; so that
-a long wire offers more resistance than a short one, and a
-thin wire more than a thick one. Before any current can
-flow in a circuit the electro-motive force must overcome
-the resistance, and we might say that the volts drive the
-amperes through the resistance. The unit of resistance is
-the ohm, and the definition of a volt is that electro-motive
-force which will cause a current of one ampere to flow
-through a conductor having a resistance of one ohm. These
-units of measurement are named after three famous scientists,
-Volta, Ampère, and Ohm.</p>
-
-<figure id="fig_8" class="figcenter" style="max-width: 26em;">
- <img src="images/i_046.jpg" width="2023" height="685" alt=" ">
- <figcaption class="caption"><span class="smcap">Fig. 8.</span>—Cells connected in Parallel.
-</figcaption></figure>
-
-<p>A number of cells coupled together form a battery, and
-different methods of coupling are used to get different<span class="pagenum" id="Page_34">34</span>
-results. In addition to the resistance of the circuit outside
-the cell, the cell itself offers an internal resistance, and part
-of the electro-motive force is used up in overcoming this
-resistance. If we can decrease this internal resistance we
-shall have a larger current at our disposal, and one way of
-doing this is to increase the size of the plates. This of
-course means making the cell larger, and very large cells
-take up a lot of room and are troublesome to move about.
-We can get the same effect however by coupling. If we
-connect together all the positive terminals and all the
-negative terminals of several cells, that is, copper to copper
-and zinc to zinc in Daniell cells, we get the same result as
-if we had one very large cell. The current is much larger,
-but the electro-motive force remains the same as if only
-one cell were used, or in other words we have more amperes
-but no more volts. This is called connecting in “parallel,”
-and the method is shown in <a href="#fig_8">Fig. 8</a>. On the other hand,
-if, as is usually the case, we want a larger electro-motive
-force, we connect the positive terminal of one cell to the
-negative terminal of the next, or copper to zinc all through.
-In this way we add together the electro-motive forces of all
-the cells, but the amount of current remains that of a single
-cell; that is, we get more volts but no more amperes. This is
-called connecting in “series,” and the arrangement is shown<span class="pagenum" id="Page_35">35</span>
-in <a href="#fig_9">Fig. 9</a>. We can also increase both volts and amperes by
-combining the two methods.</p>
-
-<figure id="fig_9" class="figcenter" style="max-width: 24em;">
- <img src="images/i_047.png" width="1879" height="835" alt=" ">
- <figcaption class="caption"><span class="smcap">Fig. 9.</span>—Cells connected in Series.
-</figcaption></figure>
-
-<p>A voltaic cell gives us a considerable quantity of
-electricity at low pressure, the electro-motive force of a
-Leclanché cell being about 1½ volts, and that of a Daniell
-cell about 1 volt. We may perhaps get some idea of the
-electrical conditions existing during a thunderstorm from
-the fact that to produce a spark one mile long through air
-at ordinary pressure we should require a battery of more
-than a thousand million Daniell cells. Cells such as we
-have described in this chapter are called primary cells, as
-distinguished from accumulators, which are called secondary
-cells. Some of the practical applications of primary cells
-will be described in later chapters.</p>
-
-<p>Besides the voltaic cell, in which the current is produced
-by chemical action, there is the thermo-electric battery, or
-thermopile, which produces current directly from heat
-energy. About 1822 Seebeck was experimenting with
-voltaic pairs of metals, and he found that a current could
-be produced in a complete metallic circuit consisting of
-different metals joined together, by keeping these joinings
-at different temperatures. <a href="#fig_10">Fig. 10</a> shows a simple arrangement
-for demonstrating this effect, which is known as the<span class="pagenum" id="Page_36">36</span>
-“Seebeck effect.” A slab of bismuth, BB, has placed upon it
-a bent strip of copper, C. If one of the junctions of the
-two metals is heated as shown, a current flows; and the
-same effect is produced
-by cooling one of the
-junctions. This current
-continues to flow
-as long as the two junctions
-are kept at different
-temperatures. In
-1834 another scientist,
-Peltier, discovered that
-if a current was passed
-across a junction of two different metals, this junction was
-either heated or cooled, according to the direction in which
-the current flowed. In <a href="#fig_10">Fig. 10</a> the current across the
-heated junction tends to cool the junction, while the Bunsen
-burner opposes this cooling, and keeps up the temperature.
-A certain amount of the heat energy is thus transformed
-into electrical
-energy. At the
-other junction
-the current
-produces a
-heating effect,
-so that some of
-the electrical
-energy is retransformed
-into heat.</p>
-
-<figure id="fig_10" class="figleft" style="max-width: 16em;">
- <img src="images/i_048.png" width="1266" height="711" alt=" ">
- <figcaption class="caption"><span class="smcap">Fig. 10.</span>—Diagram to illustrate the
-Seebeck effect.
-</figcaption></figure>
-
-<figure id="fig_11" class="figright" style="max-width: 20em;">
- <img src="images/i_048b.png" width="1534" height="684" alt=" ">
- <figcaption class="caption"><span class="smcap">Fig. 11.</span>—Diagram to show arrangement of two
-different metals in Thermopile.
-</figcaption></figure>
-
-<p>A thermopile consists of a number of alternate bars or
-strips of two unlike metals, joined together as shown
-diagrammatically in <a href="#fig_11">Fig. 11</a>. The arrangement is such
-that the odd junctions are at one side, and the even ones<span class="pagenum" id="Page_37">37</span>
-at the other. The odd junctions are heated, and the even
-ones cooled, and a current flows when the circuit is completed.
-By using a larger number of junctions, and by
-increasing the difference of temperature between them, the
-voltage of the current may be increased. Thermopiles are
-nothing like so efficient as voltaic cells, and they are more
-costly. They are used to a limited extent for purposes
-requiring a very small and constant current, but for
-generating considerable quantities of current at high
-pressure they are quite useless. The only really important
-practical use of the thermopile is in the detection and
-measurement of very minute differences of temperature,
-which are beyond the capabilities of the ordinary thermometer.
-Within certain limits, the electro-motive force of a
-thermopile is exactly proportionate to the difference of
-temperature. The very slightest difference of temperature
-produces a current, and by connecting the wires from a
-specially constructed thermopile to a delicate instrument
-for measuring the strength of the current, temperature
-differences of less than one-millionth of a degree can be
-detected.</p>
-<hr class="chap x-ebookmaker-drop">
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_38">38</span></p>
-
-<h2 class="nobreak" id="toclink_38"><a id="chapter_V"></a>CHAPTER V<br>
-
-<span class="subhead">THE ACCUMULATOR</span></h2>
-</div>
-
-<p class="in0"><span class="firstword">If</span> we had two large water tanks, one of which could be
-emptied only by allowing the bottom to fall completely out,
-and the other by means of a narrow pipe, it is easy to see
-which would be the more useful to us as a source of water
-supply. If both tanks were filled, then from the first we
-could get only a sudden uncontrollable rush of water, but
-from the other we could get a steady stream extending over
-a long period, and easily controlled. The Leyden jar stores
-electricity, but in yielding up its store it acts like the first
-tank, giving a sudden discharge in the form of a bright
-spark. We cannot control the discharge, and therefore we
-cannot make it do useful work for us. For practical
-purposes we require a storing arrangement that will act like
-the second tank, giving us a steady current of electricity
-for a long period, and this we have in the accumulator or
-storage cell.</p>
-
-<p>A current of electricity has the power of decomposing
-certain liquids. If we pass a current through water, the
-water is split up into its two constituent gases, hydrogen
-and oxygen, and this may be shown by the apparatus seen
-in <a href="#fig_12">Fig. 12</a>. It consists of a glass vessel with two strips of
-platinum to which the current is led. The vessel contains
-water to which has been added a little sulphuric acid to
-increase its conducting power, and over the strips are inverted
-two test-tubes filled with the acidulated water. The<span class="pagenum" id="Page_39">39</span>
-platinum strips, which are called <em>electrodes</em>, are connected
-to a battery of Daniell cells. When the current passes,
-the water is decomposed, and oxygen collects at the electrode
-connected to the positive terminal of the battery, and
-hydrogen at the other electrode. The two gases rise up
-into the test-tubes and displace the water in them, and the
-whole process is called the electrolysis of water. If now
-we disconnect the battery and join the two electrodes by
-a wire, we find that a current flows from the apparatus
-as from a voltaic cell, but
-in the opposite direction
-from the original battery
-current.</p>
-
-<figure id="fig_12" class="figright" style="max-width: 14em;">
- <img src="images/i_051.png" width="1070" height="1191" alt=" ">
- <figcaption class="caption"><span class="smcap">Fig. 12.</span>—Diagram showing Electrolysis
-of Water.
-</figcaption></figure>
-
-<p>It will be remembered
-that one of the troubles
-with a simple voltaic cell
-was polarization, caused
-by the accumulation of
-hydrogen; and that this
-weakened the current by
-setting up an opposing
-electro-motive force tending
-to produce another
-current in the opposite
-direction. In the present
-case a similar opposing or back electro-motive force is
-produced, and as soon as the battery current is stopped
-and the electrodes are connected, we get a current in the
-reverse direction, and this current continues to flow until
-the two gases have recombined, and the electrodes have
-regained their original condition. Consequently we can
-see that in order to electrolyze water, our battery must
-have an electro-motive force greater than that set up in
-opposition to it, and at least two Daniell cells are required.</p>
-
-<p><span class="pagenum" id="Page_40">40</span></p>
-
-<p>This apparatus thus may be made to serve to some
-extent as an accumulator or storage cell, and it also serves
-to show that an accumulator does not store up or accumulate
-electricity. In a voltaic cell we have chemical energy
-converted into electrical energy, and here we have first
-electrical energy converted into chemical energy, and then
-the chemical energy converted back again into electrical
-energy. This is a rough-and-ready way of putting the
-matter, but it is good enough for practical purposes, and at
-any rate it makes it quite clear that what an accumulator
-really stores up is not electricity, but energy, which is given
-out in the form of electricity.</p>
-
-<p>The apparatus just described is of little use as a source
-of current, and the first really practical accumulator was
-made in 1878 by Gaston Planté. The electrodes were two
-strips of sheet lead placed one upon the other, but separated
-by some insulating material, and made into a roll. This
-roll was placed in dilute sulphuric acid, and one strip or
-plate connected to the positive, and the other to the
-negative terminal of the source of current. The current
-was passed for a certain length of time, and then the accumulator
-partly discharged; after which current was passed
-again, but in the reverse direction, followed by another period
-of discharge. This process, which is called <em>forming</em>,
-was continued for several days, and its effect was to change
-one plate into a spongy condition, and to form a coating
-of peroxide of lead on the other. When the plates were
-properly formed the accumulator was ready to be fully
-charged and put into use. The effect of charging was to
-rob one plate of its oxygen, and to transfer this oxygen to
-the other plate, which thus received an overcharge of the
-gas. During the discharge of the accumulator the excess
-of oxygen went back to the place from which it had been
-taken, and the current continued until the surfaces of both<span class="pagenum" id="Page_41">41</span>
-plates were reduced to a chemically inactive state. The
-accumulator could be charged and discharged over and
-over again as long as the plates remained in good order.</p>
-
-<p>In 1881, Faure hit upon the idea of coating the plates
-with a paste of red-lead, and this greatly shortened the
-time of forming. At first it was found difficult to make the
-paste stick to the plates, but this trouble was got rid of by
-making the plates in the form of grids, and pressing the
-paste into the perforations. Many further improvements
-have been made from time to time, but instead of tracing
-these we will go on at once to the description of a present-day
-accumulator. There are now many excellent accumulators
-made, but we have not space to consider more than
-one, and we will select that known as the “Chloride”
-accumulator.</p>
-
-<p>The positive plate of this accumulator is of the Planté
-type, but it is not simply a casting of pure lead, but is made
-by a building-up process which allows of the use of a
-lead-antimony mixture for the grids. This gives greater
-strength, and the grids themselves are unaffected by the
-chemical changes which take place during the charging and
-discharging of the cell. The active material, that is the
-material which undergoes chemical change, is pure lead
-tape coiled up into rosettes, which are so designed that the
-acid can circulate through the plates. These rosettes are
-driven into the perforations of the grid by a hydraulic press,
-and during the process of forming they expand and thus
-become very firmly fixed. The negative plate has a frame
-made in two parts, which are riveted together after the
-insertion of the active material, which is thus contained in
-a number of small cages. The plate is covered outside
-with a finely perforated sheet of lead, which prevents the
-active material from falling out. It is of the utmost
-importance that the positive and negative plates should be<span class="pagenum" id="Page_42">42</span>
-kept apart when in the cell, and in the Chloride accumulator
-this is ensured by the use of a patent separator made of
-a thin sheet of wood the size of the plates. Before being
-used the wood undergoes a special treatment to remove all
-substances which might be harmful, and it then remains
-unchanged either in appearance or composition. Other
-insulating substances, such as glass rods or ebonite forks,
-can be used as separators, but it is claimed that the wood
-separator is not only more satisfactory, but that in some
-unexplained way it actually helps to keep up the capacity
-of the cell. The plates are placed in glass, or lead-lined
-wood or metal boxes, and are suspended from above the
-dilute sulphuric acid with which the cells are filled. A
-space is left below the plates for the sediment which
-accumulates during the working of the cell.</p>
-
-<p>In all but the smallest cells several pairs of plates are
-used, all the positive plates being connected together and
-all the negative plates. This gives the same effect as two
-very large plates, on the principle of connecting in parallel,
-spoken of in <a href="#chapter_IV">Chapter IV</a>. A single cell, of whatever size,
-gives current at about two volts, and to get higher voltages
-many cells are connected in series, as with primary cells.
-The capacity is generally measured in ampere-hours. For
-instance, an accumulator that will give a current of eight
-amperes for one hour, or of four amperes for two hours, or
-one ampere for eight hours, is said to have a capacity of
-eight ampere-hours.</p>
-
-<p>Accumulators are usually charged from a dynamo or
-from the public mains, and the electro-motive force of
-the charging current must be not less than 2½ volts for
-each cell, in order to overcome the back electro-motive
-force of the cells themselves. It is possible to charge
-accumulators from primary cells, but except on a very
-small scale the process is comparatively expensive. Non-polarizing<span class="pagenum" id="Page_43">43</span>
-cells, such as the Daniell, must be used for this
-purpose.</p>
-
-<p>The practical applications of accumulators are almost
-innumerable, and year by year they increase. As the most
-important of these are connected with the use of electricity
-for power and light, it will be more convenient to speak of
-them in the chapters dealing with this subject. Minor
-uses of accumulators will be referred to briefly from time
-to time in other chapters.</p>
-<hr class="chap x-ebookmaker-drop">
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_44">44</span></p>
-
-<h2 class="nobreak" id="toclink_44"><a id="chapter_VI"></a>CHAPTER VI<br>
-
-<span class="subhead">MAGNETS AND MAGNETISM</span></h2>
-</div>
-
-<p class="in0"><span class="firstword">In</span> many parts of the world there is to be found a kind of
-iron ore, some specimens of which have the peculiar power
-of attracting iron, and of turning to the north if suspended
-freely. This is called the <em>lodestone</em>, and it has been
-known from very remote times. The name Magnetism has
-been given to this strange property of the lodestone, but the
-origin of the name is not definitely known. There is an
-old story about a shepherd named Magnes, who lived in
-Phrygia in Asia Minor. One day, while tending his sheep
-on Mount Ida, he happened to touch a dark coloured rock
-with the iron end of his crook, and he was astonished and
-alarmed to find that the rock was apparently alive, for it
-gripped his crook so firmly that he could not pull it away.
-This rock is said to have been a mass of lodestone, and
-some people believe that the name magnet comes from the
-shepherd Magnes. Others think that the name is derived
-from Magnesia, in Asia Minor, where the lodestone was
-found in large quantities; while a third theory finds the
-origin in the Latin word <i lang="la">magnus</i>, heavy, on account of the
-heavy nature of the lodestone. The word lodestone itself
-comes from the Saxon <i lang="osx">laeden</i>, meaning to lead.</p>
-
-<p>It is fairly certain that the Chinese knew of the lodestone
-long before Greek and Roman times, and according
-to ancient Chinese records this knowledge extends as far
-back as 2600 <span class="allsmcap">B.C.</span> Humboldt, in his <cite>Cosmos</cite>, states that a<span class="pagenum" id="Page_45">45</span>
-miniature figure of a man which always turned to the south
-was used by the Chinese to guide their caravans across the
-plains of Tartary as early as 1000 <span class="allsmcap">B.C.</span> The ancient Greek
-and Roman writers frequently refer to the lodestone.
-Thales, of whom we spoke in <a href="#chapter_I">Chapter I</a>., believed that its
-mysterious power was due to the possession of a soul, and
-the Roman poet Claudian imagined that iron was a food
-for which the lodestone was hungry. Our limited space
-will not allow of an account of the many curious speculations
-to which the lodestone has given rise, but the following
-suggestion of one Famianus Strada, quoted from
-Houston’s <cite>Electricity in Every-Day Life</cite>, is really too
-good to be omitted.</p>
-
-<p>“Let there be two needles provided of an equal Length
-and Bigness, being both of them touched by the same
-lodestone; let the Letters of the Alphabet be placed on the
-Circles on which they are moved, as the Points of the
-Compass under the needle of the Mariner’s Chart. Let
-the Friend that is to travel take one of these with him, first
-agreeing upon the Days and Hours wherein they should
-confer together; at which times, if one of them move the
-Needle, the other Needle, by Sympathy, will move unto
-the same letter in the other instantly, though they are
-never so far distant; and thus, by several Motions of the
-Needle to the Letters, they may easily make up any Words
-or Sense which they have a mind to express.” This is
-wireless telegraphy in good earnest!</p>
-
-<p>The lodestone is a natural magnet. If we rub a piece
-of steel with a lodestone we find that it acquires the same
-properties as the latter, and in this way we are able to
-make any number of magnets, for the lodestone does not
-lose any of its own magnetism in the process. Such
-magnets are called artificial magnets. Iron is easier to
-magnetize than steel, but it soon loses its magnetism,<span class="pagenum" id="Page_46">46</span>
-whereas steel retains it; and the harder the steel the better
-it keeps its magnetism. Artificial magnets, therefore, are
-made of specially hardened steel. In this chapter we shall
-refer only to steel magnets, as they are much more convenient
-to use than the lodestone, but it should be
-remembered that both act in exactly the same way. We
-will suppose that we have a pair of bar magnets, and a
-horse-shoe magnet, as shown in <a href="#fig_13">Fig. 13</a>.</p>
-
-<figure id="fig_13" class="figleft" style="max-width: 16em;">
- <img src="images/i_058.png" width="1272" height="1120" alt=" ">
- <figcaption class="caption"><span class="smcap">Fig. 13.</span>—Horse-shoe and Bar Magnets,
-with Keepers.
-</figcaption></figure>
-
-<p>If we roll a bar magnet amongst iron filings we find
-that the filings remain
-clinging to it in two
-tufts, one at each
-end, and that few or
-none adhere to the
-middle. These two
-points towards which
-the filings are attracted
-are called the
-poles of the magnet.
-Each pole attracts
-filings or ordinary
-needles, and one or
-two experiments will
-show that the attraction
-becomes evident while the magnet is still some little
-distance away. If, however, we test our magnet with other
-substances, such as wood, glass, paper, brass, etc., we see
-that there is no attraction whatever.</p>
-
-<p>If one of our bar magnets is suspended in a sort of
-stirrup of copper wire attached to a thread, it comes to rest
-in a north and south direction, and it will be noticed that
-the end which points to the north is marked, either with a
-letter N or in some other way. This is the north pole of
-the magnet, and of course the other is the south pole. If<span class="pagenum" id="Page_47">47</span>
-now we take our other magnet and bring its north pole
-near each pole of the suspended magnet in turn, we find
-that it repels the other north pole, but attracts the south
-pole. Similarly, if we present the south pole, it repels the
-other south pole, but attracts the north pole. From these
-experiments we learn that both poles of a magnet attract
-filings or needles, and that in the case of two magnets
-unlike poles attract, but similar poles repel one another.
-It will be noticed that this corresponds closely with the
-results of our experiments in <a href="#chapter_I">Chapter I</a>., which showed
-that an electrified body attracts unelectrified bodies, such
-as bits of paper or pith balls, and that unlike charges
-attract, and similar charges repel each other. So far as we
-have seen, however, a magnet attracts only iron or steel,
-whereas an electrified body attracts any light substance.
-As a matter of fact, certain other substances, such as nickel
-and cobalt, are attracted by a magnet, but not so readily as
-iron and steel; while bismuth, antimony, phosphorus, and
-a few other substances are feebly repelled.</p>
-
-<p>The simplest method of magnetizing a piece of steel by
-means of one of our bar magnets is the following: Lay the
-steel on the table, and draw one pole of the magnet along
-it from end to end; lift the magnet clear of the steel, and
-repeat the process several times, always starting at the
-same end and treating each surface of the steel in turn. A
-thin, flat bar of steel is the best for the purpose, but steel
-knitting needles may be made in this way into useful
-experimental magnets.</p>
-
-<p>We have seen that a magnet has two poles or points
-where the magnetism is strongest. It might be thought
-that by breaking a bar magnet in the middle we should get
-two small bars each with a single pole, but this is not the
-case, for the two poles are inseparable. However many
-pieces we break a magnet into, each piece is a perfect<span class="pagenum" id="Page_48">48</span>
-magnet having a north and south pole. Thus while we
-can isolate a positive or a negative charge of electricity, we
-cannot isolate north or south magnetism.</p>
-
-<p>If we place the north pole of a bar magnet near to, but
-not touching, a bar of soft iron, as in <a href="#plate_IIa">Plate II.<i>a</i></a>, we find that
-the latter becomes a magnet, as shown by its ability to
-support filings; and that as soon as the magnet is removed
-the filings drop off, showing that the iron has lost its
-magnetism. If the iron is tested while the magnet is in
-position it is found to have a south pole at the end nearer
-the magnet, and a north pole at the end farther away; and
-if the magnet is reversed, so as to bring its south pole
-nearer the iron, the poles of the latter are found to reverse
-also. The iron has gained its new properties by magnetic
-induction, and we cannot fail to notice the similarity between
-this experiment and that in <a href="#fig_2">Fig. 2</a>, <a href="#chapter_II">Chapter II</a>., which
-showed electro-static induction. A positively or a negatively
-electrified body induces an opposite charge at the
-nearer end, and a similar charge at the further end of a
-conductor, and a north or a south pole of a magnet
-induces opposite polarity at the nearer end, and a
-similar polarity at the further end of a bar of iron. In
-<a href="#chapter_II">Chapter II</a>. we showed that the attraction of a pith ball
-by an electrified body was due to induction, and from what
-we have just learnt about magnetic induction the reader
-will have no difficulty in understanding why a magnet
-attracts filings or needles.</p>
-
-<figure id="plate_IIa" class="figcenter" style="max-width: 26em;">
- <p class="caption">PLATE II.</p>
- <img src="images/i_061.jpg" width="2060" height="1021" alt=" ">
- <figcaption class="caption">
-
-<p>(<i>a</i>) EXPERIMENT TO SHOW MAGNETIC INDUCTION.</p>
-</figcaption></figure>
-
-<figure id="plate_IIb" class="figcenter" style="max-width: 26em;">
- <img src="images/i_061b.jpg" width="2060" height="1348" alt=" ">
- <figcaption class="caption">(<i>b</i>) EXPERIMENT TO SHOW THE PRODUCTION OF MAGNETISM BY AN
-ELECTRIC CURRENT.
-</figcaption></figure>
-
-<p>Any one who experiments with magnets must be struck
-with the distance at which one magnet can influence filings
-or another magnet. If a layer of iron filings is spread on a
-sheet of paper, and a magnet brought gradually nearer
-from above, the filings soon begin to move about restlessly,
-and when the magnet comes close enough they fly up to it
-as if pulled by invisible strings. A still more striking<span class="pagenum" id="Page_49">49</span>
-experiment consists in spreading filings thinly over a sheet
-of cardboard and moving a magnet to and fro underneath
-the sheet. The result is most amusing. The filings seem
-to stand up on their hind legs, and they march about like
-regiments of soldiers. Here again invisible strings are
-suggested, and we might wonder whether there really is
-anything of the kind. Yes, there is. To put the matter
-in the simplest way, the magnet acts by means of strings
-or lines of force, which emerge from it in definite directions,
-and in a most interesting way we can see some of these
-lines of force actually at work.</p>
-
-<p>Place a magnet, or any arrangement of magnets, underneath
-a sheet of glass, and sprinkle iron filings from a
-muslin bag thinly and evenly all over the glass. Then tap
-the glass gently with a pencil, and the filings at once
-arrange themselves in a most remarkable manner. All the
-filings become magnetized by induction, and when the tap
-sets them free for an instant from the friction of the glass
-they take up definite positions under the influence of the
-force acting upon them. In this way we get a map of
-the general direction of the magnetic lines of force, which
-are our invisible strings.</p>
-
-<p>Many different maps may be made in this way, but we
-have space for only two. <a href="#plate_III">Plate III.<i>a</i></a> shows the lines of two
-opposite poles. Notice how they appear to stream across
-from one pole to the other. It is believed that there is a
-tension along the lines of force not unlike that in stretched
-elastic bands, and if this is so it is easy to see from the
-figure why opposite poles attract each other.</p>
-
-<p><a href="#plate_III">Plate III.<i>b</i></a> shows the lines of force of two similar poles.
-In this case they do not stream from pole to pole, but turn
-aside as if repelling one another, and from this figure we
-see why there is repulsion between two similar poles. It
-can be shown, although in a much less simple manner, that<span class="pagenum" id="Page_50">50</span>
-lines of electric force proceed from electrified bodies, and
-in electric attraction and repulsion between two charged
-bodies the lines of force take paths which closely resemble
-those in our two figures. A space filled with lines of
-magnetic force is called a <em>magnetic field</em>, and one filled
-with lines of electric force is called an <em>electric field</em>.</p>
-
-<p>A horse-shoe magnet, which is simply a bar of steel
-bent into the shape of a horse-shoe before being magnetized,
-gradually loses its magnetism if left with its poles
-unprotected, but this loss is prevented if the poles are
-connected by a piece of soft iron. The same loss occurs
-with a bar magnet, but as the two poles cannot be connected
-in this way it is customary to keep two bar magnets side
-by side, separated by a strip of wood; with opposite poles
-together and a piece of soft iron across the ends. Such
-pieces of iron are called <em>keepers</em>, and <a href="#fig_13">Fig. 13</a> shows a
-horse-shoe magnet and a pair of bar magnets with their
-keepers. It may be remarked that a magnet never should
-be knocked or allowed to fall, as rough usage of this kind
-causes it to lose a considerable amount of its magnetism.
-A magnet is injured also by allowing the keeper to slam on
-to it; but pulling the keeper off vigorously does good
-instead of harm.</p>
-
-<p>If a magnetized needle is suspended so that it is free to
-swing either horizontally or vertically, it not only comes
-to rest in a north and south direction, but also it tilts with
-its north-pointing end downwards. If the needle were
-taken to a place south of the equator it would still tilt, but
-the south-pointing end would be downwards. In both
-cases the angle the needle makes with the horizontal is
-called the <em>magnetic dip</em>.</p>
-
-<figure id="plate_III" class="figcenter" style="max-width: 26em;">
- <p class="caption b1">PLATE III.</p>
- <p class="caption smaller">(<i>a</i>) LINES OF MAGNETIC FORCE OF TWO OPPOSITE POLES.</p>
- <img src="images/i_065.jpg" width="2080" height="3126" alt=" ">
- <figcaption class="caption">
- <p class="smaller">(<i>b</i>) LINES OF MAGNETIC FORCE OF TWO SIMILAR POLES.</p>
-</figcaption></figure>
-
-<p>It is evident that a suspended magnetized needle would
-not invariably come to rest pointing north and south unless
-it were compelled to do so, and a little consideration shows<span class="pagenum" id="Page_51">51</span>
-that the needle acts as if it were under the influence of a
-magnet. Dr. Gilbert of Colchester, of whom we spoke in
-<a href="#chapter_I">Chapter I</a>., gave a great deal of time to the study of
-magnetic phenomena, and in 1600 he announced what may
-be regarded as his greatest discovery: <em>The terrestrial
-globe itself is a great magnet</em>. Here, then, is the explanation
-of the behaviour of the magnetized needle. The Earth
-itself is a great magnet, having its poles near to the
-geographical north and south poles. But a question at
-once suggests itself: “Since similar poles repel one another,
-how is it that the north pole of a magnet turns towards the
-north magnetic pole of the earth?” This apparent difficulty
-is caused by a confusion in terms. If the Earth’s
-north magnetic pole really has north magnetism, then the
-north-pointing end of a magnet must be a south pole; and
-on the other hand, if the north-pointing end of a magnet
-has north magnetism, then the Earth’s north magnetic pole
-must be really a south pole. It is a troublesome matter to
-settle, but it is now customary to regard the Earth’s north
-magnetic pole as possessing south magnetism, and the
-south magnetic pole as possessing north magnetism. In
-this way the north-pointing pole of a magnet may be looked
-upon as a true north pole, and the south-pointing pole as a
-true south pole.</p>
-
-<p>Magnetic dip also is seen to be a natural result of the
-Earth’s magnetic influence. Here in England, for instance,
-the north magnetic pole is much nearer than the south
-magnetic pole, and consequently its influence is the
-stronger. Therefore a magnetized needle, if free to do
-so, dips downwards towards the north. At any place
-where the south magnetic pole is the nearer the direction
-of the dip of course is reversed. If placed immediately
-over either magnetic pole the needle would take up a
-vertical position, and at the magnetic equator it would not<span class="pagenum" id="Page_52">52</span>
-dip at all, for the influence of the two magnetic poles would
-be equal. A little study of <a href="#fig_14">Fig. 14</a>, which represents a
-dipping needle at different parts of the earth, will make
-this matter clearer. N and S represent the Earth’s north
-and south magnetic poles, and the arrow heads are the
-north poles of the needles.</p>
-
-<figure id="fig_14" class="figleft" style="max-width: 16em;">
- <img src="images/i_068.png" width="1265" height="927" alt=" ">
- <figcaption class="caption"><span class="smcap">Fig. 14.</span>—Diagram to illustrate Magnetic Dip.
-</figcaption></figure>
-
-<p>Since the Earth is a magnet, we should expect it to
-be able to induce magnetism in a bar of iron, just as our
-artificial magnets do, and we can show that this is actually
-the case. If a steel poker is held pointing to and dipping
-down towards the
-north, and struck
-sharply with a piece
-of wood while in this
-position, it acquires
-magnetic properties
-which can be tested
-by means of a small
-compass needle. It
-is an interesting fact
-that iron pillars and
-railings which have
-been standing for a
-long time in one position are found to be magnetized.
-In the northern hemisphere the bases of upright iron
-pillars are north poles, and their upper ends south poles,
-and in the southern hemisphere the polarity is reversed.</p>
-
-<p>The most valuable application of the magnetic needle
-is in the compass. An ordinary pocket compass for inland
-use consists simply of a single magnetized needle pivoted
-so as to swing freely over a card on which are marked the
-thirty-two points of the compass. Ships’ compasses are
-much more elaborate. As a rule a compound needle is
-used, consisting of eight slender strips of steel, magnetized<span class="pagenum" id="Page_53">53</span>
-separately, and suspended side by side. A compound
-needle of this kind is very much more reliable than a
-single needle. The material of which the card is made
-depends upon whether the illumination for night work is
-to come from above or below. If the latter, the card must
-be transparent, and it is often made of thin sheet mica;
-but if the light comes from above, the card is made of
-some opaque material, such as very stout paper. The
-needle and card are contained in a sort of bowl made of
-copper. In order to keep this bowl in a horizontal position,
-however the ship may be pitching and rolling, it is supported
-on gimbals, which are two concentric rings attached
-to horizontal pivots, and moving in axes at right angles
-to one another. Further stability may be obtained by
-weighting the bottom of the bowl with lead. There are
-also liquid compasses, in which the card is floated on the
-surface of dilute alcohol, and many modern ships’ compasses
-have their movements regulated by a gyrostat.</p>
-
-<p>The large amount of iron and steel used in the construction
-of modern vessels has a considerable effect upon
-the compass needle, and unless the compass is protected
-from this influence its readings are liable to serious errors.
-The most satisfactory way of giving this protection is by
-placing on each side of the compass a large globe of soft
-iron, twelve or more inches in diameter.</p>
-
-<p>On account of the fact that the magnetic poles of the
-Earth do not coincide with the geographical north and
-south poles, a compass needle seldom points exactly north
-and south, and the angle between the magnetic meridian
-and the geographical meridian is called the <em>declination</em>.
-The discovery that the declination varies in different parts
-of the world was made by Columbus in 1492. For purposes
-of navigation it is obviously very important that the
-declination at all points of the Earth’s surface should be<span class="pagenum" id="Page_54">54</span>
-known, and special magnetic maps are prepared in which
-all places having the same declination are joined by a
-line.</p>
-
-<p>It is an interesting fact that the Earth’s magnetism is
-subject to variation. The declination and the dip slowly
-change through long periods of years, and there are also
-slight annual and even daily variations.</p>
-
-<p>At one time magnets were credited with extraordinary
-effects upon the human body. Small doses of lodestone,
-ground to powder and mixed with water, were supposed
-to prolong life, and Paracelsus, a famous alchemist and
-physician, born in Switzerland in 1493, believed in the
-potency of lodestone ointment for wounds made with steel
-weapons. Baron Reichenbach, 1788–1860, believed that
-he had discovered the existence of a peculiar physical force
-closely connected with magnetism, and he gave this force
-the name <em>Od</em>. It was supposed to exist everywhere,
-and, like magnetism, to have two poles, positive and
-negative; the left side of the body being od-positive, and
-the right side od-negative. Certain individuals, known as
-“sensitives,” were said to be specially open to its influence.
-These people stated that they saw strange flickering lights
-at the poles of magnets, and that they experienced peculiar
-sensations when a magnet was passed over them. Some
-of them indeed were unable to sleep on the left side, because
-the north pole of the Earth, being od-negative, had
-a bad effect on the od-negative left side. The pretended
-revelations of these “sensitives” created a great stir at the
-time, but now nobody believes in the existence of <em>Od</em>.</p>
-
-<p>Professor Tyndall was once invited to a seance, with
-the object of convincing him of the genuineness of spiritualism.
-He sat beside a young lady who claimed to have
-spiritualistic powers, and his record of his conversation with
-her is amusing. The Reichenbach craze was in full swing<span class="pagenum" id="Page_55">55</span>
-at the time, and Tyndall asked if the lady could see any of
-the weird lights supposed to be visible to “sensitives.”</p>
-
-<div class="blockquot">
-
-<p>“<i>Medium.</i>—Oh yes; but I see the light around all
-bodies.</p>
-
-<p><i>I.</i>—Even in perfect darkness?</p>
-
-<p><i>Medium.</i>—Yes; I see luminous atmospheres round
-all people. The atmosphere which surrounds
-Mr. R. C. would fill this room with light.</p>
-
-<p><i>I.</i>—You are aware of the effects ascribed by Baron
-Reichenbach to magnets?</p>
-
-<p><i>Medium.</i>—Yes; but a magnet makes me terribly ill.</p>
-
-<p><i>I.</i>—Am I to understand that, if this room were
-perfectly dark, you could tell whether it contained
-a magnet, without being informed of
-the fact?</p>
-
-<p><i>Medium.</i>—I should know of its presence on entering
-the room.</p>
-
-<p><i>I.</i>—How?</p>
-
-<p><i>Medium.</i>—I should be rendered instantly ill.</p>
-
-<p><i>I.</i>—How do you feel to-day?</p>
-
-<p><i>Medium.</i>—Particularly well; I have not been so
-well for months.</p>
-
-<p><i>I.</i>—Then, may I ask you whether there is, at the
-present moment, a magnet in my possession?</p>
-
-<p>The young lady looked at me, blushed, and
-stammered, ‘No; I am not <i>en rapport</i> with
-you.’</p>
-
-<p><em>I sat at her right hand, and a left-hand pocket,
-within six inches of her person, contained a
-magnet.</em>”</p>
-</div>
-
-<p>Tyndall adds, “Our host here deprecated discussion
-as it ‘exhausted the medium.’”</p>
-<hr class="chap x-ebookmaker-drop">
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_56">56</span></p>
-
-<h2 class="nobreak" id="toclink_56"><a id="chapter_VII"></a>CHAPTER VII<br>
-
-<span class="subhead">THE PRODUCTION OF MAGNETISM BY ELECTRICITY</span></h2>
-</div>
-
-<figure id="fig_15" class="figcenter" style="max-width: 19em;">
- <img src="images/i_072.png" width="1519" height="866" alt=" ">
- <figcaption class="caption"><span class="smcap">Fig. 15.</span>—Diagram to illustrate Magnetic effect of an Electric Current.
-</figcaption></figure>
-
-<p class="in0"><span class="firstword">In</span> the previous chapter attention was drawn to the fact
-that there are many close parallels between electric and
-magnetic phenomena, and in this chapter it will be shown
-that magnetism can be produced by electricity. In the
-year 1819 Professor Oersted, of the University of Copenhagen,
-discovered that a freely swinging magnetized needle,
-such as a compass needle, was deflected by a current of
-electricity flowing through a wire. In <a href="#fig_15">Fig. 15</a>, A, a
-magnetic needle is shown at rest in its usual north and
-south direction, and over it is held a copper wire, also
-pointing north and south. A current of electricity is now
-sent through the wire, and the needle is at once deflected,
-<a href="#fig_15">Fig. 15</a>, B. The direction of the current is indicated by<span class="pagenum" id="Page_57">57</span>
-an arrow, and the direction in which the needle has moved
-is shown by the two small arrows. If the direction of the
-current is reversed, the needle will be deflected in the
-opposite direction. From this experiment we see that the
-current has brought magnetic influences into play, or in
-other words has produced magnetism. If iron filings are
-brought near the wire while the current is flowing, they
-are at once attracted and cling to the wire, but as soon as
-the current is stopped
-they drop off. This
-shows us that the wire
-itself becomes a magnet
-during the passage of
-the current, and that it
-loses its magnetism
-when the current ceases
-to flow.</p>
-
-<figure id="fig_16" class="figright" style="max-width: 15em;">
- <img src="images/i_073.jpg" width="1180" height="1174" alt=" ">
- <figcaption class="caption"><span class="smcap">Fig. 16.</span>—Magnetic Field round wire
-conveying a Current.
-</figcaption></figure>
-
-<p>Further, it can be
-shown that two freely
-moving parallel wires
-conveying currents attract
-or repel one
-another according to
-the direction of the currents.
-If both currents are flowing in the same direction
-the wires attract one another, but if the currents flow in
-opposite directions the wires repel each other. <a href="#fig_16">Fig. 16</a>
-shows the direction of the lines of force of a wire conveying
-a current and passed through a horizontal piece of cardboard
-covered with a thin layer of iron filings; and from this
-figure it is evident that the passage of the current produces
-what we may call magnetic whirls round the wire.</p>
-
-<p>A spiral of insulated wire through which a current is
-flowing shows all the properties of a magnet, and if free to<span class="pagenum" id="Page_58">58</span>
-move it comes to rest pointing north and south. It is
-attracted or repelled by an ordinary magnet according to
-the pole presented to it and the direction of the current,
-and two such spirals show mutual attraction and repulsion.
-A spiral of this kind is called a <em>solenoid</em>, and in addition
-to the properties already mentioned it has the peculiar
-power of drawing or sucking into its interior a rod of
-iron. Solenoids have various practical applications, and in
-later chapters we shall refer to them again.</p>
-
-<p>If several turns of cotton-covered wire are wound round
-an iron rod, the passing of a current through the wire
-makes the rod into a magnet (<a href="#plate_IIa">Plate II.<i>b</i></a>), but the magnetism
-disappears as soon as the current ceases to flow. A
-magnet made by the passage of an electric current is called
-an <em>electro-magnet</em>, and it has all the properties of the
-magnets mentioned in the previous chapter. A bar of steel
-may be magnetized in the same way, but unlike the iron
-rod it retains its magnetism after the current is interrupted.
-This provides us with a means of magnetizing a piece of
-steel much more strongly than is possible by rubbing with
-another magnet. Steel magnets, which retain their
-magnetism, are called <em>permanent</em> magnets, as distinguished
-from electro-magnets in which soft iron is used, so
-that their magnetism lasts only as long as the current
-flows.</p>
-
-<p>Electro-magnets play an extremely important part in
-the harnessing of electricity; in fact they are used in one
-form or another in almost every kind of electrical
-mechanism. In later chapters many of these uses will be
-described, and here we will mention only the use of
-electro-magnets for lifting purposes. In large engineering
-works powerful electro-magnets, suspended from some
-sort of travelling crane, are most useful for picking up and
-carrying about heavy masses of metal, such as large<span class="pagenum" id="Page_59">59</span>
-castings. No time is lost in attaching the casting to the
-crane; the magnet picks it up directly the current is
-switched on, and lets it go the instant the current is
-stopped. In any large steel works the amount of scrap
-material produced is astonishingly great, hundreds of tons
-of turnings and similar scrap accumulating in a very short
-time. A huge mound of turnings is awkward to deal with
-by ordinary manual labour, but a combination of electro-magnet
-and crane solves the difficulty completely, lifting
-and loading the scrap into carts or trucks at considerable
-speed, and without requiring much attention.</p>
-
-<p>Some time ago a disastrous fire occurred at an
-engineering works in the Midlands, the place being almost
-entirely burnt out. Amongst the débris was, of course, a
-large amount of metal, and as this was too valuable to be
-wasted, an electro-magnet was set to work on the wreckage.
-The larger pieces of metal were picked up in the ordinary
-way, and then the remaining rubbish was shovelled against
-the face of the magnet, which held on to the metal but
-dropped everything else, and in this way some tons of
-metal were recovered.</p>
-
-<p>The effect produced upon a magnetized needle by a
-current of electricity affords a simple means of detecting the
-existence of such a current. An ordinary pocket compass
-can be made to show the presence of a moderate current, but
-for the detection of extremely small currents a much more
-sensitive apparatus is employed. This is called a <em>galvanometer</em>,
-and in its simplest form it consists essentially of
-a delicately poised magnetic needle placed in the middle of
-a coil of several turns of wire. The current thus passes
-many times round the needle, and this has the effect of
-greatly increasing the deflection of the needle, and hence
-the sensitiveness of the instrument. Although such an
-arrangement is generally called a galvanometer, it is really<span class="pagenum" id="Page_60">60</span>
-a galvanoscope, for it does not measure the current but only
-shows its presence.</p>
-
-<p>We have seen that electro-motive force is measured in
-volts, and that the definition of a volt is that electro-motive
-force which will cause a current of one ampere to flow
-through a conductor having a resistance of one ohm. If we
-make a galvanometer with a long coil of very thin wire
-having a high resistance, the amount of current that will
-flow through it will be proportionate to the electro-motive
-force. Such a galvanometer, fitted with a carefully
-graduated scale, in this way will indicate the number of
-volts, and it is called a <em>voltmeter</em>. If we have a galvanometer
-with a short coil of very thick wire, the resistance put
-in the way of the current is so small that it may be left out
-of account, and by means of a graduated scale the number
-of amperes may be shown; such an instrument being called
-an <em>amperemeter</em>, or <em>ammeter</em>.</p>
-
-<p>For making exact measurements of electric currents the
-instruments just described are not suitable, as they are not
-sufficiently accurate; but their working shows the principle
-upon which currents are measured. The actual instruments
-used in electrical engineering and in scientific work are
-unfortunately too complicated to be described here.</p>
-<hr class="chap x-ebookmaker-drop">
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_61">61</span></p>
-
-<h2 class="nobreak" id="toclink_61"><a id="chapter_VIII"></a>CHAPTER VIII<br>
-
-<span class="subhead">THE INDUCTION COIL</span></h2>
-</div>
-
-<p class="in0"><span class="firstword">The</span> voltaic cell and the accumulator provide us with
-currents of electricity of considerable volume, but at low
-pressure or voltage. For many purposes, however, we require
-a comparatively small amount of current at very high
-pressure, and in such cases we use an apparatus called
-the <em>induction coil</em>. Just as an electrified body and a
-magnet will induce electrification and magnetism respectively,
-so a current of electricity will induce another current;
-and an induction coil is simply an arrangement by which a
-current in one coil of wire is made to induce a current in
-another coil.</p>
-
-<p>Suppose we have two coils of wire placed close together,
-one connected to a battery of voltaic cells, with
-some arrangement for starting and stopping the current
-suddenly, and the other to a galvanometer. As soon as
-we send the current through the first coil, the needle of
-the galvanometer moves, showing that there is a current
-flowing through the second coil; but the needle quickly
-comes back to its original position, showing that this
-current was only momentary. So long as we keep the
-current flowing through the first coil the galvanometer
-shows no further movement, but as soon as we stop the
-current the needle again shows by its movements that
-another momentary current has been produced in the
-second coil. This experiment shows us that a current<span class="pagenum" id="Page_62">62</span>
-induces another current only at the instant it is started or
-stopped, or, as we say, at the instant of making or breaking
-the circuit.</p>
-
-<p>The coil through which we send the battery current is
-called the “primary coil,” and the one in which a current is
-induced is called the “secondary coil.” The two momentary
-currents in the secondary coil do not both flow in the same
-direction. The current induced on making the circuit
-flows in a direction opposite to that of the current in the
-primary coil; and the current induced on breaking the
-circuit flows in the same direction as that in the primary
-coil. If the two coils are exactly alike, the induced current
-will have the same voltage as the primary current; but
-if the secondary coil has twice as many turns of wire
-as the primary coil, the induced current will have twice
-the voltage of the primary current. In this way, by
-multiplying the turns of wire in the secondary coil, we
-can go on increasing the voltage of the induced current,
-and this is the principle upon which the induction coil
-works.</p>
-
-<p>We may now describe the construction of such a coil.
-The primary coil is made of a few turns of thick copper
-wire carefully insulated, and inside it is placed a core consisting
-of a bundle of separate wires of soft iron. Upon
-this coil, but carefully insulated from it, is wound the
-secondary coil, consisting of a great number of turns of
-very fine wire. In large induction coils the secondary coil
-has thousands of times as many turns as the primary, and
-the wire forming it may be more than a hundred miles in
-length. The ends of the secondary coil are brought to
-terminals so that they can be connected up to any apparatus
-as desired.</p>
-
-<figure id="fig_17" class="figright" style="max-width: 13em;">
- <img src="images/i_079.png" width="1008" height="920" alt=" ">
- <figcaption class="caption hang"><span class="smcap">Fig. 17.</span>—Diagram showing working of
-Contact-Breaker for Induction Coil.
-</figcaption></figure>
-
-<p>In order that the induced currents shall follow each
-other in quick succession, some means of rapidly making<span class="pagenum" id="Page_63">63</span>
-and breaking the circuit is required, and this is provided
-by an automatic contact breaker. It consists of a small
-piece of soft iron, A, <a href="#fig_17">Fig. 17</a>, fixed to a spring, B, having
-a platinum tip at C. The adjustable screw, D, also has a
-platinum tip, E. Normally the two platinum tips are just
-touching one another, and matters are arranged so that
-their contact completes the circuit. When the apparatus
-is connected to a suitable battery a current flows through
-the primary coil, and the iron core, F, becomes an electro-magnet,
-which draws A towards it. The platinum tips
-are thus no longer in contact and the circuit is broken.
-Immediately this occurs the
-iron core loses its magnetism
-and ceases to attract A, which
-is then moved back again by
-the spring B, so that the
-platinum tips touch, the circuit
-is once more completed, and
-the process begins over again.
-All this takes place with the
-utmost rapidity, and the speed
-at which the contact-breaker
-works is so great as to produce
-a musical note. There
-are many other types of contact-breakers, but in every
-case the purpose is the same, namely, to make and
-break the primary circuit as rapidly as possible.</p>
-
-<p>The efficiency of the coil is greatly increased by a
-condenser which is inserted in the primary circuit. It
-consists of alternate layers of tinfoil and paraffined
-paper, and its action is like that of a Leyden jar.
-A switch is provided to turn the battery current on or
-off, and there is also a reversing switch or commutator,
-by means of which the direction of the current may be<span class="pagenum" id="Page_64">64</span>
-reversed. The whole arrangement is mounted on a
-suitable wooden base, and its general appearance is shown
-in <a href="#fig_18">Fig. 18</a>.</p>
-
-<figure id="fig_18" class="figcenter clear" style="max-width: 19em;">
- <img src="images/i_080.jpg" width="1473" height="851" alt=" ">
- <figcaption class="caption">
-
-<p class="floatl"><i>By permission of</i>]</p>
-<p class="floatr">[<i>Harry W. Cox, Ltd.</i></p>
-
-<p class="floatc"><span class="smcap">Fig. 18.</span>—Typical Induction Coil.</p>
-</figcaption></figure>
-
-<p>By means of a large induction coil we can obtain a
-voltage hundreds or even thousands of times greater than
-that of the original battery current, but on account of the
-great resistance of a very long, thin wire, the amperage is
-much smaller. The induction coil produces a rapid
-succession of sparks, similar to those obtained from a
-Wimshurst machine. A coil has been constructed capable
-of giving sparks 42½ inches in length, and having a
-secondary coil with 340,000 turns of wire, the total
-length of the wire being 280 miles. Induction coils are
-largely employed for scientific purposes, and they are
-used in wireless telegraphy and in the production of
-X-rays.</p>
-
-<p>The principle of the induction coil can be applied
-also to the lowering of the voltage of a current. If
-we make the secondary coil with less, instead of more
-turns of wire than the primary coil, the induced current
-will be of lower voltage than the primary current, but<span class="pagenum" id="Page_65">65</span>
-its amperage will be correspondingly higher. This fact
-is taken advantage of in cases where it is desirable
-to transform a high voltage current from the public
-mains down to a lower voltage current of greater
-amperage.</p>
-<hr class="chap x-ebookmaker-drop">
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_66">66</span></p>
-
-<h2 class="nobreak" id="toclink_66"><a id="chapter_IX"></a>CHAPTER IX<br>
-
-<span class="subhead">THE DYNAMO AND THE ELECTRIC MOTOR</span></h2>
-</div>
-
-<p class="in0"><span class="firstword">Most</span> of my readers will have seen the small working
-models of electric tramcars which can be bought at any
-electrical supply stores. These usually require a current of
-about one ampere at three or four volts. If we connect
-such a car to the battery recommended for it, and keep it
-running continuously, we find that the battery soon begins
-to show signs of exhaustion. Now if we imagine our
-little car increased to the size of an electric street car, and
-further imagine, say, a hundred such cars carrying heavy
-loads day after day from morning to night, we shall realize
-that a battery of cells capable of supplying the current
-necessary to run these cars would be so colossal as to be
-utterly impracticable. We therefore must look beyond the
-voltaic cell for a source of current for such a purpose, and
-this source we find in a machine called the “dynamo,”
-from the Greek word <em>dynamis</em>, meaning force.</p>
-
-<p>Oersted’s discovery of the production of magnetism by
-electricity naturally suggested the possibility of producing
-electricity from magnetism. In the year 1831 one of the
-most brilliant of our British scientists, Michael Faraday,
-discovered that a current of electricity could be induced in
-a coil of wire either by moving the coil towards or away
-from a magnet, or by moving a magnet towards or away
-from the coil. This may be shown in a simple way by
-connecting the ends of a coil of insulated wire to a galvanometer,<span class="pagenum" id="Page_67">67</span>
-and moving a bar magnet in and out of the coil;
-when the galvanometer shows that a current is induced in
-the coil on the insertion of the magnet, and again on its
-withdrawal. We have seen that a magnet is surrounded
-by a field of magnetic force, and Faraday found that the
-current was induced when the lines of force were cut across.</p>
-
-<p>Utilizing this discovery Faraday constructed the first
-dynamo, which consisted of a copper plate or disc rotated
-between the poles of a powerful horse-shoe magnet, so as
-to cut the lines of force. The current flowed either from
-the shaft to the rim, or <i lang="la">vice versa</i>, according to the direction
-of rotation; and it was conducted away by means of two
-wires with spring contacts, one pressing against the shaft,
-and the other against the circumference of the disc. This
-machine was miserably inefficient, but it was the very first
-dynamo, and from it have been slowly evolved the mighty
-dynamos used to-day in electric power stations throughout
-the world. There is a little story told of Faraday
-which is worth repeating even if it is not true. Speaking
-of his discovery that a magnet could be made to produce
-an electric current, a lady once said to him, “This is all
-very interesting, but what is the use of it?” “Madam,”
-replied Faraday, “what is the use of a baby?” In
-Faraday’s “baby” dynamo, as in all others, some kind of
-power must be used to produce the necessary motion, so
-that all dynamos are really machines for converting
-mechanical energy into electrical energy.</p>
-
-<p>The copper disc in this first dynamo did not prove
-satisfactory, and Faraday soon substituted for it rotating
-coils of wire. In 1832 a dynamo was constructed in which
-a length of insulated wire was wound upon two bobbins
-having soft iron cores, and a powerful horse-shoe magnet
-was fixed to a rotating spindle in such a position that its
-poles faced the cores of the bobbins. This machine gave<span class="pagenum" id="Page_68">68</span>
-a fair current, but it was found that the magnet gradually
-lost its magnetism on account of the vibration caused by
-its rotation. The next step was to make the magnet a
-fixture, and to rotate the bobbins of wire. This was a
-great improvement, and the power of machines built on
-this principle was much increased by having a number of
-rotating coils and several magnets. One such machine
-had 64 separate
-coils rotating
-between the
-poles of 40 large
-magnets. Finally,
-permanent
-magnets were
-superseded by
-electro-magnets,
-which
-gave a much
-more powerful
-field of force.</p>
-
-<figure id="fig_19" class="figcenter" style="max-width: 19em;">
- <img src="images/i_084.png" width="1519" height="1503" alt=" ">
- <figcaption class="caption"><span class="smcap">Fig. 19.</span>—Diagram showing principle of Dynamo
-producing Alternating Current.
-</figcaption></figure>
-
-<p>Having seen
-something of
-the underlying
-principle and of
-the history of
-the dynamo, we
-must turn our attention to its actual working. <a href="#fig_19">Fig. 19</a> is a
-rough representation of a dynamo in its simplest form. The
-two poles of the magnet are shown marked north and south,
-and between them revolves the coil of wire A¹ A², mounted
-on a spindle SS. This revolving coil is called the armature.
-To each of the insulated rings RR is fixed one end of the
-coil, and BB are two brushes of copper or carbon, one
-pressing on each ring. From these brushes the current is<span class="pagenum" id="Page_69">69</span>
-led away into the main circuit, and in this case we may
-suppose that the current is used to light a lamp.</p>
-
-<p>In speaking of the induction coil we saw that the
-currents induced on making and on breaking the circuit
-flowed in opposite directions, and similarly, Faraday found
-that the currents induced in a coil of wire on inserting and
-on withdrawing his magnet flowed in opposite directions.
-In the present case the magnet is stationary and the coil
-moves, but the effect is just the same. Now if we suppose
-the armature to be revolving in a clockwise direction, then
-A¹ is descending and entering the magnetic field in front of
-the north pole, consequently a current is induced in the
-coil, and of course in the main circuit also, in one direction.
-Continuing its course, A¹ passes away from this portion of
-the magnetic field, and thus a current is induced in the
-opposite direction. In this way we get a current which
-reverses its direction every half-revolution, and such a
-current is called an alternating current. If, as in our
-diagram, there are only two magnetic poles, the current
-flows backwards and forwards once every revolution, but
-by using a number of magnets, arranged so that the coil
-passes in turn the poles of each, it can be made to flow
-backwards and forwards several times. One complete
-flow backwards and forwards is called a period, and the
-number of periods per second is called the periodicity or
-frequency of the current. A dynamo with one coil or set
-of coils gives what is called “single-phase” current, that is, a
-current having one wave which keeps flowing backwards
-and forwards. If there are two distinct sets of coils we get
-a two-phase current, in which there are two separate waves,
-one rising as the other falls. Similarly, by using more
-sets of coils, we may obtain three-phase or polyphase
-currents.</p>
-
-<figure id="fig_20" class="figcenter" style="max-width: 19em;">
- <img src="images/i_086.png" width="1511" height="1651" alt=" ">
- <figcaption class="caption"><span class="smcap">Fig. 20.</span>—Diagram showing principle of Dynamo
-producing Continuous Current.
-</figcaption></figure>
-
-<p>Alternating current is unsuitable for certain purposes,<span class="pagenum" id="Page_70">70</span>
-such as electroplating; and by making a small alteration in
-our dynamo we get a continuous or direct current, which
-does not reverse its direction. <a href="#fig_20">Fig. 20</a> shows the new
-arrangement. Instead of the two rings in <a href="#fig_19">Fig. 19</a>, we have
-now a single ring divided into two parts, each half being
-connected to one end of the revolving coil. Each brush,
-therefore, remains on one portion of the ring for half a
-revolution, and
-then passes
-over on to the
-other portion.
-During one
-half-revolution
-we will suppose
-the current to
-be flowing from
-brush B¹ in the
-direction of the
-lamp. Then
-during the next
-half-revolution
-the current
-flows in the opposite
-direction;
-but brush B¹
-has passed on
-to the other half
-of the ring, and so the current is still leaving by it.
-In this way the current must always flow in the same
-direction in the main circuit, leaving by brush B¹ and
-returning by brush B². This arrangement for making the
-alternating current into a continuous current is called a
-<em>commutator</em>.</p>
-
-<figure id="plate_IV" class="figcenter" style="max-width: 26em;">
- <p class="caption">PLATE IV.</p>
- <img src="images/i_087.jpg" width="2024" height="2679" alt=" ">
- <figcaption class="caption">
-
-<p class="floatl"><i>By permission of</i></p>
-<p class="floatr"><i>Lancashire Dynamo & Motor Co. Ltd.</i></p>
-
-<p class="floatc">A TYPICAL DYNAMO AND ITS PARTS.</p>
-</figcaption></figure>
-
-<p>In actual practice a dynamo has a set of electro-magnets,
-and the armature consists of many coils of wire mounted
-on a core of iron, which has the effect of concentrating the
-lines of force. The armature generally revolves in small
-dynamos, but in large ones it is usually a fixture, while the
-electro-magnets revolve. <a href="#plate_IV">Plate IV</a>. shows a typical dynamo
-and its parts.</p>
-
-<p>As we saw in an earlier chapter, an electro-magnet has
-magnetic powers only while a current is being passed
-through its winding, and so some means of supplying
-current to the electro-magnets in a dynamo must be provided.
-It is a remarkable fact that it is almost impossible
-to obtain a piece of iron which has not some traces of
-magnetism, and so when a dynamo is first set up there is
-often sufficient magnetism in the iron of the electro-magnets
-to produce a very weak field. The rapid cutting of the
-feeble lines of force of this field sets up a weak current,
-which, acting upon the electro-magnets, gradually brings
-them up to full strength. Once the dynamo is generating
-current it keeps on feeding its magnets by sending either
-the whole or a part of its current through them. After
-it has once been set going the dynamo is always able to
-start again, because the magnet cores retain enough
-magnetism to set up a weak field. If there is not enough
-magnetism in the cores to start a dynamo for the first
-time, a current from some outside source is sent round the
-magnets.</p>
-
-<p>The foregoing remarks apply to continuous current
-dynamos only. Alternating current can be used for exciting
-electro-magnets, but in this case the magnetic field produced
-is alternating also, so that each pole of the magnet has
-north and south magnetism alternately. This will not do
-for dynamo field magnets, and therefore an alternating
-current dynamo cannot feed its own magnets. The electro-magnets
-in such dynamos are supplied with current from a<span class="pagenum" id="Page_72">72</span>
-separate continuous current dynamo, which may be of quite
-small size.</p>
-
-<p>It is a very interesting fact that electric current can
-be generated by a dynamo in which the earth itself is
-used to provide the magnetic field, no permanent or electro-magnets
-being used at all. A simple form of dynamo
-of this kind consists of a rectangular loop of copper wire
-rotating about an axis pointing east and west, so that
-the loop cuts the lines of force of the Earth’s magnetic
-field.</p>
-
-<p>The dynamo provides us with a constant supply of
-electric current, but this current is no use unless we can
-make it do work for us. If we reverse the usual order of
-things in regard to a dynamo, and supply the machine with
-current instead of mechanical power, we find that the
-armature begins to revolve rapidly, and the machine is
-no longer a dynamo, but has become an electric motor.
-This shows us that an electric motor is simply a dynamo
-reversed. Let us suppose that we wish to use the
-dynamo in <a href="#fig_20">Fig. 20</a> as a motor. In order to supply the
-current we will take away the lamp and substitute a
-second continuous-current dynamo. We know from
-<a href="#chapter_VII">Chapter VII</a>. that when a current is sent through a coil
-of wire the coil becomes a magnet with a north and a
-south pole. The coil in our dynamo becomes a magnet
-as soon as the current is switched on, and the attraction
-between its poles and the opposite poles of the magnet
-causes it to make half a revolution. At this point the
-commutator reverses the current, and consequently the
-polarity of the coil, so that there is now repulsion where
-previously there was attraction, and the coil makes another
-half-revolution. So the process goes on until the armature
-attains a very high speed. In general construction there is
-practically no difference between a dynamo and a motor,<span class="pagenum" id="Page_73">73</span>
-but there are differences in detail which adapt each to its
-own particular work. By making certain alterations in
-their construction electric motors can be run with alternating
-current.</p>
-
-<p>The fact that a dynamo could be reversed and run as a
-motor was known probably as early as 1838, but the great
-value of this reversibility does not seem to have been
-realized until 1873. At an industrial exhibition held at
-Vienna in that year, it so happened that a workman or
-machinery attendant connected two cables to a dynamo
-which was standing idle, and he was much surprised to
-find that it at once began to revolve at a great speed. It
-was then seen that the cables led to another dynamo which
-was running, and that the current from this source had
-made the first dynamo into a motor. There are many
-versions of this story, but the important point in all
-is that this was the first occasion on which general
-attention was drawn to the possibilities of the electric
-motor.</p>
-
-<p>The practical advantages afforded by the electric motor
-are many and great. Once we have installed a sufficiently
-powerful dynamo and a steam or other engine to drive it,
-we can place motors just where they are required, either
-close to the dynamo or miles away, driving them simply by
-means of a connecting cable. In factories, motors can be
-placed close to the machines they are required to drive,
-anywhere in the building, thus doing away with all complicated
-and dangerous systems of shafting and belts. In
-many cases where it would be either utterly impossible or
-at least extremely inconvenient to use any form of steam,
-gas, or oil engine, electric motors can be employed without
-the slightest difficulty. In order to realize this, one only
-has to think of the positions in which electrically-driven
-ventilating fans are placed, or of the unpleasantly familiar<span class="pagenum" id="Page_74">74</span>
-electric drill of the dentist. An electric motor is small and
-compact, gives off no fumes and practically no heat, makes
-very little noise, is capable of running for very long periods
-at high speed and with the utmost steadiness, and requires
-extremely little attention.</p>
-<hr class="chap x-ebookmaker-drop">
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_75">75</span></p>
-
-<h2 class="nobreak" id="toclink_75"><a id="chapter_X"></a>CHAPTER X<br>
-
-<span class="subhead">ELECTRIC POWER STATIONS</span></h2>
-</div>
-
-<p class="in0"><span class="firstword">It</span> is apparently a very simple matter to fit up a power
-station with a number of very large dynamos driven by
-powerful engines, and to distribute the current produced by
-these dynamos to all parts of a town or district by means
-of cables, but as a matter of fact it is a fairly complicated
-engineering problem. First of all the source of power for
-driving the dynamos has to be considered. In private and
-other small power plants, gas, petrol or oil engines are
-generally used, but for large stations the choice lies between
-steam and water power. In this country steam power is
-used almost exclusively. Formerly the ordinary reciprocating
-steam engines were always employed, and though these
-are still in very extensive use, they are being superseded in
-many cases by steam turbines. The turbine is capable of
-running at higher speeds than the reciprocating engine, and
-at the greatest speeds it runs with a great deal less noise,
-and with practically no vibration at all. More than this,
-turbines take up much less room, and require less oil and
-attendance. The turbines are coupled directly to the
-dynamos, so that the two machines appear almost as one.
-In the power station shown on <a href="#plate_V">Plate V</a>. a number of alternating
-current dynamos coupled to steam turbines are seen.</p>
-
-<p>A large power station consumes enormous quantities of
-coal, and for convenience of supply it is situated on the
-bank of a river or canal, or, if neither of these is available,<span class="pagenum" id="Page_76">76</span>
-as close to the railway as possible. The unloading of the
-coal barges or trucks is done mechanically, the coal passing
-into a large receiving hopper. From here it is taken to
-another hopper close to the furnaces by means of coal
-elevators and conveyors, which consist of a number of
-buckets fixed at short intervals on an endless travelling
-chain. From the furnace hopper the coal is fed into the
-furnaces by mechanical stokers, and the resulting ash and
-clinker falls into a pit below the furnaces, from which it is
-carted away.</p>
-
-<p>The heat produced in the furnaces is used to generate
-steam, and from the boilers the steam passes to the engines
-along a steam pipe. After doing its work in the engines,
-the steam generally passes to a condenser, in which it is
-cooled to water, freed from oil and grease, and returned to
-the boilers to be transformed once more into steam. As
-this water from the condenser is quite warm, less heat is
-required to raise steam from it than would be the case if
-the boiler supply were kept up with cold water. The
-power generated by the engines is used to drive the
-dynamos, and stout copper cables convey the current from
-these to what are called “bus” bars. There are two of
-these, one receiving the positive cable from the dynamos,
-and the other the negative cable, and the bars run from end
-to end of a large main switchboard. From this switchboard
-the current is distributed by other cables known as feeders.</p>
-
-<p>The nature of the current generated at a power station
-is determined to a great extent by the size of the district to
-be supplied. Generally speaking, where the current is not
-to be transmitted beyond a radius of about two miles from
-the station, continuous current is generated; while alternating
-current is employed for the supply of larger areas. In
-some cases both kinds of current are generated at one
-station.</p>
-
-<figure id="plate_V" class="figcenter" style="max-width: 40em;">
- <p class="caption">PLATE V.</p>
- <img src="images/i_095.jpg" width="3146" height="2015" alt=" ">
- <figcaption class="caption">
-
-<p class="floatl"><i>By permission of</i></p>
-<p class="floatr"><i>C. A. Parsons & Co.</i></p>
-
-<p class="floatc">LOTS ROAD ELECTRIC POWER STATION, CHELSEA.</p>
-</figcaption></figure>
-
-<p>If continuous current is to be used, it is generated
-usually at a pressure of from 400 to 500 volts, the average
-being about 440 volts; and the supply is generally on what
-is known as the three-wire system. Three separate wires
-are employed. The two outer wires are connected
-respectively to the positive and the negative bus bars
-running along the main switchboard, these bars receiving
-positive or negative current directly from the dynamos.
-The outer wires therefore carry current at the full voltage
-of the system. Between them is a third and smaller wire,
-connected to a third bar, much smaller than the outer bars,
-and known as the mid-wire bar. This bar is not connected
-to the dynamos, but to earth, by means of a large plate of
-copper sunk into the ground. Connexion between the
-mid-wire bar and the outer bars is made by two machines
-called “balancers,” one connecting the mid-wire bar and the
-positive bus bar, and the other the mid-wire bar and the
-negative bus bar. If the pressure between the outer bars
-is 440 volts, then the pressure between the mid-wire bar and
-either of the outer bars will be 220 volts, that is just half.</p>
-
-<p>The balancers serve the purpose of balancing the
-voltage on each side, and they are machines capable of
-acting either as motors or dynamos. In order to comply
-with Board of Trade regulations, electric appliances of all
-kinds intended for ordinary domestic purposes, including
-lamps, and heating and cooking apparatus, are supplied
-with current at a pressure not exceeding 250 volts. In a
-system such as we are describing, all these appliances are
-connected between the mid-wire and one or other of the
-outer wires, thus receiving current at 220 volts. In
-practice it is impossible to arrange matters so that the
-lamps and other appliances connected with the positive side
-of the system shall always take the same amount of current
-as those connected with the negative side, and there is<span class="pagenum" id="Page_78">78</span>
-always liable to be a much greater load on one side or the
-other. If, for instance, a heavy load is thrown on the
-negative side, the voltage on that side will drop. The
-balancer on the positive side then acts as an electric motor,
-drives the balancer on the negative side as a dynamo, and
-thus provides the current required to raise the voltage on
-the negative side until the balance is restored. The working
-of the balancers, which need not be described in further
-detail, is practically automatic. Electric motors, for driving
-electric trams or machinery of any kind, are connected
-between the outer wires, so that they receive the full 440
-volts of the system.</p>
-
-<p>In any electric supply system the demand for current
-does not remain constant, but fluctuates more or less. For
-instance, in a system including an electric tramway, if a car
-breaks down and remains a fixture for a short time, all cars
-behind it are held up, and a long line of cars is quickly
-formed. When the breakdown is repaired, all the cars
-start practically at the same instant, and consequently a
-sudden and tremendous demand for current is made. In a
-very large tramway system in a fairly level city, the
-fluctuations in the demand for current, apart from accidents,
-are not very serious, for they tend to average themselves;
-but in a small system, and particularly if the district is hilly,
-the fluctuations are very great, and the current demand
-may vary as much as from 400 to 2000 amperes. Again,
-in a system supplying power and light, the current demand
-rises rapidly as the daylight fails on winter afternoons,
-because, while workshop and other motors are still in full
-swing, thousands of electric lamps are switched on more or
-less at the same time. The power station must be able to
-deal with any exceptional demands which are likely to
-occur, and consequently more current must be available
-than is actually required under average conditions. Instead<span class="pagenum" id="Page_79">79</span>
-of having generating machinery large enough to meet all
-unusual demands, the generators at a station using continuous
-current may be only of sufficient size to supply a little
-more than the average demand, any current beyond this
-being supplied by a battery of storage cells. The battery
-is charged during periods when the demand for current is
-small, and when a heavy load comes on, the current from
-the battery relieves the generators of the sudden strain.
-To be of any service for such a purpose the storage battery
-of course must be very large. <a href="#plate_VI">Plate VI</a>. shows a large
-battery of no cells, and some idea of the size of the
-individual cells may be obtained from the fact that each
-weighs about 3900 lb.</p>
-
-<p>Alternating current is produced at almost all power
-stations supplying large districts. It is generated at high
-pressure, from 2000 volts upwards, the highest pressure
-employed in this country being about 11,000 volts. Such
-pressures are of course very much too high for electric
-lamps or motors, and the object of generating current of
-this kind is to secure the greatest economy in transmission
-through the long cables. Electric energy is measured in
-watts, the watts being obtained by multiplying together
-the pressure or voltage of the current, and its rate of flow
-or amperage. From this it will be seen that, providing the
-product of voltage and amperage remains the same, it
-makes no difference, so far as electric energy is concerned,
-whether the current be of high voltage and low amperage,
-or of low voltage and high amperage. Now in transmitting
-a current through a long cable, there is a certain
-amount of loss due to the heating of the conductor. This
-heating is caused by the current flow, not by the pressure;
-and the heavier the current, the greater the heating, and
-the greater the loss. This being so, it is clear that by
-decreasing the current flow, and correspondingly increasing<span class="pagenum" id="Page_80">80</span>
-the pressure, the loss in transmission will be reduced; and
-this is why alternating current is generated at high pressure
-when it is to be transmitted to a distance.</p>
-
-<p>The kind of alternating current generated is usually
-that known as three-phase current. Formerly single-phase
-current was in general use, but it has been superseded by
-three-phase current because the latter is more economical
-to generate and to distribute, and also more satisfactory for
-electric motors. The actual voltage of the current sent out
-from the station varies according to the distance to which
-the current is to be conveyed. In the United States and
-in other countries where current has to be conveyed to
-places a hundred or even more miles from the station,
-pressures as high as 120,000 volts are in use. It is possible
-to produce alternating current at such pressures directly
-from the dynamos, but in practice this is never done, on
-account of the great liability to breakdown of the insulation.
-Instead, the current is generated at from 2000 to 10,000 or
-11,000 volts, and raised to the required pressure, before
-leaving the station, by means of a step-up transformer.
-We have seen that an induction coil raises, or steps up, the
-voltage of the current supplied to it. A step-up transformer
-works on the same principle as the induction coil, and in
-passing through it the current is raised in voltage, but
-correspondingly lowered in amperage. Of course, if the
-pressure of the current generated by the dynamos is already
-sufficiently high to meet the local requirements, the transformer
-is not used.</p>
-
-<figure id="plate_VI" class="figcenter" style="max-width: 42em;">
- <p class="caption">PLATE VI.</p>
- <img src="images/i_101.jpg" width="3301" height="2190" alt=" ">
- <figcaption class="caption">
-
-<p class="floatl"><i>By permission of</i></p>
-<p class="floatr"><i>Chloride Electrical Storage Co. Ltd.</i></p>
-
-<p class="floatc">POWER STATION BATTERY OF ACCUMULATORS.</p>
-</figcaption></figure>
-
-<p>For town supply the current from the power station is
-led along underground cables to a number of sub-stations,
-situated in different parts of the town, and generally underground.
-At each sub-station the current passes through a
-step-down transformer, which also acts on the principle of the
-induction coil, but in the reverse way, so that the voltage is
-lowered instead of being raised. From the transformer the
-current emerges at the pressure required for use, but it is
-still alternating current; and if it is desired to have a
-continuous-current supply this alternating current must be
-converted. One of the simplest arrangements for this
-purpose consists of an electric motor and a dynamo, the
-two being coupled together. The motor is constructed to
-run on the alternating current from the transformer, and
-it drives the dynamo, which is arranged to generate continuous
-current. There is also a machine called a “rotary
-converter,” which is largely used instead of the motor
-generator. This machine does the work of both motor and
-dynamo, but its action is too complicated to be described
-here. From the sub-stations the current, whether converted
-or not, is distributed as required by a network of
-underground cables.</p>
-
-<p>In many parts of the world, especially in America,
-water power is utilized to a considerable extent instead of
-steam for the generation of electric current. The immense
-volume of water passing over the Falls of Niagara develops
-energy equal to about seven million horse-power, and a
-small amount of this energy, roughly about three-quarters
-of a million horse-power, has been harnessed and made to
-produce electric current for light and power. The water
-passes down a number of penstocks, which are tubes or
-tunnels about 7 feet in diameter, lined with brick and
-concrete; and at the bottom of these tubes are placed
-powerful water turbines. The falling water presses upon
-the vanes of the turbines, setting them revolving at great
-speed, and the power produced in this way is used to drive
-a series of very large alternating current dynamos. The
-current is conveyed at a pressure of about 60,000 volts
-to various towns within a radius of 200 or 300 miles,
-and it is anticipated that before very long the supply will<span class="pagenum" id="Page_82">82</span>
-be extended to towns still more distant. Many other
-American rivers have been harnessed in a similar way,
-though not to the same extent; and Switzerland and
-Norway are utilizing their water power on a rapidly
-increasing scale. In England, owing to the abundance of
-coal, little has been done in this direction. Scotland is
-well favoured in the matter of water power, and it is
-estimated that the total power available is considerably
-more than enough to run the whole of the railways of that
-country. Very little of this power has been utilized however,
-and the only large hydro-electric installation is the
-one at Kinlochleven, in Argyllshire. It is a mistake to
-suppose that water power means power for nothing, but
-taking things all round the cost of water power is considerably
-lower than that of steam.</p>
-<hr class="chap x-ebookmaker-drop">
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_83">83</span></p>
-
-<h2 class="nobreak" id="toclink_83"><a id="chapter_XI"></a>CHAPTER XI<br>
-
-<span class="subhead">ELECTRICITY IN LOCOMOTION</span></h2>
-</div>
-
-<p class="in0"><span class="firstword">The</span> electric tramcar has become such a necessary feature
-of our everyday life that it is very difficult to realize how
-short a time it has been with us. To most of us a horse-drawn
-tramcar looks like a relic of prehistoric times, and
-yet it is not so many years since the horse tram was in full
-possession of our streets. Strikes of tramway employees
-are fortunately rare events, but a few have occurred during
-the past two or three years in Leeds and in other towns,
-and they have brought home to us our great dependence
-upon the electric tram. During the Leeds strike the streets
-presented a most curious appearance, and the city seemed
-to have made a jump backward to fifty years ago. Every
-available article on wheels was pressed into service to bring
-business men into the city from the outlying districts, and
-many worthy citizens were seen trying to look dignified
-and unconcerned as they jogged along in conveyances
-which might have come out of the Ark. On such an
-occasion as this, if we imagine the electric light supply
-stopped also, we can form some little idea of our indebtedness
-to those who have harnessed electricity and made it
-the greatest power of the twentieth century.</p>
-
-<p>There are three distinct electric tramway systems; the
-trolley or overhead system, the surface contact system,
-and the conduit system. The trolley system has almost
-driven the other two from the field, and it is used almost<span class="pagenum" id="Page_84">84</span>
-exclusively throughout Great Britain and Ireland. On the
-Continent and in the United States the conduit system still
-survives, but probably it will not be long before the trolley
-system is universally employed.</p>
-
-<p>The superiority of the trolley system lies in the fact that
-it is cheaper to construct and to maintain than the other
-two, and also in its much greater reliability under all
-working conditions. The overhead wire is not one continuous
-cable, but is divided into sections of about half a
-mile in length, each section being supplied with current
-from a separate main. At each point where the current
-is fed to the trolley wire a sort of metal box may be seen
-at the side of the street. These boxes are called “feeder
-pillars,” and each contains a switch by means of which the
-current can be cut off from that particular section, for
-repairing or other purposes. Above the car is fixed an arm
-provided with a trolley wheel which runs along the wire,
-and this wheel takes the current from the wire. From the
-wheel the current passes down the trolley arm to the
-controller, which is operated by the driver, and from there
-to the motors beneath the car. Leaving the motors it
-passes to the wheels and then to the rails, from which it
-is led off at intervals by cables and so returned to the
-generating station. The current carried by the rails is at
-a pressure of only a few volts, so that there is not the
-slightest danger of shock from them. There are generally
-two electric motors beneath the car, and the horse-power
-of each varies from about fifteen to twenty-five.</p>
-
-<p>The controller consists mainly of a number of graduated
-resistances. To start the car the driver moves a handle
-forward notch by notch, thus gradually cutting out the
-resistance, and so the motors receive more and more
-current until they are running at full speed. The movement
-of the controller handle also alters the connexion of<span class="pagenum" id="Page_85">85</span>
-the motors. When the car is started the motors are
-connected in series, so that the full current passes through
-each, while the pressure is divided between them; but
-when the car is well on the move the controller connects
-the motors in parallel, so that each receives the full pressure
-of the current.</p>
-
-<p>The conduit and surface contact systems are much the
-same as the trolley system except in the method of supplying
-the current to the cars. In the conduit system two
-conductors conveying the current are placed in an underground
-channel or conduit of concrete strengthened by iron
-yokes. The top of the conduit is almost closed in so as to
-leave only a narrow slot, through which passes the current
-collector of the car. This current collector, or “plough” as
-it is called, carries two slippers which make contact with the
-conductors, and thus take current from them. In this
-system the current returns along one of the conductors, so
-that no current passes along the track rails. This is the
-most expensive of the three systems, both in construction
-and maintenance.</p>
-
-<p>The surface contact or stud system is like the conduit
-system in having conductors placed in a sort of underground
-trough, but in this case contact with the conductors
-is made by means of metal studs fixed at intervals in the
-middle of the track. The studs are really the tops of
-underground boxes each containing a switch, which, when
-drawn up to a certain position, connects the stud to the
-conductors. These switches are arranged to be moved by
-magnets fixed beneath the car, and thus when the car
-passes over a stud the magnets work the switch and connect
-the stud to the conductors, so that the stud is then
-“alive.” The current is taken from the studs by means of
-sliding brushes or skates which are carried by the car.
-The studs are thus alive only when the car is passing over<span class="pagenum" id="Page_86">86</span>
-them, and at all other times they are dead, and not in any
-way dangerous.</p>
-
-<p>The weight and speed of electric cars make it important
-to have a thoroughly reliable system of brakes. First of
-all there are ordinary mechanical brakes, which press
-against the wheels. Then there are electro-magnetic
-slipper brakes which press on the rails instead of on the
-wheels of the car. These brakes are operated by electro-magnets
-of great power, the current necessary to excite the
-magnets being taken from the motors. Finally there is
-a most interesting and ingenious method of regenerative
-control. Before a car can be stopped after it has attained
-considerable speed a certain amount of energy has to be
-got rid of in some way. With the ordinary mechanical or
-electro-magnetic brakes this energy is wasted, but in the
-regenerative method it is turned into electric current, which
-is sent back into the circuit. If an electric motor is supplied
-with mechanical power instead of electric current it becomes
-a dynamo, and generates current instead of using it. In
-the regenerative system, when a car is “coasting” down a
-hill it drives the wheels, and the wheels drive the motors,
-so that the latter become dynamos and generate current
-which is sent back to the power station. In this way some
-of the abnormal amount of current taken by a car in climbing
-a hill is returned when the car descends the hill. The
-regenerative system limits the speed of the car, so that it
-cannot possibly get beyond control.</p>
-
-<figure id="plate_VII" class="figcenter" style="max-width: 39em;">
- <p class="caption">PLATE VII.</p>
- <img src="images/i_109.jpg" width="3105" height="1980" alt=" ">
- <figcaption class="caption">
-
-<p class="floatl"><i>By permission of</i></p>
-<p class="floatr"><i>Siemens Brothers Dynamo Works Ltd.</i></p>
-
-<p class="floatc">ELECTRIC COLLIERY RAILWAY.</p>
-</figcaption></figure>
-
-<p>A large tramway system spreads outwards from the
-centre of a city to the suburbs, and usually terminates at
-various points on the outskirts of these suburbs. It often
-happens that there are villages lying some distance beyond
-these terminal points, and it is very desirable that there
-should be some means of transport between these villages
-and the city. An extension of the existing tramway is not
-practicable in many cases, because the traffic would not be
-sufficient to pay for the heavy outlay, and also because the
-road may not be of sufficient width to admit of cars running
-on a fixed track. The difficulty may be overcome satisfactorily
-by the use of trackless trolley cars. With these
-cars the costly business of laying a rail track is altogether
-avoided, only a system of overhead wires being necessary.
-As there is no rail to take the return current, a second
-overhead wire is required. The car is fitted with two
-trolley arms, and the current is taken from one wire by the
-first arm, sent through the controller and the motors, and
-returned by the second arm to the other wire, and so back
-to the generating station. The trolley poles are so arranged
-that they allow the car to be steered round obstructions
-or slow traffic, and the car wheels are usually fitted
-with solid rubber tyres. Trackless cars are not capable of
-dealing with a large traffic, but they are specially suitable
-where an infrequent service, say a half-hourly one, is enough
-to meet requirements.</p>
-
-<p>We come now to electric railways. These may be
-divided into two classes, those with separate locomotives
-and those without. The separate locomotive method is
-largely used for haulage purposes in collieries and large
-works of various kinds. In <a href="#plate_VII">Plate VII</a>. is seen an electric
-locomotive hauling a train of coal waggons in a colliery
-near the Tyne, and it will be seen that the overhead
-system is used, the trolley arm and wheel being replaced
-by sliding bows. In a colliery railway it is generally
-impossible to select the most favourable track from the
-railway constructor’s point of view, as the line must be
-arranged to serve certain points. This often means taking
-the line sometimes through low tunnels or bridges where
-the overhead wire must be low, and sometimes over public
-roads where the wire must be high; and the sliding bow<span class="pagenum" id="Page_88">88</span>
-is better able than the trolley arm and wheel to adapt
-itself to these variations. In the colliery where this locomotive
-is used the height of the overhead wire ranges
-from 10 feet 6 inches through tunnels or bridges, to 21 feet
-where the public road is crossed. The locomotive weighs
-33½ tons, and has four electric motors each developing
-50 horse-power with the current employed. It will be
-noticed that the locomotive has two sets of buffers. This
-is because it has to deal with both main line waggons and
-the smaller colliery waggons, the upper set of buffers being
-for the former, and the lower and narrower set for the
-latter. <a href="#plate_VIII">Plate VIII</a>. shows a 50-ton locomotive on the
-British Columbia Electric Railway, and a powerful locomotive
-in use in South America. In each case it will be
-seen that the trolley wheel is used.</p>
-
-<p>In this country electric railways for passenger traffic
-are mostly worked on what is known as the multiple-unit
-system, in which no separate locomotives are used, the
-motors and driving mechanism being placed on the cars
-themselves. There are also other cars without this equipment,
-so that a train consists of a single motor-car with or
-without trailer, or of two motor-cars with trailer between,
-or in fact of any other combination. When a train
-contains two or more motor-cars all the controllers, which
-are very similar to those on electric tramcars, are electrically
-connected so as to be worked together from one
-master controller. This system allows the length of the
-train to be adjusted to the number of passengers, so that
-no power is wasted in running empty cars during periods
-of small traffic. In suburban railways, where the stopping-places
-are many and close together, the efficiency of the
-service depends to a large extent upon the time occupied
-in bringing the trains from rest to full speed. In this
-respect the electric train has a great advantage over the<span class="pagenum" id="Page_89">89</span>
-ordinary train hauled by a steam locomotive, for it can
-pick up speed at three or more times the rate of the latter,
-thus enabling greater average speeds and a more frequent
-service to be maintained.</p>
-
-<p>Electric trains are supplied with current from a central
-generating station, just as in the case of electric tramcars,
-but on passenger lines the overhead wire is in most cases
-replaced by a third rail. This live rail is placed upon
-insulators just outside the track rail, and the current is
-collected from it by sliding metal slippers which are carried
-by the cars. The return current may pass along the
-track rails as in the case of trolley tramcars, or be conveyed
-by another insulated conducting rail running along
-the middle of the track.</p>
-
-<p>The electric railways already described are run on continuous
-current, but there are also railways run on alternating
-current. A section of the London, Brighton, and South
-Coast Railway is electrically operated by alternating
-current, the kind of current used being that known as
-single-phase. The overhead system is used, and the
-current is led to the wire at a pressure of about 6000 volts.
-This current is collected by sliding bows and conveyed to
-transformers carried on the trains, from which it emerges
-at a pressure of about 300 volts, and is then sent through
-the motors. The overhead wires are not fixed directly to
-the supports as in the case of overhead tramway wires, but
-instead two steel cables are carried by the supports, and
-the live wires are hung from these. The effect of this
-arrangement is to make the sliding bows run steadily and
-evenly along the wires without jumping or jolting. If ever
-electricity takes the place of steam for long distance railway
-traffic, this system, or some modification of it, probably
-will be employed.</p>
-
-<p>Mention must be made also of the Kearney high speed<span class="pagenum" id="Page_90">90</span>
-electric mono-railway. In this system the cars, which are
-electrically driven, are fitted above and below with grooved
-wheels. The lower wheels run on a single central rail
-fixed to sleepers resting on the ground, and the upper
-wheels run on an overhead guide rail. It is claimed that
-speeds of 150 miles an hour are attainable with safety and
-economy in working. This system is yet only just out of
-the experimental stage, but its working appears to be
-exceedingly satisfactory.</p>
-
-<p>A self-contained electric locomotive has been constructed
-by the North British Locomotive Company. It
-is fitted with a steam turbine which drives a dynamo generating
-continuous current, and the current is used to drive
-four electric motors. This locomotive has undergone extensive
-trials, but its practical value as compared with the
-ordinary type of electric locomotive supplied with current
-from an outside source is not yet definitely established.</p>
-
-<p>At first sight it appears as though the electric storage
-cell or accumulator ought to provide an almost perfect
-means of supplying power for self-propelled electric vehicles
-of all kinds. In practice, however, it has been found that
-against the advantages of the accumulator there are to be
-set certain great drawbacks, which have not yet been
-overcome. Many attempts have been made to apply
-accumulator traction to electric tramway systems, but they
-have all failed, and the idea has been abandoned. There
-are many reasons for the failure of these attempts. The
-weight of a battery of accumulators large enough to run a
-car with a load of passengers is tremendous, and this is
-of course so much dead weight to be hauled along, and it
-becomes a very serious matter when steep hills have to be
-negotiated. When a car is started on a steep up-gradient
-a sudden and heavy demand for current is made, and this
-puts upon the accumulators a strain which they are not<span class="pagenum" id="Page_91">91</span>
-able to bear without injury. Another great drawback is
-the comparatively short time for which accumulators can
-give a heavy current, for this necessitates the frequent
-return of the cars to the central station in order to have
-the batteries re-charged. Finally, accumulators are sensitive
-things, and the continuous heavy vibration of a tramcar
-is ruinous to them.</p>
-
-<p>The application of accumulators to automobiles is much
-more feasible, and within certain limits the electric motor-car
-may be considered a practical success. The electric
-automobile is superior to the petrol-driven car in its delightfully
-easy and silent running, and its freedom from all
-objectionable smells. On the other hand high speeds
-cannot be attained, and there is the trouble of having the
-accumulators re-charged, but for city work this is not a
-serious matter. Two sets of accumulators are used, so that
-one can be left at the garage to be charged while the other
-is in use, the replacing of the exhausted set by the freshly
-charged one being a matter of only a few minutes. The
-petrol-driven car is undoubtedly superior in every way for
-touring purposes. Petrol can now be obtained practically
-anywhere, whereas accumulator charging stations are comparatively
-few and far between, especially in country
-districts; and there is no comparison as regards convenience
-between the filling of a petrol tank and the charging of a
-set of accumulators, for one process takes a few minutes
-and the other a few hours.</p>
-
-<p>Accumulator-driven locomotives are not in general use,
-but for certain special purposes they have proved very
-satisfactory. A large locomotive of this kind was used for
-removing excavated material and for taking in the iron
-segments, sleepers, rails, and other materials in the construction
-of the Great Northern, Piccadilly, and Brompton
-Tube Railway. This locomotive is 50 feet 6 inches long,<span class="pagenum" id="Page_92">92</span>
-and it carries a battery of eighty large “chloride” cells, the
-total weight of locomotive and battery being about 64
-tons. It is capable of hauling a load of 60 tons at a
-rate of from 7 to 9 miles an hour on the level.</p>
-
-<p>Amongst the latest developments of accumulator traction
-is a complete train to take the place of a steam locomotive
-hauling a single coach on the United Railways of Cuba.
-According to the <cite>Scientific American</cite> the train consists
-of three cars, each having a battery of 216 cells, supplying
-current at 200 volts to the motors. Each car has accommodation
-for forty-two passengers, and the three are arranged
-to work on the multiple-unit system from one master controller.
-The batteries will run from 60 to 100 miles for
-each charging of seven hours.</p>
-<hr class="chap x-ebookmaker-drop">
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_93">93</span></p>
-
-<h2 class="nobreak" id="toclink_93"><a id="chapter_XII"></a>CHAPTER XII<br>
-
-<span class="subhead">ELECTRIC LIGHTING</span></h2>
-</div>
-
-<p class="in0"><span class="firstword">In</span> the first year of the nineteenth century one of the
-greatest of England’s scientists, Sir Humphry Davy,
-became lecturer on chemistry to the Royal Institution,
-where his brilliant lectures attracted large and enthusiastic
-audiences. He was an indefatigable experimenter, and in
-order to help on his work the Institution placed at his
-disposal a very large voltaic battery consisting of 2000
-cells. In 1802 he found that if two rods of carbon, one
-connected to each terminal of his great battery, were
-first made to touch one another and then gradually
-separated, a brilliant arch of light was formed between
-them. The intense brilliance of this electric arch, or <em>arc</em>
-as it came to be called, naturally suggested the possibility
-of utilizing Davy’s discovery for lighting purposes, but the
-maintaining of the necessary current proved a serious
-obstacle. The first cost of a battery of the required size
-was considerable, but this was a small matter compared
-with the expense of keeping the cells in good working
-order. Several very ingenious and more or less efficient
-arc lamps fed by battery current were produced by various
-inventors, but for the above reason they were of little use
-except for experimental purposes, and the commercial
-success of the arc lamp was an impossibility until the
-dynamo came to be a really reliable source of current.
-Since that time innumerable shapes and forms of arc lamps<span class="pagenum" id="Page_94">94</span>
-have been devised, while the use of such lamps has increased
-by leaps and bounds. To-day, wherever artificial
-illumination on a large scale is required, there the arc lamp
-is to be found.</p>
-
-<p>When the carbon rods are brought into contact and
-then slightly separated, a spark passes between them.
-Particles of carbon are torn off by the spark and volatilized,
-and these incandescent particles form a sort of bridge which
-is a sufficiently good conductor for the current to pass
-across it from one rod to the other. When the carbons
-are placed horizontally, the glowing mass is carried upwards
-by the ascending currents of heated air, and it assumes the
-arch-like form from which it gets its name. If the carbons
-are vertical the curve is not produced, a more or less
-straight line being formed instead. The electric arc may be
-formed between any conducting substances, but for practical
-lighting purposes carbon is found to be most suitable.</p>
-
-<p>Either continuous or alternating currents may be used
-to form the arc. With continuous current, if the carbon
-rods are fully exposed to the air, they gradually consume
-away, and minute particles of carbon are carried across
-from the positive rod to the negative rod, so that the former
-wastes at about twice the rate of the latter. The end of
-the positive rod becomes hollowed out so as to resemble a
-little crater, and the end of the negative rod becomes more
-or less pointed. The fact that with continuous current the
-positive rod consumes away twice as fast as the negative
-rod, may be taken advantage of to decrease the cost of new
-carbons, by replacing the wasted positive rod with a new
-one, and using the unconsumed portion of the old positive
-rod as a new negative rod.<a id="FNanchor_1" href="#Footnote_1" class="fnanchor">1</a> If alternating current is used,
-each rod in turn becomes the positive rod, so that no crater<span class="pagenum" id="Page_95">95</span>
-is formed, and both the carbons have the same shape and
-are consumed at the same rate. A humming noise is liable
-to be produced by the alternating current arc, but by careful
-construction of the lamp this noise is reduced to the
-minimum.</p>
-
-<div class="footnote">
-
-<p><a id="Footnote_1" href="#FNanchor_1" class="label">1</a> In actual practice the positive carbon is made double the thickness of the
-negative, so that the two consume at about the same rate.</p>
-
-</div>
-
-<p>If the carbons are enclosed in a suitable globe the
-rate of wasting is very much less. The oxygen inside the
-globe becomes rapidly consumed, and although the globe
-is not air-tight, the heated gases produced inside it check
-the entrance of further supplies of fresh air as long as the
-lamp is kept burning. When the light is extinguished,
-and the lamp cools down, fresh air enters again freely.</p>
-
-<p>Arc lamp carbons may be either solid or cored. The
-solid form is made entirely of very hard carbon, while the
-cored form consists of a narrow tube of carbon filled up with
-soft graphite. Cored carbons usually burn more steadily
-than the solid form. In what are known as flame arc
-lamps the carbons are impregnated with certain metallic
-salts, such as calcium. These lamps give more light for
-the same amount of current. The arc is long and flame-like,
-and usually of a striking yellow colour, but it is not so
-steady as the ordinary arc.</p>
-
-<figure id="fig_21" class="figright" style="max-width: 13em;">
- <img src="images/i_120.png" width="1027" height="1092" alt=" ">
- <figcaption class="caption hang"><span class="smcap">Fig. 21.</span>—Diagram showing simple
-method of carbon regulation for Arc Lamps.
-</figcaption></figure>
-
-<p>As the carbon rods waste away, the length of the arc
-increases, and if this increase goes beyond a certain limit
-the arc breaks and the current ceases. If the arc is to be
-kept going for any length of time some arrangement for
-pushing the rods closer together must be provided, in order
-to counteract the waste. In arc lamps this pushing together,
-or “feeding” as it is called, is done automatically, as
-is also the first bringing together and separating of the rods
-to start or strike the arc. <a href="#fig_21">Fig. 21</a> shows a simple arrangement
-for this purpose. A is the positive carbon, and B
-the negative. C is the holder for the positive carbon, and
-this is connected to the rod D, which is made of soft iron.<span class="pagenum" id="Page_96">96</span>
-This rod is wound with two separate coils of wire as shown,
-coil E having a low resistance, and coil F a high one.
-These two coils are solenoids, and D is the core,
-(<a href="#chapter_VII">Chapter VII</a>.). When the lamp is not in use, the weight of
-the holder keeps the positive carbon in contact with the
-negative carbon. When switched on, the current flows
-along the cable to the point H. Here it has two paths
-open to it, one through coil E to the positive carbon, and
-the other through coil F and back to the source of supply.
-But coil E has a much lower
-resistance than coil F, and
-so most of the current
-chooses the easier path
-through E, only a small
-amount of current taking
-the path through the other
-coil. Both coils are now
-magnetized, and E tends to
-draw the rod D upwards,
-while F tends to pull it
-downwards. Coil E, however,
-has much greater power
-than coil F, because a much
-larger amount of current is
-passing through it; and so it overcomes the feeble pull of F,
-and draws up the rod. The raising of D lifts the positive
-carbon away from the negative carbon, and the arc is struck.
-The carbons now begin to waste away, and very slowly the
-distance between them increases. The path of the current
-passing through coil E is from carbon A to carbon B by
-way of the arc, and as the length of the gap between A
-and B increases, the resistance of this path also increases.
-The way through coil E thus becomes less easy, and as
-time goes on more and more current takes the alternative<span class="pagenum" id="Page_97">97</span>
-path through coil F. This results in a decrease in the
-magnetism of E, and an increase in that of F, and at a
-certain point F becomes the more powerful of the two, and
-pulls down the rod. In this way the positive carbon is
-lowered and brought nearer to the negative carbon.
-Directly the diminishing distance between A and B reaches
-a certain limit, coil E once more asserts its superiority, and
-by overcoming the pull of F it stops the further approach
-of the carbons. So, by the opposing forces of the two
-coils, the carbons are maintained between safe limits, in
-spite of their wasting away.</p>
-
-<figure id="plate_IXa" class="figcenter" style="max-width: 29em;">
- <p class="caption">PLATE IX.</p>
- <img src="images/i_121.jpg" width="2246" height="1522" alt=" ">
-</figure>
-
-<figure id="plate_IXb" class="figcenter" style="max-width: 22em;">
- <img src="images/i_121b.jpg" width="1689" height="2160" alt=" ">
- <figcaption class="caption">
-
-<p class="floatl"><i>By permission of</i></p>
-<p class="floatr"><i>Union Electric Co. Ltd.</i></p>
-
-<p class="floatc">NIGHT PHOTOGRAPHS, TAKEN BY THE LIGHT OF THE ARC LAMPS.</p>
-</figcaption></figure>
-
-<p>The arc lamp is largely used for the illumination
-of wide streets, public squares, railway stations, and the
-exteriors of theatres, music-halls, picture houses, and large
-shops. The intense brilliancy of the light produced may be
-judged from the accompanying photographs (<a href="#plate_IXa">Plate IX</a>.),
-which were taken entirely by the light of the arc lamps.
-Still more powerful arc lamps are constructed for use in
-lighthouses. The illuminating power of some of these
-lamps is equal to that of hundreds of thousands of
-candles, and the light, concentrated by large reflectors, is
-visible at distances varying from thirty to one hundred
-miles.</p>
-
-<p>Arc lamps are also largely used for lighting interiors,
-such as large showrooms, factories or workshops. For
-this kind of lighting the dazzling glare of the outdoor
-lamp would be very objectionable and harmful to the eyes,
-so methods of indirect lighting are employed to give a soft
-and pleasant light. Most of the light in the arc lamp comes
-from the positive carbon, and for ordinary outdoor lighting
-this carbon is placed above the negative carbon. In lamps
-for interior lighting the arrangement is frequently reversed,
-so that the positive carbon is below. Most of the light is
-thus directed upwards, and if the ceiling is fairly low and<span class="pagenum" id="Page_98">98</span>
-of a white colour the rays are reflected by it, and a soft and
-evenly diffused lighting is the result. Some light comes
-also from the negative carbon, and those downward rays
-are reflected to the ceiling by a reflector placed beneath the
-lamp. Where the ceiling is very high or of an unsuitable
-colour, a sort of artificial ceiling in the shape of a large
-white reflector is placed above the lamp to produce the
-same effect. Sometimes the lamp is arranged so that part
-of the light is reflected to the ceiling, and part transmitted
-directly through a semi-transparent reflector below the
-lamp. The composition of the light of the arc lamp is very
-similar to that of sunlight, and by the use of such lamps the
-well-known difficulty of judging and matching colours by
-artificial light is greatly reduced. This fact is of great
-value in drapery establishments, and the arc lamp has
-proved a great success for lighting rooms used for night
-painting classes.</p>
-
-<p>The powerful searchlights used by warships are arc
-lamps provided with special arrangements for projecting
-the light in any direction. A reflector behind the arc concentrates
-the light and sends it out as a bundle of parallel
-rays, and the illuminating power is such that a good searchlight
-has a working range of nearly two miles in clear
-weather. According to the size of the projector, the
-illumination varies from about 3000 to 30,000 or 40,000
-candle-power. For some purposes, such as the illuminating
-of narrow stretches of water, a wider beam is required, and
-this is obtained by a diverging lens placed in front of the
-arc. In passing through this lens the light is dispersed or
-spread out to a greater or less extent according to the
-nature of the lens. Searchlights are used in navigating
-the Suez Canal by night, for lighting up the buoys along
-the sides of the canal. The ordinary form of searchlight
-does this quite well, but at the same time it illuminates<span class="pagenum" id="Page_99">99</span>
-equally an approaching vessel, so that the pilot on this
-vessel is dazzled by the blinding glare. To avoid this
-dangerous state of things a split reflector is used, which
-produces two separate beams with a dark space between
-them. In this way the sides of the canal are illuminated,
-but the light is not thrown upon oncoming vessels, so that
-the pilots can see clearly.</p>
-
-<p>Glass reflectors are much more efficient than metallic
-ones, but they have the disadvantage of being easily put
-out of action by gunfire. This defect is remedied by protecting
-the glass reflector by a screen of wire netting.
-This is secured at the back of the reflector, and even if the
-glass is shattered to a considerable extent, as by a rifle
-bullet, the netting holds it together, and keeps it quite
-serviceable. Reflectors protected in this way are not put
-out of action by even two or three shots fired through
-them. Searchlight arcs and reflectors are enclosed in metal
-cylinders, which can be moved in any direction, vertically
-or horizontally.</p>
-
-<p>In the arc lamps already described, a large proportion
-of the light comes from the incandescent carbon electrodes.
-About the year 1901 an American electrician, Mr. P. C.
-Hewitt, brought out an arc lamp in which the electrodes
-took no part in producing the light, the whole of which
-came from a glowing stream of mercury vapour. This
-lamp, under the name of the Cooper-Hewitt mercury
-vapour lamp, has certain advantages over other electric
-illuminants, and it has come into extensive use.</p>
-
-<figure id="fig_22" class="figcenter" style="max-width: 23em;">
- <img src="images/i_126.png" width="1775" height="1198" alt=" ">
- <figcaption class="caption"><span class="smcap">Fig. 22.</span>—Sketch of Mercury Vapour Lamp.
-</figcaption></figure>
-
-<p>It consists of a long glass tube, exhausted of air, and
-containing a small quantity of mercury. Platinum wires to
-take the current from the source of supply are sealed in at
-each end. The tube is attached to a light tubular framework
-of metal suspended from the ceiling, and this frame
-is arranged so that it can be tilted slightly downwards by<span class="pagenum" id="Page_100">100</span>
-pulling a chain. As shown in <a href="#fig_22">Fig. 22</a>, the normal position
-of the lamp is not quite horizontal, but tilted slightly downwards
-towards the end of the tube having the bulb containing
-the mercury. The platinum wire at this end dips
-into the mercury, so making a metallic contact with it.
-The lamp is lighted by switching on the current and pulling
-down the chain. The altered angle makes the mercury
-flow along the tube towards the other platinum electrode,
-and as soon as it touches this a conducting path for the
-current is formed from end to end of the tube. The lamp
-is now allowed to fall back to its original angle, so that the
-mercury returns to its bulb. There is now no metallic connexion
-between the electrodes, but the current continues
-to pass through the tube as a vacuum discharge. Some
-of the mercury is immediately vaporized and rendered
-brilliantly incandescent, and so the light is produced. The
-trouble of pulling down the chain is avoided in the
-automatic mercury vapour lamp, which is tilted by an
-electro-magnet. This magnet is automatically cut out of<span class="pagenum" id="Page_101">101</span>
-circuit as soon as the tilting is completed and the arc
-struck.</p>
-
-<p>The average length of the tube in the ordinary form of
-mercury vapour lamp is about 30 inches, and a light of
-from 500 to 3000 candle-power is produced, according to
-the current used. Another form, known as the “Silica”
-lamp, is enclosed in a globe like that of an ordinary electric
-arc lamp. The tube is only about 5 or 6 inches in
-length, and it is made of quartz instead of glass, the
-arrangements for automatically tilting the tube being
-similar to those in the ordinary form of lamp.</p>
-
-<p>The light of the mercury vapour lamp is different from
-that of all other lamps. Its peculiarity is that it contains
-practically no red rays, most of the light being yellow, with
-a certain proportion of green and blue. The result is a
-light of a peacock-blue colour. The absence of red rays
-alters colour-values greatly, scarlet objects appearing
-black; and on this account it is impossible to match colours
-by this light. In many respects, however, the deficiency
-in red rays is a great positive advantage. Every one who
-has worked by mercury vapour light must have noticed
-that it enables very fine details to be seen with remarkable
-distinctness. This property is due to an interesting fact.
-Daylight and ordinary artificial light is a compound or
-mixture of rays of different colours. It is a well-known
-optical fact that a simple lens is unable to bring all these
-rays to the same focus; so that if we sharply focus an image
-by red light, it is out of focus or blurred by blue light. This
-defect of the lens is called “chromatic aberration.” The
-eye too suffers from chromatic aberration, so that it cannot
-focus sharply all the different rays at the same time. The
-violet rays are brought to a focus considerably in front of
-the red rays, and the green and the yellow rays come in
-between the two. The eye therefore automatically and<span class="pagenum" id="Page_102">102</span>
-unconsciously effects a compromise, and focuses for the
-greenish-yellow rays. The mercury vapour light consists
-very largely of these rays, and consequently it enables the
-image to be focused with greater sharpness; or, in other
-words, it increases the acuteness of vision. Experiments
-carried out by Dr. Louis Bell and Dr. C. H. Williams
-demonstrated this increase in visual sharpness very conclusively.
-Type, all of exactly the same size, was examined
-by mercury vapour light, and by the light from an electric
-incandescent lamp with tungsten filament. The feeling of
-sharper definition produced by the mercury vapour light
-was so strong that many observers were certain that the
-type was larger, and they were convinced that it was
-exactly the same only after careful personal examination.</p>
-
-<p>Mercury vapour light apparently imposes less strain
-upon the eyes than ordinary artificial light, and this
-desirable feature is the result of the absence of the red rays,
-which, besides having little effect in producing vision, are
-tiring to the eyes on account of their heating action. The
-light is very highly actinic, and for this reason it is largely
-used for studio and other interior photographic work. In
-cases where true daylight colour effects are necessary, a
-special fluorescent reflector is used with the lamp. By
-transforming the frequency of the light waves, this reflector
-supplies the missing red and orange rays, the result being
-a light giving normal colour effects.</p>
-
-<p>Another interesting vapour lamp may be mentioned
-briefly. This has a highly exhausted glass tube containing
-neon, a rare gas discovered by Sir William Ramsay. The
-light of this lamp contains no blue rays, and it is of a
-striking red colour. Neon lamps are used chiefly for
-advertising purposes, and they are most effective for
-illuminated designs and announcements, the peculiar and
-distinctive colour of the light attracting the eye at once.</p>
-
-<p><span class="pagenum" id="Page_103">103</span></p>
-
-<p>An electric current meets with some resistance in
-passing through any substance, and if the substance is a
-bad conductor the resistance is very great. As the current
-forces its way through the resistance, heat is produced, and
-a very thin wire, which offers a high resistance, may be
-raised to a white heat by an electric current, and it then
-glows with a brilliant light. This fact forms the basis of
-the electric incandescent or glow lamp.</p>
-
-<p>In the year 1878, Thomas A. Edison set himself the
-task of producing a perfect electric incandescent lamp,
-which should be capable of superseding gas for household
-and other interior lighting. The first and the greatest
-difficulty was that of finding a substance which could be
-formed into a fine filament, and which could be kept
-in a state of incandescence without melting or burning
-away. Platinum was first chosen, on account of its very
-high melting-point, and the fact that it was not acted
-upon by the gases of the air. Edison’s earliest lamps
-consisted of a piece of very thin platinum wire in the
-shape of a spiral, and enclosed in a glass bulb from which
-the air was exhausted. The ends of the spiral were
-connected to outside wires sealed into the bulb. It was
-found, however, that keeping platinum continuously at a
-high temperature caused it to disintegrate slowly, so that
-the lamps had only a short life. Fine threads or filaments
-of carbon were then tried, and found to be much more
-durable, besides being a great deal cheaper. The carbon
-filament lamp quickly became a commercial success, and
-up to quite recent years it was the only form of electric
-incandescent lamp in general use.</p>
-
-<p>In 1903 a German scientist, Dr. Auer von Welsbach,
-of incandescent gas mantle fame, produced an electric lamp
-in which the filament was made of the metal osmium, and
-this was followed by a lamp using the metal tantalum for<span class="pagenum" id="Page_104">104</span>
-the filament, the invention of Siemens and Halske. For a
-while the tantalum lamp was very successful, but more
-recently it has been superseded in popularity by lamps
-having a filament of the metal tungsten. The success of
-these lamps has caused the carbon lamp to decline in
-favour. The metal filaments become incandescent much
-more easily than the carbon filament, and for the same
-candle-power the metal filament lamp consumes much less
-current than the carbon lamp.</p>
-
-<p>The construction of tungsten lamps is very interesting.
-Tungsten is a very brittle metal, and at first the lamps
-were fitted with a number of separate filaments. These
-were made by mixing tungsten powder with a sort of paste,
-and then squirting the mixture through very small
-apertures, so that it formed hair-like threads. Early in
-1911 lamps having a filament consisting of a single continuous
-piece of drawn tungsten wire were produced. It
-had been known for some time that although tungsten was
-so brittle at ordinary temperatures, it became quite soft
-and flexible when heated to incandescence in the lamp, and
-that it lost this quality again as soon as it cooled down.
-A process was discovered by which the metal could be
-made permanently ductile, by mechanical treatment while
-in the heated state. In this process pure tungsten powder
-is pressed into rods and then made coherent by heating.
-While still hot it is hammered, and finally drawn out into
-fine wires through diamond dies. The wire is no thicker
-than a fine hair, and it varies in size from about 0·012 mm.
-to about 0·375 mm., according to the amount of current it
-is intended to take. It is mounted by winding it continuously
-zigzag shape round a glass carrier, which has at
-the top and the bottom a number of metal supports
-arranged in the form of a star, and insulated by a central
-rod of glass. One star is made of strong, stiff material,<span class="pagenum" id="Page_105">105</span>
-and the other consists of fine wires of some refractory
-metal, molybdenum being used in the Osram lamps.
-These supports act as springs, and keep the wire securely
-in its original shape, no matter in what position the lamp
-is used. The whole is placed in a glass bulb, which is
-exhausted of air and sealed up.</p>
-
-<p>For some purposes lamps with specially small bulbs are
-required, and in these the tungsten wire is made in the
-shape of fine spirals, instead of in straight pieces, so that
-it takes up much less room. In the “Axial” lamp the
-spiral is mounted in such a position that most of the light
-is sent out in one particular direction.</p>
-
-<p>The latest development in electric incandescent lamps is
-the “half-watt” lamp. The watt is the standard of electrical
-energy, and it is the rate of work represented by a current
-of one ampere at a pressure of 1 volt. With continuous
-currents the watts are found very simply by multiplying
-together the volts and the amperes. For instance, a
-dynamo giving a current of 20 amperes at a pressure
-of 50 volts would be called a 1000-watt dynamo.
-With alternating currents the calculation is more complicated,
-but the final result is the same. The ordinary form
-of tungsten lamp gives about one candle-power for every
-watt, and is known as a one-watt lamp. As its name
-suggests, the half-watt lamp requires only half this amount
-of energy to give the same candle-power, so that it is very
-much more economical in current. In this lamp the
-tungsten filament is wound in a spiral, but instead of being
-placed in the usual exhausted bulb, it is sealed into a bulb
-containing nitrogen gas. The increased efficiency is
-obtained by running the filament at a temperature from
-400° to 600° C. higher than that at which the filament in
-the ordinary lamp is used.</p>
-
-<p>In spite of the great advances in artificial lighting<span class="pagenum" id="Page_106">106</span>
-made during recent years, no one has yet succeeded in
-producing light without heat. This heat is not wanted,
-and it represents so much waste energy. It has often been
-said that the glow-worm is the most expert of all illuminating
-engineers, for it has the power of producing at will a
-light which is absolutely without heat. Perhaps the nearest
-approach to light without heat is the so-called “cold light”
-invented by M. Dussaud, a French scientist. His device
-consists of a revolving ring of exactly similar tungsten
-lamps. Each of these lamps has current passed through it
-in turn, and the duration of the current in each is so short,
-being only a fraction of a second, that the lamp has not
-sufficient time to develop any appreciable amount of heat.
-The light from the ring of lamps is brought to a focus, and
-passed through a lens to wherever it is required. Electric
-incandescent lamps are made in a variety of sizes, each one
-being intended for a certain definite voltage. If a lamp
-designed for, say, 8 volts, is used on a circuit of
-32 volts, its candle-power is greatly increased, while the
-amount of current consumed is not increased in proportion.
-In this way the lamp becomes a more efficient source
-of light, but the “over-running,” as it is called, has a
-destructive effect on the filament, so that the life of the
-lamp is greatly shortened. In the Dussaud system however
-the time during which each lamp has current passing
-through it is so short, followed by a period of rest, that
-the destructive effect of over-running is reduced to the
-minimum; so that by using very high voltages an extremely
-brilliant light is safely obtained with a comparatively
-small consumption of current. It might be thought
-that the constant interchange of lamps would result in an
-unsteady effect, but the substitution of one lamp for another
-is carried out so rapidly that the eye gets the impression of
-perfect steadiness. The Dussaud system is of little use<span class="pagenum" id="Page_107">107</span>
-for ordinary lighting purposes, but for lighthouse illumination,
-photographic studio work, and the projection of
-lantern slides and cinematograph films, it appears to be of
-considerable value.</p>
-
-<p>Electric light has many advantages over all other
-illuminants. It gives off very little heat, and does not use
-up the oxygen in the air of a room as gas does; while by
-means of flexible wires the lamps can be put practically
-anywhere, so that the light may be had just where it is
-wanted. Another great advantage is that the light may be
-switched on without any trouble about matches, and there
-is none of the danger from fire which always exists with
-a flame.</p>
-
-<p>The current for electric lamps is generally taken from
-the public mains, but in isolated country houses a dynamo
-has to be installed on the premises. This is usually driven
-by a small engine running on petrol or paraffin. In order
-to avoid having to run the engine and dynamo continually,
-the current is not taken directly from the dynamo, but from
-a battery of accumulators. During the day the dynamo is
-used to charge the accumulators, and these supply the
-current at night without requiring any attention.</p>
-
-<p>Electric lighting from primary cells is out of the
-question if a good light is wanted continuously for long
-periods, for the process is far too costly and troublesome.
-If a light of small candle-power is required for periods of
-from a few minutes to about an hour, with fairly long
-intervals of rest, primary cells may be made a success.
-Large dry cells are useful for this purpose, but probably
-the most satisfactory cell is the sack Leclanché. This is
-similar in working to the ordinary Leclanché cell used for
-bells, but the carbon mixture is placed in a canvas bag or
-sack, instead of in a porous pot, and the zinc rod is replaced
-by a sheet of zinc surrounding the sack. These cells give<span class="pagenum" id="Page_108">108</span>
-about 1½ volt each, so that four, connected in series,
-are required to light a 6-volt lamp. The lamps must
-take only a very small current, or the cells will fail
-quickly. Small metal filament lamps taking from a third
-to half an ampere are made specially for this purpose, and
-these always should be used. A battery of sack Leclanché
-cells with a miniature lamp of this kind forms a convenient
-outfit for use as a night-light, or for lighting a dark cupboard,
-passage or staircase. Lamps with ruby glass, or
-with a ruby cap to slip over the bulb, may be obtained for
-photographic purposes. If the outfit is wanted for use as
-a reading-lamp it is better to have two separate batteries,
-and to use them alternately for short periods. With this
-arrangement each battery has a short spell of work followed
-by a rest, and the light may be kept on for longer periods
-without overworking the cells.</p>
-<hr class="chap x-ebookmaker-drop">
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_109">109</span></p>
-
-<h2 class="nobreak" id="toclink_109"><a id="chapter_XIII"></a>CHAPTER XIII<br>
-
-<span class="subhead">ELECTRIC HEATING</span></h2>
-</div>
-
-<p class="in0"><span class="firstword">The</span> light of the electric incandescent lamp is produced by
-the heating to incandescence of a thin filament of metal or
-carbon, and the heat itself is produced by the electric
-current forcing its way through the great resistance opposed
-to it by the filament. In such lamps the amount of heat
-produced is too small to be of much practical use, but by
-applying the same principle on a larger scale we get an
-effective electric heater.</p>
-
-<p>The most familiar and the most attractive of all electric
-heaters is the luminous radiator. This consists of two or
-more large incandescent lamps, having filaments of carbon.
-The lamps are made in the form of long cylinders, the
-glass being frosted, and they are set, generally in a vertical
-position, in an ornamental case or frame of metal. This
-case is open at the front, and has a metal reflector behind.
-The carbon filaments are raised to an orange-red heat by
-the passage of the current, and they then radiate heat rays
-which warm the bulbs and any other objects in their path.
-The air in contact with these heated bodies is warmed,
-and gradually fills the room. This form of heater, with
-its bright glowing lamps, gives a room a very cheerful
-appearance.</p>
-
-<p>In the non-luminous heaters, or “convectors” as they are
-called, the heating elements consist of strips of metal or
-wires having a very high resistance. These are placed in<span class="pagenum" id="Page_110">110</span>
-a frame and made red-hot by the current. Cold air enters
-at the bottom of the frame, becomes warm by passing over
-the heating elements, and rises out at top and into the
-room. More cold air enters the frame and is heated in
-the same way, and in a very short time the whole of the
-air of the room becomes warmed. The full power of the
-heater is used in the preliminary warming of the room, but
-afterwards the temperature may be kept up with a much
-smaller consumption of current, and special regulating
-switches are provided to give different degrees of heat.
-Although these heaters are more powerful than the
-luminous radiators, they are not cheerful looking; but in
-some forms the appearance is improved by an incandescent
-lamp with a ruby glass bulb, which shines through the
-perforated front of the frame.</p>
-
-<p>The Bastian, or red glow heater, has thin wires wound
-in a spiral and enclosed in tubes made of quartz. These
-tubes are transparent both to light and heat, and so the
-pleasant glow of the red-hot wire is visible. A different
-type of heater, the hot oil radiator, is very suitable for
-large rooms. This has a wire of high resistance immersed
-in oil, which becomes hot and maintains a steady
-temperature.</p>
-
-<p>Electric cooking appliances, like the heaters just described,
-depend upon the heating of resistance wires or
-strips of metal. The familiar electric kettle has a double
-bottom, and in the cavity thus formed is placed the resistance
-material, protected by strips of mica, a mineral
-substance very largely used in electrical appliances of all
-kinds on account of its splendid insulating qualities.
-Electric irons are constructed in much the same way as
-kettles, and sometimes they are used with stands which
-cut off the current automatically when the iron is laid down
-upon them, so that waste and overheating are prevented.<span class="pagenum" id="Page_111">111</span>
-There are also a great many varieties of electric ovens,
-grillers, hot-plates, water-heaters, glue-pots, and foot and
-bed warmers. These of course differ greatly in construction,
-but as they all work on the same principle there
-is no need to describe them.</p>
-
-<p>Electric hot-plates are used in an interesting way in
-Glasgow, to enable the police on night duty to have a hot
-supper. The plates are fitted to street telephone signal
-boxes situated at points where a number of beats join. By
-switching on current from the
-public mains the policemen
-are able to warm their food
-and tea, and a supper interval
-of twenty minutes is allowed.
-Even policemen are sometimes
-absent-minded, and to
-avoid the waste of current and
-overheating of the plate that
-would result if a “bobby”
-forgot to switch off, an arrangement
-is provided which
-automatically switches off the
-current when the plate is not
-in use.</p>
-
-<figure id="fig_23" class="figright" style="max-width: 13em;">
- <img src="images/i_137.jpg" width="1025" height="1188" alt=" ">
- <figcaption class="caption"><span class="smcap">Fig. 23.</span>—Diagram to illustrate principle
-of Electric Furnace.
-</figcaption></figure>
-
-<p>We must turn now to
-electric heating on a much larger scale, in the electric
-furnaces used for industrial purposes. The dazzling
-brilliance of the light from the electric arc lamp is due
-to the intense heat of the stream of vaporized carbon
-particles between the carbon rods, the temperature of this
-stream being roughly about 5400° F. This great heat
-is made use of in various industries in the electric arc
-furnace. <a href="#fig_23">Fig. 23</a> is a diagram of a simple furnace of this
-kind. A is a vertical carbon rod which can be raised or<span class="pagenum" id="Page_112">112</span>
-lowered, and B is a bed of carbon, forming the bottom of
-the furnace, and acting as a second rod. A is lowered
-until it touches B, the current, either continuous or alternating,
-is switched on, and A is then raised. The arc is
-thus struck between A and B, and the material contained
-in the furnace is subjected to intense heat. When the
-proper stage is reached the contents of the furnace are
-drawn off at C, and fresh material is fed in from above,
-so that if desired the process may be kept going continuously.
-Besides the electric arc furnace there are also
-resistance furnaces, in which the heat is produced by
-the resistance of a conductor to a current passing
-through it. This conductor may be the actual substance
-to be heated, or some other resisting material placed close
-to it.</p>
-
-<p>It will be of interest to mention now one or two of
-the uses of electric furnaces. The well-known substance
-calcium carbide, so much used for producing acetylene gas
-for lighting purposes, is a compound of calcium and
-carbon; it is made by raising a mixture of lime and coke
-to an intense heat in an electric furnace. The manufacture
-of calcium carbide is carried on on a very large scale at
-Niagara, with electric power obtained from the Falls, and
-at Odda in Norway, where the power is supplied by the
-river Tysse. Carborundum, a substance almost as hard
-as the diamond, is largely used for grinding and polishing
-purposes. It is manufactured by sending a strong current
-through a furnace containing a core of coke surrounded by
-a mixture of sand, sawdust, and carbon. The core becomes
-incandescent, and the heating is continued until the
-carbon combines with the sand, the process taking about
-a day. Graphite, a kind of carbon, occurs naturally in the
-form of plumbago, which is used for making black lead
-pencils. It is obtained by mining, but many of the mines<span class="pagenum" id="Page_113">113</span>
-are already worked out, and others will be exhausted
-before long. By means of the electric furnace, graphite
-can now be made artificially, by heating anthracite
-coal, and at Niagara a quantity running into thousands
-of tons is produced every year. Electric furnaces are
-now largely employed, particularly in France, in the
-production of the various alloys of iron which are used
-in making special kinds of steel; and they are used also
-to a considerable extent in the manufacture of quartz
-glass.</p>
-
-<p>For many years past a great deal of time and money
-has been spent in the attempt to make artificial diamonds.
-Quite apart from its use in articles of jewellery, the
-diamond has many very important industrial applications,
-its value lying in its extreme hardness, which is not
-equalled by any other substance. The very high price
-of diamonds however is at present a serious obstacle to
-their general use. If they could be made artificially on a
-commercial scale they would become much cheaper, and
-this would be of the greatest importance to many industries,
-in which various more or less unsatisfactory substitutes are
-now used on account of their much smaller cost. Recent
-experiments seem to show that electricity will solve the
-problem of diamond making. Small diamonds, one-tenth
-of an inch long, have been made in Paris by means of the
-electric arc furnace. The furnace contains calcium carbide,
-surrounded by a mixture of carbon and lime, and the arc,
-maintained by a very powerful current, is kept in operation
-for several hours. A black substance, something like coke,
-is formed round the negative carbon, and in this are found
-tiny diamonds. The diamonds continue to increase slowly
-in size during the time that the arc is at work, and it is estimated
-that they grow at the rate of about one-hundredth
-of an inch per hour. So far only small diamonds have<span class="pagenum" id="Page_114">114</span>
-been made, but there seems to be no reason why large ones
-should not be produced, by continuing the process for three
-or four days.</p>
-
-<p>A chapter on electric heating would not be complete
-without some mention of electric welding. Welding is the
-process of uniting two pieces of metal by means of a combination
-of heat and pressure, so that a strong and permanent
-joint is produced. The chief difficulty in welding is
-that of securing and keeping up the proper temperature,
-and some metals are much more troublesome than others
-in this respect. Platinum, iron, and steel are fairly easy to
-weld, but most of the other metals, and alloys of different
-metals, require very exact regulation of temperature. It
-is almost impossible to obtain this exact regulation by
-ordinary methods of heating, but the electric current makes
-it a comparatively easy matter. The principle of ordinary
-electric welding is very simple. The ends of the two
-pieces of metal are placed together, and a powerful current
-is passed through them. This current meets with a high
-resistance at the point of contact of the two pieces, and so
-heat is produced. When the proper welding temperature
-is reached, and the metal is in a sort of pasty condition,
-the two pieces are pressed strongly together, and the
-current is switched off. The pieces are now firmly united
-together. The process may be carried out by hand, the
-welding smith switching the current on and off, and applying
-pressure at the right moment by means of hydraulic
-power. There are also automatic welders, which perform
-the same operations without requiring any manual control.
-Alternating current is used, of low voltage but very high
-amperage.</p>
-
-<p>Steel castings are sometimes found to have small
-defects, such as cracks or blow-holes. These are not
-discarded as useless, but are made quite sound by welding<span class="pagenum" id="Page_115">115</span>
-additional metal into the defective places by means of the
-electric arc. The arc is formed between the casting and a
-carbon rod, and the tremendous heat reduces the surface of
-the metal to a molten condition. Small pieces or rods of
-metal are then welded in where required.</p>
-<hr class="chap x-ebookmaker-drop">
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_116">116</span></p>
-
-<h2 class="nobreak" id="toclink_116"><a id="chapter_XIV"></a>CHAPTER XIV<br>
-
-<span class="subhead">ELECTRIC BELLS AND ALARMS</span></h2>
-</div>
-
-<p class="in0"><span class="firstword">The</span> most familiar of all electrically worked appliances is
-probably the electric bell, which in some form or other is
-in use practically all over the world. Electric bells are
-operated by means of a current of electricity sent through
-the coils of an electro-magnet, and one of the very simplest
-forms is that known as the single-stroke bell. In this bell
-an armature or piece of soft iron is placed across, but at a
-little distance from, the poles of an electro-magnet, and to
-this piece of iron is fixed a lever terminating in a sort of
-knob which lies close to a bell or gong. When a current
-is sent round the electro-magnet the armature is attracted,
-so that the lever moves forward and strikes a sharp blow
-upon the gong. Before the gong can be sounded a second
-time the current must be interrupted in order to make the
-magnet release the armature, so that the lever may fall
-back to its original position. Thus the bell gives only one
-ring each time the circuit is closed. Bells of this kind may
-be used for signalling in exactly the same way as the Morse
-sounder, and sometimes they are made with two gongs of
-different tones, which are arranged so as to be sounded
-alternately.</p>
-
-<figure id="fig_24" class="figright" style="max-width: 11em;">
- <img src="images/i_143.png" width="853" height="1534" alt=" ">
- <figcaption class="caption"><span class="smcap">Fig. 24.</span>—Mechanism of
-Electric Bell.
-</figcaption></figure>
-
-<figure id="fig_25" class="figleft" style="max-width: 14em;">
- <img src="images/i_143b.png" width="1113" height="415" alt=" ">
- <figcaption class="caption"><span class="smcap">Fig. 25.</span>—Diagram showing principle
-of Bell-push.
-</figcaption></figure>
-
-<p>For most purposes however another form called the
-trembler bell is much more convenient. <a href="#fig_24">Fig. 24</a> is a rough
-diagram of the usual arrangement of the essential parts of
-a trembler bell. When the circuit is closed by pressing the<span class="pagenum" id="Page_117">117</span>
-bell-push, a current flows from the battery to the electro-magnet
-EE, by way of terminal T. The electro-magnet
-then attracts the soft iron armature
-A, thus causing the hammer H to
-strike the gong. But immediately
-the armature is pulled away from
-the terminal T¹ the circuit is
-broken and the magnet loses its
-attraction for the armature, which
-is moved back again into contact
-with T¹ by the spring S. The
-circuit is thus again closed, the
-armature is again attracted, and
-the hammer strikes the gong a
-second time. This process goes
-on over and over again at a great
-speed as long as the bell-push is
-kept pressed down, resulting in
-an extremely rapid succession of
-strokes upon the gong. It will
-be noticed that the working of
-this bell is very similar to that of the automatic contact-breaker
-used for induction coils (<a href="#chapter_VIII">Chapter VIII</a>.). For
-household purposes this
-form of bell has completely
-driven out the once popular
-wire-pulled bell. Bell-pushes
-are made in a
-number of shapes and
-forms, and <a href="#fig_25">Fig. 25</a> will
-make clear the working
-principle of the familiar form which greets us from almost
-every doorway with the invitation, “Press.” In private
-offices and elsewhere the rather aggressive sound of an<span class="pagenum" id="Page_118">118</span>
-ordinary trembler bell is apt to become a nuisance, and
-in such cases a modified form which gives a quiet buzzing
-sound is often employed.</p>
-
-<p>It is frequently necessary to have an electric bell which,
-when once started, will continue ringing until it is stopped.
-Such bells are used for fire and burglar alarms and for
-many other similar purposes, and they are called continuous-ringing
-bells as distinguished from the ordinary
-trembler bells. In one common form of continuous-ringing
-bell two separate batteries are used, one to start the bell
-and the other to keep it ringing. When a momentary
-current from the first battery is sent over the bell lines the
-armature is attracted by the electro-magnet, and its movement
-allows a lever to fall upon a metal contact piece.
-This closes the circuit of the second battery, which keeps
-the bell ringing until the lever is replaced by pulling a cord
-or pressing a knob. Continuous-ringing bells are often
-fitted to alarm clocks. The alarm is set in the usual way,
-and at the appointed hour the bell begins to ring, and goes
-on ringing until its owner, able to stand the noise no longer,
-gets out of bed to stop it.</p>
-
-<p>There is another form of electric bell which has been
-devised to do away with the annoyance of bells suddenly
-ceasing to work on account of the failure of the battery.
-In this form the battery is entirely dispensed with, and
-the current for ringing the bell is taken from a very small
-dynamo fitted with a permanent steel horse-shoe magnet.
-The armature is connected to a little handle, and current
-is generated by twisting the handle rapidly to and fro
-between the thumb and finger. A special form of bell is
-required for this arrangement, which is not in general use.</p>
-
-<p>In the days of wire-pulled bells it was necessary to have
-quite a battery of bells of different tones for different rooms,
-but a single electric bell can be rung from bell-pushes<span class="pagenum" id="Page_119">119</span>
-placed in any part of a house or hotel. An indicator is
-used to show which push has been pressed, and, this like
-the bell itself, depends upon the attraction of an armature
-by an electro-magnet. Before reaching the bell the wire
-from each bell-push passes round a separate small electro-magnet,
-which is thus magnetized by the current at the
-same time that the bell is rung. In the simplest form of
-indicator the attraction of the magnet causes a little flag to
-swing backwards and forwards over its number. Another
-form is the drop indicator, in which the movement of the
-armature when attracted by the magnet allows a little flag
-to drop, thus exposing the number of the room from which
-the bell was rung. The dropped flag has to be replaced,
-either by means of a knob fixed to a rod which pushes the
-flag up again, or by pressing a push which sends the
-current through another little electro-magnet so arranged
-as to re-set the flag.</p>
-
-<p>The electric current is used to operate an almost endless
-variety of automatic alarms for special purposes. Houses
-may be thoroughly protected from undesired nocturnal
-visitors by means of a carefully arranged system of burglar
-alarms. Doors and windows are fitted with spring contacts
-so that the slightest opening of them closes a battery circuit
-and causes an alarm to sound, and even if the burglar
-succeeds in getting inside without moving a door or
-window, say by cutting out a pane of glass, his troubles are
-not by any means at an end. Other contacts are concealed
-under the doormats, and under the carpets in passages and
-stairways, so that the burglar is practically certain to tread
-on one or other of them and so rouse the house. A window
-may be further guarded by a blind contact. The blind is
-left down, and is secured at the bottom to a hook, and the
-slightest pressure upon it, such as would be given by a
-burglar trying to get through the window, sets off the alarm.<span class="pagenum" id="Page_120">120</span>
-Safes also may be protected in similar ways, and a camera
-and flashlight apparatus may be provided, so that when the
-burglar closes the circuit by tampering with the safe he
-takes his own photograph.</p>
-
-<p>The modern professional burglar is a bit of a scientist
-in his way, and he is wily enough to find and cut the wires
-leading to the contacts, so that he can open a door or
-window at his leisure without setting off the alarm. In
-order to circumvent this little game, burglar alarms are
-often arranged on the closed-circuit principle, so that the
-alarm is sounded by the breaking of the circuit. A burglar
-who deftly cut the wires of an alarm worked on this
-principle would not be particularly pleased with the results
-of his handiwork. The bells of burglar alarms may be
-arranged to ring in a bedroom or in the street, and in the
-United States, where burglar and in fact all electric
-alarms are in more general use than in England, large
-houses are sometimes connected to a police station, so that
-the alarm is given there by bell or otherwise.</p>
-
-<figure id="plate_X" class="figcenter" style="max-width: 37em;">
- <p class="caption">PLATE X.</p>
- <img src="images/i_147.jpg" width="2885" height="2046" alt=" ">
- <figcaption class="caption">
-
-<p class="floatl"><i>By permission of</i></p>
-<p class="floatr"><i>Vickers Limited.</i></p>
-
-<p class="floatc">WHERE ELECTRICAL MACHINERY IS MADE.</p>
-</figcaption></figure>
-
-<p>When an outbreak of fire is discovered it is of the
-utmost importance that the nearest fire-station should be
-notified instantly, for fire spreads with such rapidity that a
-delay of even a few minutes in getting the fire-engines to
-the spot may result in the total destruction of a building
-which otherwise might have been saved. In almost all
-large towns some system of public fire alarms is now in
-use. The signal boxes are placed in conspicuous positions
-in the streets, and sometimes also in very large buildings.
-The alarm is generally given by the starting of a clockwork
-mechanism which automatically makes and breaks a circuit
-a certain number of times. When this occurs an alarm
-bell rings at the fire-station, and the number of strokes on
-the bell, which depends upon the number of times the
-alarm mechanism makes and breaks the circuit, tells the
-attendant from which box the alarm has been given. One
-well-known form of call box has a glass front, and the
-breaking of the glass automatically closes the circuit. In
-other forms turning a handle or pulling a knob serves the
-same purpose.</p>
-
-<p>It is often required to maintain a room at one particular
-temperature, and electricity may be employed to give an
-alarm whenever the temperature rises above or falls below
-a certain point. One arrangement for this purpose consists
-of an ordinary thermometer having the top of the mercury
-tube fitted with an air-tight stopper, through which a wire
-is passed down into the tube as far as the mark indicating
-the temperature at which the alarm is desired to sound.
-Another wire is connected with the mercury in the bulb,
-and the free ends of both wires are taken to a suitable
-battery, a continuous-ringing bell being inserted in the
-circuit at some convenient point. If a rise in temperature
-takes place the mercury expands and moves up the tube,
-and at the critical temperature it touches the wire, thus
-completing the circuit and sounding the alarm. This
-arrangement only announces a rise in temperature, but by
-making the thermometer tube in the shape of a letter <span class="sans bold">U</span> an
-alarm may be given also when the temperature falls below
-a certain degree. A device known as a “thermostat” is also
-used for the same purpose. This consists of two thin strips
-of unlike metals, such as brass and steel, riveted together
-and suspended between two contact pieces. The two
-metals expand and contract at different rates, so that an
-increase in temperature makes the compound strip bend in
-one direction, and a decrease in temperature makes it bend
-in the opposite direction. When the temperature rises or
-falls beyond a certain limit the strip bends so far as to touch
-one or other of the contact pieces, and the alarm is then
-given. Either of the preceding arrangements can be used<span class="pagenum" id="Page_122">122</span>
-also as an automatic fire alarm, or if desired matters may be
-arranged so that the closing of the circuit, instead of ringing
-a bell, turns on or off a lamp, or adjusts a stove, and
-in this way automatically keeps the room at a constant
-temperature.</p>
-
-<p>Electric alarms operated by ball floats are used to some
-extent for announcing the rise or fall beyond a pre-arranged
-limit of water or other liquids, and there is a very ingenious
-electrical device by which the level of the water in a tank
-or reservoir can be ascertained at any time by indicators
-placed in convenient positions any distance away.</p>
-
-<p>In factories and other large buildings a watchman is
-frequently employed to make a certain number of rounds
-every night. Being human, a night-watchman would much
-rather sit and snooze over his fire than tramp round a dark
-and silent factory on a cold winter night; and in order to
-make sure that he pays regular visits to every point
-electricity is called in to keep an eye on him. A good
-eight-day clock is fitted with a second dial which is rotated
-by the clockwork mechanism, and a sheet of paper, which
-can be renewed when required, is placed over this dial.
-On the paper are marked divisions representing hours and
-minutes, and other divisions representing the various places
-the watchman is required to visit. A press-button is fixed
-at each point to be visited, and connected by wires with
-the clock and with a battery. As the watchman reaches
-each point on his rounds he presses the button, which is
-usually locked up so that no one else can interfere with it,
-and the current passes round an electro-magnet inside the
-clock case. The magnet then attracts an armature which
-operates a sort of fine-pointed hammer, and a perforation
-is made in the paper, thus recording the exact time at
-which the watchman visited that particular place.</p>
-
-<p>The current for ordinary electric bells is generally supplied<span class="pagenum" id="Page_123">123</span>
-by Leclanché cells, which require little attention, and
-keep in good working order for a very long time. As we
-saw in <a href="#chapter_IV">Chapter IV</a>., these bells soon polarize if used continuously,
-but as in bell work they are required to give
-current for short periods only, with fairly long intervals of
-rest, no trouble is caused on this account. These cells
-cannot be used for burglar or other alarms worked on the
-closed-circuit principle, and in such cases some form of
-Daniell cell is usually employed.</p>
-<hr class="chap x-ebookmaker-drop">
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_124">124</span></p>
-
-<h2 class="nobreak" id="toclink_124"><a id="chapter_XV"></a>CHAPTER XV<br>
-
-<span class="subhead">ELECTRIC CLOCKS</span></h2>
-</div>
-
-<p class="in0"><span class="firstword">Amongst</span> the many little worries of domestic life is the
-keeping in order of the various clocks. It ought to be a
-very simple matter to remember to wind up a clock, but
-curiously enough almost everybody forgets to do so now
-and then. We gaze meditatively at the solemn-looking
-machine ticking away on the mantelpiece, wondering
-whether we wound it up last week or not; and we wish
-the wretched thing would go without winding, instead of
-causing us all this mental effort.</p>
-
-<p>There is usually a way of getting rid of little troubles
-of this kind, and in this case the remedy is to be found in
-an electrically-driven clock. The peculiar feature about
-clocks driven by electricity is that they reverse the order
-of things in key-wound clocks, the pendulum being made
-to drive the clockwork instead of the clockwork driving
-the pendulum. No driving spring is required, and the
-motive power is supplied by a small electro-magnet.</p>
-
-<p>The actual mechanism varies considerably in different
-makes of clock. In one of the simplest arrangements there
-is a pendulum with an armature of soft iron fixed to the
-extremity of its bob. Below the pendulum is an electro-magnet,
-and this is supplied with current from a small
-battery of dry cells. A short piece of metal, called a “pallet,”
-is attached to the rod of the pendulum by means of a
-pivot; and as the pendulum swings it trails this pallet<span class="pagenum" id="Page_125">125</span>
-backwards and forwards along a horizontal spring. In this
-spring are cut two small notches, one on each side of the
-centre of the swing. As long as the pendulum is swinging
-sufficiently vigorously, the pallet slides over these notches;
-but when the swing has diminished to a certain point the
-pallet catches in one or other of the notches. This has
-the effect of pressing down the spring so that it touches a
-contact piece just below, and the battery circuit is then
-completed. The electro-magnet now comes into action
-and attracts the armature, thus giving the pendulum a pull
-which sets it swinging vigorously again. The spring is
-then freed from the pressure of the pallet, and it rises to its
-original position, so that the circuit is broken. This puts
-out of action the electro-magnet, and the latter does no
-further work until the pendulum requires another pull.
-The movement of the pendulum drives the wheelwork,
-which is similar to that of an ordinary clock, and the wheelwork
-moves the hands in the usual way. A clock of this
-kind will run without attention for several months, and
-then the battery requires to be renewed. As time-keepers,
-electrically-driven clocks are quite as good as, and often
-very much better than key-wound clocks.</p>
-
-<p>Everybody must have noticed that the numerous public
-clocks in a large town do not often agree exactly with one
-another, the differences sometimes being quite large; while
-even in one building, such as a large hotel, the different
-clocks vary more or less. This state of things is very
-unsatisfactory, for it is difficult to know which of the clocks
-is exactly right. Although large clocks are made with the
-utmost care by skilled workmen, they cannot possibly be
-made to maintain anything like the accuracy of a high-class
-chronometer, such as is used by navigators; and the only
-way to keep a number of such clocks in perfect agreement
-is to control their movements from one central or master<span class="pagenum" id="Page_126">126</span>
-clock. This can be done quite satisfactorily by electricity.
-The master-clock and the various sub-clocks are connected
-electrically, so that a current can be sent from the master-clock
-to all the others. Each sub-clock is fitted with an
-electro-magnet placed behind the figure XII at the top of
-the dial. At the instant when the master-clock reaches the
-hour, the circuit is closed automatically, and the current
-energizes these magnets. The minute hands of all the
-sub-clocks are gripped by the action of the magnets, and
-pulled exactly to the hour; the pulling being backward or
-forward according to whether the clocks are fast or slow.
-In this way all the clocks in the system are in exact agreement
-at each hour. The same result may be attained by
-adjusting all the sub-clocks so that they gain a little, say a
-few seconds in the hour. In this case the circuit is closed
-about half a minute before the hour. As each sub-clock
-reaches the hour, its electro-magnet comes into action, and
-holds the hands so that they cannot proceed. When the
-master-clock arrives at the hour the circuit is broken, the
-magnets release their captives, and all the clocks move
-forward together.</p>
-
-<p>It is possible to control sub-clocks so that their pendulums
-actually beat exactly with the pendulum of the master-clock;
-but only a small number of clocks can be controlled
-in this way, and they must be of the best quality. The
-method is similar to that used for hourly corrections, the
-main difference being that the circuit is closed by the
-pendulum of the master-clock at each end of its swing, so
-that the pendulums of the sub-clocks are accelerated or
-held back as may be required.</p>
-
-<p>In the correcting systems already described the sub-clocks
-are complete in themselves, so that they work quite
-independently, except at the instant of correction. For
-hotels, schools, and other large buildings requiring clocks<span class="pagenum" id="Page_127">127</span>
-at a number of different points, a simpler arrangement is
-adopted. Only one complete clock is used, this being the
-master-clock, which may be wound either electrically or by
-key. The sub-clocks are dummies, having only a dial with
-its hands, and an electro-magnetic arrangement behind the
-dial for moving the hands. The sub-clocks are electrically
-connected with the master-clock, and the mechanism of this
-clock is arranged to close the circuit automatically every
-half-minute. Each time this occurs the magnet of each
-sub-clock moves forward the hands half a minute, and in
-this way the dummy clocks are made to travel on together
-by half-minute steps, exactly in unison with the master-clock.</p>
-<hr class="chap x-ebookmaker-drop">
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_128">128</span></p>
-
-<h2 class="nobreak" id="toclink_128"><a id="chapter_XVI"></a>CHAPTER XVI<br>
-
-<span class="subhead">THE TELEGRAPH</span></h2>
-</div>
-
-<p class="in0"><span class="firstword">We</span> come now to one of the most important inventions of
-the nineteenth century, the electric telegraph. From very
-early times men have felt the necessity for some means of
-rapidly communicating between two distant points. The
-first really practical method of signalling was that of lighting
-beacon fires on the tops of hills, to spread some important
-tidings, such as the approach of an enemy. From this
-simple beginning arose more complicated systems of signalling
-by semaphore, flags, or flashing lights. All these
-methods proved incapable of dealing with the rapidly
-growing requirements of commerce, for they were far too
-slow in action, and in foggy weather they were of no use
-at all. We are so accustomed to walking into a telegraph
-office, filling up a form, and paying our sixpence or more,
-that it is very difficult for us to realize the immense importance
-of the electric telegraph; and probably the best
-way of doing this is to try to imagine the state of things
-which would result if the world’s telegraphic instruments
-were put out of action for a week or two.</p>
-
-<p>The earliest attempts at the construction of an electric
-telegraph date back to a time long before the discovery of
-the electric current. As early as 1727 it was known that
-an electric discharge could be transmitted to a considerable
-distance through a conducting substance such as a moistened
-thread or a wire, and this fact suggested the possibility of<span class="pagenum" id="Page_129">129</span>
-a method of electric signalling. In 1753 a writer in <cite>Scott’s
-Magazine</cite> brought forward an ingenious scheme based upon
-the attraction between an electrified body and any light
-substance. His telegraph was worked by an electric
-machine, and it consisted of twenty-six separate parallel
-wires, every wire having a metal ball suspended from it at
-each end. Close to each ball was placed a small piece of
-paper upon which was written a letter of the alphabet.
-When any wire was charged, the paper letters at each end
-of it were attracted towards the metal balls, and in this
-way words and sentences were spelled out. Many other
-systems more or less on the same lines were suggested
-during the next fifty years, but although some of them had
-considerable success in an experimental way, they were all
-far too unreliable to have any commercial success.</p>
-
-<p>With the invention of the voltaic cell, inventors’ ideas
-took a new direction. In 1812 a telegraph based upon the
-power of an electric current to decompose water was
-devised by a German named Sömmering. He used a
-number of separate wires, each connected to a gold pin
-projecting from below into a glass vessel filled with
-acidulated water. There were thirty-five wires in all, for
-letters and numbers, and when a current was sent along
-any wire bubbles of gas formed at the pin at the end of it,
-and so the letters or numbers were indicated. This telegraph,
-like its predecessors, never came into practical use.
-Oersted’s discovery in 1829 of the production of magnetism
-by electricity laid the foundation of the first really practical
-electric telegraphs, but little progress was made until the
-appearance of the Daniell cell, in 1836. The earlier forms
-of voltaic cells polarized so rapidly that it was impossible
-to obtain a constant current from them, but the non-polarizing
-Daniell cell at once removed all difficulty in this
-respect. In the year 1837 three separate practical telegraphs<span class="pagenum" id="Page_130">130</span>
-were invented: by Morse in the United States, by
-Wheatstone and Cooke in England, and by Steinheil in
-Munich.</p>
-
-<figure id="fig_26" class="figleft" style="max-width: 16em;">
- <img src="images/i_158.png" width="1215" height="2021" alt=" ">
- <figcaption class="caption"><span class="smcap">Fig. 26.</span>—Dial of Five-Needle Telegraph.
-</figcaption></figure>
-
-<p>The first telegraph of Wheatstone and Cooke consisted
-of five magnetic needles
-pivoted on a vertical
-dial. The letters of the
-alphabet were marked
-on the dial, and the
-needles were deflected
-by currents made to
-pass through wires by
-the depression of keys,
-so that two needles
-would point towards
-the required letter.
-<a href="#fig_26">Fig. 26</a> is a sketch of
-the dial of this apparatus.
-This telegraph was
-tried successfully on the
-London and North-Western
-Railway, over
-a wire a mile and a half
-in length. Wheatstone
-and Cooke afterwards
-invented a single-needle
-telegraph in which the
-letters were indicated
-by movements of the needle to the right or to the left,
-according to the direction of a current sent through a coil
-of wire. Wheatstone subsequently produced an apparatus
-which printed the letters on paper.</p>
-
-<p>In the United States, Morse had thought out a scheme
-of telegraphy in 1832, but it was not until 1837 that he got<span class="pagenum" id="Page_131">131</span>
-his apparatus into working order. He was an artist by
-profession, and for a long time he was unable to develop
-his ideas for lack of money. After many efforts he succeeded
-in obtaining a State grant of £6000 for the
-construction of a telegraph line between Baltimore and
-Washington, and the first message over this line was sent
-in 1844, the line being thrown open to the public in the
-following year. Amongst the features of this telegraph
-were a receiving instrument which automatically recorded
-the messages on a moving paper ribbon, by means of a
-pencil actuated by an electro-magnet; and an apparatus
-called a relay, which enabled the recording instrument to
-be worked when the current was enfeebled by the resistance
-of a very long wire. Morse also devised a telegraphic
-code which is practically the same as that in use to-day.</p>
-
-<p>The great discovery of the German Steinheil was that
-a second wire for the return of the current was not necessary,
-and that the earth could be used for this part of the circuit.</p>
-
-<p>In reading the early history of great inventions one is
-continually struck with the indifference or even hostility
-shown by the general public. In England the electric
-telegraph was practically ignored until the capture of a
-murderer by means of it literally forced the public to see
-its value. The murder was committed near Slough, and
-the murderer succeeded in taking train for London.
-Fortunately the Great Western Railway had a telegraph
-line between Slough and London, and a description telegraphed
-to Paddington enabled the police to arrest the
-murderer on his arrival. In the United States too there
-was just the same indifference. The rate for messages on
-the line between Baltimore and Washington was one cent
-for four words, and the total amount taken during the first
-four days was one cent!</p>
-
-<p>One of the simplest forms of telegraph is the single-needle<span class="pagenum" id="Page_132">132</span>
-instrument. This consists of a magnetic needle
-fixed to a spindle at the back of an upright board through
-which the spindle is passed. On the same spindle, but in
-front of the board, is fixed a dial needle, which, of course,
-moves along with the magnetic needle. A coil of wire is
-passed round the magnetic needle, and connected to a
-commutator for reversing
-the direction of the
-current. By turning a
-handle to the left a
-current is made to flow
-through the coil, and
-the magnetic needle
-moves to one side; but
-if the handle is turned
-to the right the current
-flows through the coil
-in the opposite direction,
-and the needle
-moves to the other
-side. Instead of a
-handle, two keys may
-be used, the movement
-of the needle varying
-according to which key
-is pressed. A good
-operator can transmit
-at the rate of about twenty words a minute with this
-instrument. The Morse code, which consists of combinations
-of dots and dashes, is used, a movement of the
-dial needle to the left meaning a dot, and one to the right
-a dash. The code as used in the single-needle instrument
-is shown in <a href="#fig_27">Fig. 27</a>.</p>
-
-<figure id="fig_27" class="figcenter" style="max-width: 16em;">
- <img src="images/i_160.png" width="1228" height="1747" alt=" ">
- <figcaption class="caption"><span class="smcap">Fig. 27.</span>—Code for Single-Needle Telegraph.
-</figcaption></figure>
-
-<p>Needle instruments are largely used in railway signal<span class="pagenum" id="Page_133">133</span>
-cabins, but for general telegraphic work an instrument
-called the Morse sounder is employed. This consists of
-an electro-magnet which, when a current is passed through
-it, attracts a small piece of iron fixed to one end of a
-pivoted lever. The other end of this lever moves between
-two stops. At the transmitting station the operator closes
-a battery circuit by pressing a key, when the electro-magnet
-of the sounder at the receiving station attracts the iron,
-and the lever flies from one stop to the other with a sharp
-click, returning again as soon as the circuit is broken. A
-dot is signalled when the lever falls back immediately after
-the click, and a dash when it makes a short stay before
-returning. <a href="#fig_28">Fig. 28</a> shows the code of signals for the
-Morse telegraph.</p>
-
-<figure id="fig_28" class="figcenter" style="max-width: 26em;">
- <img src="images/i_161.png" width="2064" height="875" alt=" ">
- <figcaption class="caption"><span class="smcap">Fig. 28.</span>—The Morse Code.
-</figcaption></figure>
-
-<p>In passing through a very long wire an electric current
-becomes greatly reduced in strength owing to the resistance
-of the wire. If two telegraph stations are a great distance
-apart the energy of the current thus may be unequal to the
-task of making the electro-magnet move the lever of the
-sounder so as to produce a click, but this difficulty is overcome
-by the use of an ingenious arrangement called a
-“relay.” It consists of a very small electro-magnet which<span class="pagenum" id="Page_134">134</span>
-attracts a light bar, the movement of the bar being made
-to close the circuit of another battery at the receiving
-station. The feeble current works the relay, and the
-current in the local circuit operates the sounder.</p>
-
-<p>The word “telegraph,” which is derived from the Greek
-<em>tele</em>, far off, and <em>grapho</em>, I write, strictly signifies writing at
-a distance. The needle instrument and the sounder do not
-write in any way, but by modifying the construction of the
-sounder it can be made to record the messages it receives.
-A small wheel is fitted to the free end of the lever of the
-sounder, and an ink-well is placed so that the wheel dips
-into it when the lever is in the normal position. When
-the circuit is closed the lever moves just as in the ordinary
-sounder, but instead of clicking against a stop it presses
-the inked wheel against a paper ribbon which is kept
-slowly moving forward by clockwork. In this way the
-wheel continues to mark a line along the paper as long as
-the circuit remains closed, and according to the time the
-transmitting key is kept down a short mark or dot, or a
-long mark or dash, is produced. The clockwork which
-moves the paper ribbon is started automatically by the
-current, and it continues working until the message is
-finished.</p>
-
-<figure id="fig_29" class="figcenter" style="max-width: 27em;">
- <img src="images/i_163.jpg" width="2101" height="2229" alt=" ">
- <figcaption class="caption"><p><span class="smcap">Fig. 29.</span>—A Morse Message.</p>
-
-<p>(<i>a</i>) Perforated Tape. <span class="in4">(<i>b</i>) Printed Tape.</span></p>
-
-<p class="p1">TRANSLATION.</p>
-
-<p class="justify"><i>Series of alternate dots and dashes indicating commencement of message.</i></p>
-
-<p class="justify">Sec (<i>section</i>) A. D. T. (<i>Daily Telegraph</i>) Fm (<i>from</i>) Berri, Antivari.</p>
-
-<p class="justify"><i>Then follow the letters</i> G. Q., <i>signifying fresh line</i>.</p>
-
-<p class="justify">They hd (<i>had</i>) bn (<i>been</i>) seen advancing in t (<i>the</i>) distance and wr (<i>were</i>)
-recognised by thr (<i>their</i>) usual uniform wh (<i>which</i>) consists o (<i>of</i>) a white fez.</p>
-
-<p class="justify"><em>Finally double dots indicating full stop.</em></p>
-</figcaption></figure>
-
-<p>A good Morse operator can maintain a speed of about
-thirty words a minute, but this is far too slow for certain
-kinds of telegraphic work, such as the transmission of press
-news, and for such work the Wheatstone automatic transmitter
-is used. First of all the messages are punched on
-a paper ribbon. This is done by passing the ribbon from
-right to left by clockwork through a punching machine
-which is provided with three keys, one for dots, one for
-dashes, and the other for spaces. If the left-hand key is
-pressed, two holes opposite to one another are made,
-representing a dot; and if the right-hand key is pressed,<span class="pagenum" id="Page_136">136</span>
-two diagonal holes are punched, representing a dash. In
-<a href="#fig_29">Fig. 29</a>, which shows a piece of ribbon punched in this
-way, a third line of holes will be noticed between the outside
-holes representing the dots and dashes. These holes
-are for the purpose of guiding the paper ribbon steadily
-along through the transmitting machine. The punched
-ribbon is then drawn by clockwork through a Wheatstone
-transmitter. In this machine two oscillating needles, connected
-with one pole of a battery, are placed below the
-moving ribbon. Each time a hole passes, these needles
-make contact with a piece of metal connected with the
-other pole of the battery, thus making and breaking the
-circuit with much greater rapidity than is possible with
-the Morse key. At the receiving station the messages are
-recorded by a form of Morse inker, coming out in dots and
-dashes as though sent by hand. Below the punched ribbon
-in <a href="#fig_29">Fig. 29</a> is shown the corresponding arrangement of dots
-and dashes. The same punched ribbon may be used
-repeatedly when the message has to be sent on a number
-of different lines. The Wheatstone automatic machine is
-capable of transmitting at the rate of from 250 to 400
-words a minute. <a href="#fig_29">Fig. 29</a> is a fragment of a <cite>Daily
-Telegraph</cite> Balkan War special, as transmitted to the
-<cite>Yorkshire Post</cite> over the latter’s private wire from London
-to Leeds. In the translation it will be seen that many
-common words are abbreviated.</p>
-
-<p>One weak point of telegraphy with Wheatstone instruments
-is that the messages are received in Morse
-code, and have to be translated. During recent years
-telegraphs have been invented which actually produce
-their messages in ordinary written or printed characters.
-A very ingenious instrument is the Hughes printing
-telegraph, which turns out messages in typewritten form.
-Its mechanism is too complicated to be described here,<span class="pagenum" id="Page_137">137</span>
-but in general it consists of a transmitter having a keyboard
-something like that of a typewriter, by means of
-which currents of electricity are made to press a sheet of
-paper at the right instant against a revolving type-wheel
-bearing the various characters. This telegraph has been
-modified and brought to considerable perfection, and in
-one form or another it is used in European countries and
-in the United States.</p>
-
-<p>In the Pollak-Virag system of telegraphy the action of
-light upon sensitized photographic paper is utilized. An
-operator punches special groupings of holes on a paper
-ribbon about 1 inch wide, by means of a perforating
-machine resembling a typewriter, and the ribbon is then
-passed through a machine which transmits by brush contacts.
-The receiver consists of a very small mirror
-connected to two vibrating diaphragms, which control its
-movements according to the currents received, one diaphragm
-moving the mirror in a vertical direction, and the
-other in a horizontal direction. The mirror reflects a ray
-of light on to photographic bromide paper in the form of a
-moving band about 3 inches in width, and the combined
-action of the two diaphragms makes the mirror move so
-that the ray of light traces out the messages in ordinary
-alphabetical characters. As it moves forward after being
-acted upon by the light, the paper is automatically developed
-and fixed, and then passed through drying rollers. Although
-the writing is rather imperfect in formation it is quite legible
-enough for most messages, but trouble occasionally occurs
-with messages containing figures, owing to confusion arising
-from the similarity of the figures, 3, 5, and 8. The whole
-process is carried out with such rapidity that 40,000 or
-even more words can be transmitted easily in an hour.</p>
-
-<p>One of the most remarkable of present-day telegraphs
-is the Creed high-speed automatic printing telegraph.<span class="pagenum" id="Page_138">138</span>
-This has been devised to do away with hand working as
-far as possible, and to substitute quicker and more accurate
-automatic methods. In this system a perforated paper
-tape is produced by a keyboard perforator at the sending
-station. This tape is just ordinary Wheatstone tape, its
-perforations representing in the Morse code the message to
-be transmitted; and the main advantage of the Creed perforator
-over the three-key punching machine already
-described lies in the ease and speed with which it can be
-worked. The keyboard contains a separate key for each
-letter or signal of the Morse code, and the pressing of any
-key brings into operation certain punches which make the
-perforations corresponding to that particular letter. The
-perforator can be worked by any one who understands how
-to use an ordinary typewriter, and a speed of about 60
-words a minute can be maintained by a fairly skilful
-operator. If desired a number of tapes may be perforated
-at the same time.</p>
-
-<p>The tape prepared in this way is passed through a
-Wheatstone transmitter, and long or short currents, according
-to the arrangement of the perforations, are sent out
-along the telegraph line. At the receiving station these
-signals operate a receiving perforator. This machine
-produces another perforated tape, which is an exact copy
-of the tape at the sending station, and it turns out this
-duplicate tape at the rate of from 150 to 200 words a
-minute. There are two forms of this receiving perforator,
-one worked entirely by electricity, and the other by a combination
-of electricity and compressed air, both forms
-serving the same purpose. The duplicate tape is then
-passed through an automatic printer, which reproduces the
-message in large Roman characters on a paper tape. The
-printer works at a speed of from 80 to about 100 words a
-minute, and the printed tape is pasted on a telegraphic<span class="pagenum" id="Page_139">139</span>
-form by a semi-automatic process, and the message is then
-ready for delivery. <a href="#plate_XI">Plate XI</a>. shows a specimen of the
-tape from the receiving perforator, and the corresponding
-translation as turned out by the printer. This message
-formed part of a leading article in the <cite>Daily Mail</cite>. Some
-idea of the wonderful capabilities of the Creed system may
-be gained from the fact that by means of it practically the
-whole contents of the <cite>Daily Mail</cite> are telegraphed every
-night from London to Manchester and Paris, for publication
-next morning.</p>
-
-<p>One of the most remarkable features about present-day
-telegraphy is the ease with which two or more messages
-can be sent simultaneously over one line. Duplex telegraphy,
-or the simultaneous transmission of two separate
-messages in opposite directions over one wire, is now
-practised on almost every line of any importance. At first
-sight duplex telegraphy seems to be an impossibility, for if
-we have two stations, one at each end of a single wire, and
-each station fitted with a transmitter and a receiver, it
-appears as if each transmitter would affect not only the
-receiver at the opposite end of the wire, but also the
-receiver at its own end, thus causing hopeless confusion
-when both transmitters were in use at the same time.
-This actually would be the case with ordinary telegraphic
-methods, but by the use of a special arrangement all confusion
-in working is avoided.</p>
-
-<figure id="plate_XI" class="figcenter" style="max-width: 35em;">
- <p class="caption">PLATE XI.</p>
- <img src="images/i_169.jpg" width="2745" height="2077" alt=" ">
- <figcaption class="caption">
-
-<p class="floatl"><i>By permission of</i></p>
-<p class="floatr"><i>Creed, Bille & Co. Ltd.</i></p>
-
-<p class="floatc">SPECIMEN OF THE WORK OF THE CREED HIGH-SPEED PRINTING TELEGRAPH.</p>
-</figcaption></figure>
-
-<p>We have seen that a magnetic needle is deflected by a
-current passing through a coil of wire placed round it, and
-that the direction in which the needle is deflected depends
-upon the direction of the current in the coil. Now suppose
-we place round the needle two coils of wire, wound so that
-the current in one flows in a direction opposite to that of
-the current in the other. Then, if we pass two equal
-currents, one through each coil, it is evident that they will<span class="pagenum" id="Page_140">140</span>
-neutralize one another, so that the needle will not be
-deflected at all. In a duplex system one end of one of
-these coils is connected to earth, say to a copper plate
-buried in the ground, and one end of the other to the line
-wire. The two remaining ends are arranged as branches
-leading off from a single wire connected with the transmitting
-key. The whole arrangement of coils and needle
-is repeated at the other end of the line. If now the
-transmitting key at station A is pressed, the circuit is closed
-and a current flows along the single wire, and then divides
-into two where the wire branches, half of it taking the path
-through one coil and half the path through the other.
-Equal currents thus flow through the oppositely wound
-coils, and the needle at station A is not deflected. Leaving
-the coils, one of these equal currents flows away to earth,
-while the other passes out along the line wire. On its
-arrival at station B the current is able to pass through only
-one of the coils round the needle, and consequently the
-needle is deflected and the signal given. In this way the
-transmitting operator at station A is able to signal to station
-B without affecting the receiver at his own end, and
-similarly the operator at station B can transmit to A without
-affecting the B receiver. Thus there can be no confusion
-whether the transmitters are worked at different times or
-simultaneously, for each transmitter affects only the
-receiver at the opposite end of the line. The diagram in
-<a href="#fig_30">Fig. 30</a> will help to make clearer the general principle.
-K and K¹ are the two transmitting keys which close the
-circuit, and C and C¹ are the points at which the current
-divides into two. Instead of coils and needles, electro-magnets
-operating sounders may be used, such magnets
-having two separate and oppositely wound coils, acting in
-exactly the same way as the coils round the needles. The
-above description is of course only a rough outline of the<span class="pagenum" id="Page_141">141</span>
-method, and in practice matters are more complicated,
-owing to the necessity for carefully adjusted resistances and
-for condensers. There is also another and different method
-of duplexing a line, but we have not space to describe it.
-Duplex telegraphy requires two operators at each end of
-the line, one to send and the other to receive.</p>
-
-<p>Diplex telegraphy is the simultaneous transmission of
-two separate messages in the same direction over one line.
-Without going into details it may be said that for this
-purpose two different transmitting keys are required, one of
-which alters the direction, and the other the strength of the
-current though the line wire. The receivers are arranged
-so that one responds only to a strong current, and the other
-only to a current in one particular direction. A line also
-may be quadruplexed, so that it is possible to transmit
-simultaneously two messages from each end, four operators
-being required at each station, two to transmit and two
-to receive. Systems of multiplex telegraphy have been
-devised by which very large numbers of messages can be
-sent at once over a single wire, and the Baudot multiplex
-telegraph has proved very successful.</p>
-
-<figure id="fig_30" class="figcenter" style="max-width: 23em;">
- <img src="images/i_171.png" width="1775" height="826" alt=" ">
- <figcaption class="caption"><span class="smcap">Fig. 30.</span>—Diagram to illustrate principle of Duplex Telegraphy.
-</figcaption></figure>
-
-<p>The wires for telegraphic purposes may be conveyed
-either above or below the ground. Overground wires are<span class="pagenum" id="Page_142">142</span>
-carried on poles by means of insulators of porcelain or other
-non-conducting material, protected by a sort of overhanging
-screen. The wires are left bare, and they are generally
-made of copper, but iron is used in some cases. In underground
-lines the wires formerly were insulated by a covering
-of gutta-percha, but now paper is generally used. Several
-wires, each covered loosely with thoroughly dry paper, are
-laid together in a bundle, the whole bundle or cable being
-enclosed in a strong lead pipe. The paper coverings are
-made to fit loosely so that the wires are surrounded by an
-insulating layer of dry air. As many as 1200 separate
-wires are sometimes enclosed in one pipe. In order to
-keep telegraph lines in working order frequent tests are
-necessary, and the most important British Postal Telegraph
-lines are tested once a week between 7.30 and 7.45 a.m.
-The earth is generally used for the return circuit in
-telegraphy, and the ends of the return wires are connected
-either to metal plates buried in the ground to a depth at
-which the earth is permanently moist, or to iron gas or
-water pipes. The current for telegraph working on a small
-scale is usually supplied by primary cells, the Daniell cell
-being a favourite for this purpose. In large offices the
-current is generally taken from a battery of storage cells.</p>
-
-<p>During the early days of telegraphy, overhead lines
-were a source of considerable danger when thunderstorms
-were taking place. Lightning flashes often completely
-wrecked the instruments, giving severe shocks to those in
-the vicinity, and in a few cases operators were killed at
-their posts. Danger of this kind is now obviated by the
-use of contrivances known as lightning arresters. There
-are several forms of these, but only one need be mentioned.
-The main features of this are two metal plates separated
-slightly from one another, so that there is a small air gap
-between them. One plate is connected to the line wire,<span class="pagenum" id="Page_143">143</span>
-and the other to earth. Almost all lightning flashes consist
-of an oscillatory discharge, that is one which passes a
-number of times backwards and forwards between a cloud
-and the earth. A very rapidly alternating discharge of
-this kind finds difficulty in passing along the line wire,
-being greatly impeded by the coils of wire in the various
-pieces of apparatus; and although the resistance of this
-air gap is very high, the lightning discharge will cross the
-gap sooner than struggle along the line wire. In this way,
-when a flash affects the line, the discharge jumps the gap
-between the plates of the arrester and passes away harmlessly
-to earth, without entering the telegraph office at all.
-As was mentioned in <a href="#chapter_III">Chapter III</a>., the prevalence of
-magnetic storms sometimes renders telegraph lines quite
-unworkable for a time, but although such disturbances
-cause great delay and general inconvenience, they are not
-likely to be at all dangerous. It is often possible to
-maintain telegraphic communication during magnetic disturbances
-by using two lines to form a complete metallic
-loop, so that there is no earth return.</p>
-<hr class="chap x-ebookmaker-drop">
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_144">144</span></p>
-
-<h2 class="nobreak" id="toclink_144"><a id="chapter_XVII"></a>CHAPTER XVII<br>
-
-<span class="subhead">SUBMARINE TELEGRAPHY</span></h2>
-</div>
-
-<p class="in0"><span class="firstword">The</span> story of submarine telegraphy is a wonderful record
-of dogged perseverance in the face of tremendous obstacles
-and disastrous failures. It would be of no interest to trace
-the story to its very beginning, and so we will commence
-with the laying of the first cable across the English
-Channel from Dover to Calais, in 1850. A single copper
-wire covered with a layer of gutta-percha half an inch thick
-was used, and leaden weights were attached to it at intervals
-of one hundred yards, the fixing of each weight necessitating
-the stoppage of the cable-laying ship. The line was laid
-successfully, but it failed after working for a single day, and
-it afterwards turned out that a Boulogne fisherman had
-hauled up the cable with his trawl. This line proved that
-telegraphic communication between England and France
-was possible, but the enterprise was assailed with every
-imaginable kind of abuse and ridicule. It is said that some
-people really believed that the cable was worked in the
-style of the old-fashioned house bell, and that the signals
-were given by pulling the wire! In the next year another
-attempt was made by Mr. T. R. Crampton, a prominent
-railway engineer, who himself contributed half of the
-£15,000 required. The form of cable adopted by him
-consisted of four copper wires, each covered with two layers
-of gutta-percha, and the four enclosed in a covering formed
-of ten galvanized iron wires wound spirally round them.<span class="pagenum" id="Page_145">145</span>
-The line proved a permanent success, and this type of
-cable, with certain modifications, is still in use. In 1852
-three attempts were made to connect England and Ireland,
-but the first two failed owing to the employment of cables
-too light to withstand the strong tidal currents, and the
-third was somehow mismanaged as regards the paying-out,
-so that there was not enough cable to reach across. A
-heavier cable was tried in the next year, and this was a
-lasting success.</p>
-
-<p>The success of these two cables led to the laying of
-many other European cables over similar distances, but we
-must now pass on to a very much bigger undertaking, the
-laying of the Atlantic cable. In 1856 the Atlantic Telegraph
-Company was formed, with the object of establishing
-and working telegraphic communication between Ireland
-and Newfoundland, the three projectors being Messrs.
-J. W. Brett, C. T. Bright, and C. W. Field. The British
-and the United States Governments granted a subsidy, in
-return for which Government messages were to have
-priority over all others, and were to be transmitted free.
-The objections launched against the scheme were of course
-many, some of them making very amusing reading. It is
-however very strange to find so eminent a scientist as
-Professor Airy, then Astronomer Royal, seriously stating
-that it was a mathematical impossibility to submerge a
-cable safely to such depths, and that even if this could be
-done, messages could not be transmitted through such a
-great length of cable.</p>
-
-<p>It was estimated that a length of about 2500 nautical
-miles would be enough to allow for all contingencies, and
-the construction of the cable was commenced in February
-1857, and completed in June of that year. It is difficult to
-realize the gigantic nature of the task of making a cable of
-such dimensions. The length of copper wire used in<span class="pagenum" id="Page_146">146</span>
-making the conductor was 20,500 miles, while the outer
-sheathing took 367,500 miles of iron wire; the total length
-of wire used being enough to go round the Earth thirteen
-times. The cable was finally stowed away on board two
-warships, one British and the other American.</p>
-
-<p>The real troubles began with the laying of the cable.
-After landing the shore end in Valentia Bay, the paying-out
-commenced, but scarcely had five miles been laid when
-the cable caught in the paying-out machinery and parted.
-By tracing it from the shore the lost end was picked up
-and spliced, and the paying-out began again. Everything
-went well for two or three days, and then, after 380 miles
-had been laid, the cable snapped again, owing to some
-mismanagement of the brakes, and was lost at a depth of
-2000 fathoms. The cable had to be abandoned, and the
-ships returned to Plymouth.</p>
-
-<p>In the next year, 1858, another attempt was made, with
-new and improved machinery and 3000 miles of cable, and
-this time it was decided that the two ships should start
-paying-out from mid-ocean, proceeding in opposite directions
-towards the two shores after splicing their cables. On the
-voyage out the expedition encountered one of the most
-fearful storms on record, which lasted over a week, and the
-British man-of-war, encumbered with the dead weight of
-the cable, came near to disaster. Part of the cable shifted,
-and those on board feared that the whole of the huge mass
-would break away and crash through the vessel’s side.
-Sixteen days after leaving Plymouth the rendezvous was
-reached, the cables were spliced and the ships started.
-After the British ship had paid out 40 miles it was discovered
-that the cable had parted at some distance from
-the ship, and the vessels once more sought each other, and
-spliced again ready for another effort. This time the cable
-parted after each vessel had paid out a little more than<span class="pagenum" id="Page_147">147</span>
-100 miles, and the ships were forced to abandon the
-attempt.</p>
-
-<p>The failure of this second expedition naturally caused
-great discouragement, and the general feeling was that the
-whole enterprise would have to be given up. The chairman
-of the company recommended that in order to make the
-best of a bad job the remainder of the cable should be sold,
-and the proceeds divided amongst the shareholders, but
-after great efforts on the part of a dauntless few who refused
-to admit defeat, it was finally decided to make one
-more effort. No time was lost, and on 17th July 1858
-the vessels again sailed from Queenstown. As before, the
-cables were spliced in mid-ocean, and this time, after many
-anxious days, many false alarms, and one or two narrow
-escapes from disaster through faulty pieces of cable discovered
-almost too late, the cable was landed successfully
-on both shores of the Atlantic early in August.</p>
-
-<p>The Atlantic cable was now an accomplished fact, and
-dismal forebodings were turned into expressions of extravagant
-joy. The first messages passed between Queen
-Victoria and the President of the United States, and
-amongst the more important communications was one
-which prevented the sailing from Canada of two British
-regiments which had been ordered to India during the
-Mutiny. In the meantime the Indian Mutiny had been
-suppressed, and therefore these regiments were not required.
-The dispatch of this message saved a sum of about
-£50,000. The prospects of the cable company seemed
-bright, but after a short time the signals began to grow
-weaker and weaker, and finally, after about seven hundred
-messages had been transmitted, the cable failed altogether.
-This was a great blow to the general public, and we can
-imagine the bitter disappointment of the engineers and
-electricians who had laboured so hard and so long to bring<span class="pagenum" id="Page_148">148</span>
-the cable into being. It was a favourable opportunity for
-the croakers, and amongst a certain section of the public
-doubts were expressed as to whether any messages had
-been transmitted at all.</p>
-
-<p>A great consultation of experts took place with the
-object of determining the cause of the failure, and the
-unanimous opinion was that the cable had been injured by
-the use of currents of too great intensity. Some years
-elapsed before another attempt could be made, but the
-idea was never abandoned, and a great deal of study was
-given to the problems involved. Mr. Field, the most
-energetic of the original projectors, never relaxed his determination
-that the cable should be made a success, and he
-worked incessantly to achieve his ambition. It is said
-that in pursuance of his object he made sixty-four crossings
-of the Atlantic, and considering that he suffered greatly
-from sea-sickness every time this shows remarkable pluck
-and endurance.</p>
-
-<p>In 1865, new capital having been raised, preparations
-were made for another expedition. It was now decided
-to use only one vessel for laying the cable, and the <i>Great
-Eastern</i> was chosen for the task. This vessel had been
-lying idle for close on ten years, owing to her failure as a
-cargo boat, but her great size and capacity made her most
-suitable for carrying the enormous weight of the whole
-cable. In July 1865 the <i>Great Eastern</i> set sail, under
-the escort of two British warships. When 84 miles had
-been paid out, a fault occurred, and after drawing up about
-10½ miles it was found that a piece of iron wire had pierced
-the coating of the cable. The trouble was put right, and
-the paying-out continued successfully until over 700 miles
-had been laid, when another fault appeared. The cable
-was again drawn in until the fault was reached, and
-another piece of iron was found piercing clean through.<span class="pagenum" id="Page_149">149</span>
-It was evident that two such pieces of iron could not have
-got there by accident, and there was no doubt that they
-had been inserted intentionally by some malicious scoundrel,
-most likely with the object of affecting the company’s
-shares. A start was made once more, and all went well
-until about two-thirds of the distance had been covered,
-when the cable broke and had to be abandoned after
-several nearly successful attempts to recover it.</p>
-
-<p>In spite of the loss, which amounted to £600,000, the
-energetic promoters contrived to raise fresh capital, and in
-1866 the <i>Great Eastern</i> started again. This effort was
-completely successful, and on 28th July 1866 the cable
-was landed amidst great rejoicing. The following extracts
-from the diary of the engineer Sir Daniell Gooch, give us
-some idea of the landing.</p>
-
-<p>“Is it wrong that I should have felt as though my
-heart would burst when that end of our long line touched
-the shore amid the booming of cannon, the wild, half-mad
-cheers and shouts of the men?... I am given a never-dying
-thought; that I aided in laying the Atlantic cable....
-The old cable hands seemed as though they could
-eat the end; one man actually put it into his mouth and
-sucked it. They held it up and danced round it, cheering
-at the top of their voices. It was a strange sight, nay, a
-sight that filled our eyes with tears.... I did cheer, but
-I could better have silently cried.”</p>
-
-<p>This time the cable was destined to have a long and
-useful life, and later in the same year the 1865 cable was
-recovered, spliced to a new length, and safely brought to
-land, so that there were now two links between the Old
-World and the New. It was estimated that the total cost
-of completing the great undertaking, including the cost of
-the unsuccessful attempts, was nearly two and a half millions
-sterling. Since 1866 cable-laying has proceeded very<span class="pagenum" id="Page_150">150</span>
-rapidly, and to-day telegraphic communication exists between
-almost all parts of the civilized world. According
-to recent statistics, the North Atlantic Ocean is now
-crossed by no less than 17 cables, the number of cables
-all over the world being 2937, with a total length of
-291,137 nautical miles.</p>
-
-<p>Before describing the actual working of a submarine
-cable, a few words on cable-laying may be of interest.
-Before the cable-ship starts, another vessel is sent over
-the proposed course to make soundings. Galvanized steel
-pianoforte wire is used for sounding, and it is wound in
-lengths of 3 or 4 nautical miles on gun-metal drums.
-The drums are worked by an engine, and the average
-speed of working is somewhere about 100 fathoms a
-minute in descending, and 70 fathoms a minute in picking
-up. Some idea of the time occupied may be gained from
-a sounding in the Atlantic Ocean which registered a depth
-of 3233 fathoms, or nearly 3½ miles. The sinker took
-thirty-three minutes fifty seconds in descending, and forty-five
-minutes were taken in picking up. The heavy sinker
-is not brought up with the line, but is detached from the
-sounder by an ingenious contrivance and left at the bottom.
-The sounder is fitted with an arrangement to bring up a
-specimen of the bottom, and also a sample of water; and
-the temperature at any depth is ascertained by self-registering
-thermometers.</p>
-
-<p>When the soundings are complete the cable-ship takes
-up her task. The cable is coiled in tanks on board, and
-is kept constantly under water to prevent injury to the
-gutta-percha insulation by overheating. As each section
-is placed in the tank, the ends of it are led to a test-box,
-and labelled so that they can be easily recognized. Insulated
-wires run from the test-box to instruments in the
-testing-room, so that the electrical condition of the whole<span class="pagenum" id="Page_151">151</span>
-cable is constantly under observation. During the whole
-time the cable is being laid its insulation is tested continuously,
-and at intervals of five minutes signals are sent from
-the shore end to the ship, so that a fault is instantly detected.
-The cable in its tank is eased out by a number of
-men, and mechanics are posted at the cable drums and
-brakes, while constant streams of water cool the cable and
-the bearings and surfaces of the brakes. The tension, as
-shown by the dynamometer, is at all times under careful
-observation. When it becomes necessary to wind back the
-cable on account of some fault, cuts are made at intervals
-of a quarter or half a mile, tests being made at each cutting
-until the fault is localized in-board. As soon as the cable
-out-board is found “O.K.,” the ends are spliced up and the
-paying-out begins again. If the cable breaks from any
-cause, a mark-buoy is lowered instantly on the spot, and
-the cable is grappled for. This may take a day or two in
-good weather, but a delay of weeks may be caused by bad
-weather, which makes grappling impossible.</p>
-
-<p>The practical working of a submarine cable differs in
-many respects from that of a land telegraph line. The
-currents used in submarine telegraphy are extremely small,
-contrary to the popular impression. An insulated cable
-acts like a Leyden jar, in the sense that it accumulates
-electricity and does not quickly part with it, as does a bare
-overhead wire. In the case of a very long cable, such as
-one across the Atlantic, a current continues to flow from it
-for some time after the battery is disconnected. A second
-signal cannot be sent until the electricity is dissipated and
-the cable clear, and if a powerful current were employed
-the time occupied in this clearing would be considerable, so
-that the speed of signalling would be slow. Another
-objection to a powerful current is that if any flaw
-exists in the insulation of the cable, such a current is apt<span class="pagenum" id="Page_152">152</span>
-to increase the flaw, and finally cause the breakdown of
-the line.</p>
-
-<p>The feebleness of the currents in submarine telegraphy
-makes it impossible to use the ordinary land telegraph
-receiver, and a more sensitive instrument known as the
-“mirror receiver” is used. This consists of a coil of very
-fine wire, in the centre of which a tiny magnetic needle is
-suspended by a fibre of unspun silk. A magnet placed close
-by keeps the needle in one position when no current is
-flowing. As the deflections of the needle are extremely
-small, it is necessary to magnify them in some way, and
-this is done by fixing to the needle a very small mirror,
-upon which falls a ray of light from a lamp. The mirror
-reflects this ray on to a sheet of white paper marked with
-a scale, and as the mirror moves along with the needle the
-point of light travels over the paper, a very small movement
-of the needle causing the light to travel some inches.
-The receiving operator sits in a darkened room and
-watches the light, which moves to the right or to the left
-according to the direction of the current. The signals
-employed are the same as those for the single-needle
-instrument, a movement to the left indicating a dot, and
-one to the right a dash. In many instruments the total
-weight of magnet and mirror is only two or three grains,
-and the sensitiveness is such that the current from a voltaic
-cell consisting of a lady’s silver thimble with a few drops of
-acidulated water and a diminutive rod of zinc, is sufficient
-to transmit a message across the Atlantic.</p>
-
-<p>The mirror receiver cannot write down its messages,
-and for recording purposes an instrument invented by Lord
-Kelvin, and called the “siphon recorder,” is used. In this
-instrument a coil of wire is suspended between the poles of
-an electro-magnet, and to it is connected by means of a silk
-fibre a delicate glass tube or siphon. One end of the<span class="pagenum" id="Page_153">153</span>
-siphon dips into an ink-well, and capillary attraction causes
-the ink to fill the siphon. The other end of the siphon
-almost touches a moving paper ribbon placed beneath it.
-The ink and the paper are oppositely electrified, and the
-attraction between the opposite charges causes the ink to
-spurt out of the siphon in very minute drops, which fall on
-to the paper. As long as no current is passing the siphon
-remains stationary, but when a current flows from the cable
-through the coil, the latter moves to one side or the other,
-according to the direction of the current, and makes the
-siphon move also. Consequently, instead of a straight line
-along the middle of the paper ribbon, a wavy line with
-little peaks on each side of the centre is produced by the
-minute drops of ink. This recorder sometimes refuses to
-work properly in damp weather, owing to the loss of the
-opposite charges on ink and paper, but a later inventor,
-named Cuttriss, has removed this trouble by using a siphon
-kept constantly in vibration by electro-magnetism. The
-ordinary single-needle code is used for the siphon recorder.</p>
-<hr class="chap x-ebookmaker-drop">
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_154">154</span></p>
-
-<h2 class="nobreak" id="toclink_154"><a id="chapter_XVIII"></a>CHAPTER XVIII<br>
-
-<span class="subhead">THE TELEPHONE</span></h2>
-</div>
-
-<p class="in0"><span class="firstword">In</span> our younger days most of us have amused ourselves
-with a toy telephone consisting of a long piece of string
-having each end passed through the bottom of a little cardboard
-box, and secured by a knot. If the string is stretched
-tightly this arrangement enables whispered words to be
-heard at a distance of 20 or 30 yards. Simple as is
-this little toy, yet it is probable that many people would
-be rather nonplussed if asked suddenly to explain how the
-sounds travel along the string from one box to the other.
-If the toy had some complicated mechanism most likely
-every one would want to know how it worked, but the whole
-thing is so extremely simple that generally it is dismissed
-without a thought.</p>
-
-<p>If we strike a tuning-fork and then hold it close to the
-ear, we hear that it produces a sound, and at the same time,
-from a slight sensation in the hand, we become aware that
-the fork is in vibration. As the fork vibrates it disturbs
-the tiny particles of air round it and sets them vibrating,
-and these vibrations are communicated from one particle to
-another until they reach the drum of the ear, when that also
-begins to vibrate and we hear a sound. This is only
-another way of saying that the disturbances of the air
-caused by the vibrations of the tuning-fork are propagated
-in a series of waves, which we call “sound waves.” Sound is
-transmitted better through liquids than through the air, and<span class="pagenum" id="Page_155">155</span>
-better still through solids, and this is why words spoken so
-softly as to be inaudible through the air at a distance of,
-say, 100 feet, can be heard fairly distinctly at that distance
-by means of the string telephone. The sound reaches us
-along the string in exactly the same way as through the air,
-that is, by means of minute impulses passed on from particle
-to particle.</p>
-
-<p>A more satisfactory arrangement than the string telephone
-consists of two thin plates of metal connected by a
-wire which is stretched very tightly. Words spoken close
-to one plate are heard by a listener at the other plate up to
-a considerable distance. Let us try to see exactly what
-takes place when this apparatus is used. In the act of
-speaking, vibrations are set up in the air, and these in turn
-set up vibrations in the metal plate. The vibrations are
-then communicated to the wire and to the metal plate at
-the other end, and finally the vibrations of this plate produce
-vibrations in the air between the plate and the listener,
-and the sound reaches the ear.</p>
-
-<p>This simple experiment shows the remarkable fact that
-a plate of metal is able to reproduce faithfully all the vibrations
-communicated to it by the human voice, and from this
-fact it follows that if we can communicate the vibrations set
-up in one plate by the voice, to another plate at a distance
-of 100 miles, we shall be able to speak to a listener
-at the further plate just as if he were close to us. A
-stretched string or wire transmits the vibrations fairly well
-up to a certain distance, but beyond this distance the vibrations
-become weaker and weaker until no sound at all
-reaches the air. By the aid of electricity, however, we can
-transmit the vibrations to a tremendous distance, the range
-being limited only by the imperfections of our apparatus.</p>
-
-<p>The first attempt at the construction of an electric
-telephone, that is an instrument by means of which the<span class="pagenum" id="Page_156">156</span>
-vibrations set up by the voice or by a musical instrument
-are transmitted by electricity, was made in 1860 by Johann
-Philipp Reis, a teacher in a school at Friedrichsdorf, in
-Germany. His transmitting apparatus consisted of a box
-having a hole covered by a tightly stretched membrane, to
-which was attached a little strip of platinum. When the
-membrane was made to vibrate by sounds produced close
-to the box, the strip of platinum moved to and fro against
-a metal tip, which closed the circuit of a battery. The
-receiver was a long needle of soft iron round which was
-wound a coil of wire, and the ends of the needle rested on
-two little bridges of a sounding box. The vibrations of
-the membrane opened and closed the circuit at a great
-speed, and the rapid magnetization of the needle produced
-a tone of the same pitch as the one which set the membrane
-vibrating. This apparatus transmitted musical sounds and
-melodies with great accuracy, but there is considerable
-difference of opinion as to whether it was able to transmit
-speech. Professor Sylvanus Thompson distinctly states
-that Reis’s telephone could and did transmit speech, but
-other experts dispute the fact. We probably shall be quite
-safe in concluding that this telephone did transmit speech,
-but very imperfectly. In any case it is certain that the
-receiver of this apparatus is not based on the same principle
-as the modern telephone receiver.</p>
-
-<p>Some years later Graham Bell, Professor of Vocal
-Physiology in the University of Boston, turned his attention
-to the electric transmission of speech, probably being led to
-do so from his experiments in teaching the deaf and dumb.
-His apparatuses shown at an exhibition in Philadelphia in
-1876, consisted of a tube having one end open for speaking
-into, and the other closed by a tightly stretched membrane
-to which was attached a very light steel bar magnet. The
-vibrations set up in the membrane by the voice made the<span class="pagenum" id="Page_157">157</span>
-little magnet move to and fro in front of the poles of an
-electro-magnet, inserted in a battery circuit, thus inducing
-currents of electricity in the coils of the latter magnet.
-The currents produced in this way varied in direction and
-strength according to the vibratory movements of the
-membrane, and being transmitted along a wire they
-produced similar variations in current in another electro-magnet
-in the receiver. The currents produced in this
-manner in the receiver set up vibrations in a metal
-diaphragm in front of the magnet poles, and so the words
-spoken into the transmitter were reproduced.</p>
-
-<p>Since the year 1876 the telephone has developed with
-remarkable rapidity, and an attempt to trace its growth
-would involve a series of detailed descriptions of closely
-similar inventions which would be quite uninteresting to
-most readers. Now, therefore, that we have introduced
-the instruments, and seen something of its principle and
-its early forms, it will be most satisfactory to omit the
-intermediate stages and to go on to the telephone as used
-in recent years. The first telephone to come into general
-use was the invention of Graham Bell, and was an improved
-form of his early instrument just described. A case or
-tube of ebonite, which forms the handle of the instrument,
-contains a steel bar magnet having a small coil of insulated
-wire at the end nearest the mouthpiece of the tube, the
-ends of the coil passing along the tube to be connected to
-the line wires. Close to the coil end of the magnet, and
-between it and the mouthpiece, is fixed a diaphragm of
-thin sheet-iron. A complete outfit consists of two of these
-instruments connected by wires, and it will be noticed that
-no battery is employed.</p>
-
-<p>The air vibrations set up by the voice make the
-diaphragm vibrate also, so that it moves backwards and
-forwards. These movements are infinitesimally small, but<span class="pagenum" id="Page_158">158</span>
-they are sufficient to affect the lines of force of the magnet
-to such an extent that rapidly alternating currents of varying
-degrees of strength are set up in the coil and sent along
-the line wire. On arriving at the receiver these currents
-pass through the coil and produce rapid variations in the
-strength of the magnet, so that instead of exerting a
-uniform attraction upon the iron diaphragm, the magnet
-pulls it with constantly varying force, and thus sets it
-vibrating. The air in front of the diaphragm now begins
-to vibrate, and the listener hears a reproduction of the
-words spoken into the transmitter. The way in which the
-fluctuations of the current make the second diaphragm
-vibrate exactly in accordance with the first is very remarkable,
-and it is important to notice that the listener does not
-hear the actual voice of the speaker, but a perfect reproduction
-of it; in fact, the second diaphragm speaks.</p>
-
-<p>The reader probably will be surprised to be told that
-the transmitter and the receiver of a magneto-electric
-telephone are respectively a dynamo and electric motor of
-minute proportions. We provide a dynamo with mechanical
-motion and it gives us electric current, and by sending
-this current through an electric motor we get mechanical
-motion back again. In the transmitter of the telephone
-just described, the mechanical motion is in the form of
-vibrations of the metal diaphragm, which set up currents of
-electricity in the coil of wire round the magnet, so that the
-transmitter is really a tiny dynamo driven by the voice.
-The receiver is provided with electric current from the
-transmitter, and it converts this into mechanical motion in
-the diaphragm, so that the receiver is a little electric motor.</p>
-
-<p>Transmitters of the type just described work well over
-short distances, but the currents they produce are too feeble
-for transmission over a very long wire, and on this account
-they have been superseded by transmitters on the microphone<span class="pagenum" id="Page_159">159</span>
-principle. A microphone is an instrument for
-making extremely small sounds plainly audible. If a
-current is passed through a box containing loose bits of
-broken carbon, it meets with great resistance, but if the
-bits of carbon are compressed their conducting power is
-considerably increased. Even such slight differences in
-pressure as are produced by vibrating the box will affect
-the amount of current passing through the carbon. If this
-current is led by wires to an ordinary telephone receiver
-the arrangement becomes a simple form of microphone.
-The vibrations of the box vary
-the resistance of the carbon,
-and the corresponding variations
-in the current set up
-vibrations in the receiver, but
-in a magnified form. The
-smallest sound vibrations alter
-the resistance of the carbon,
-and as these vibrations are
-magnified in the receiver, the
-reproduced sound is magnified
-also. The footsteps of a fly
-may be heard quite distinctly by means of a good microphone,
-and the ticks of a watch sound like the strokes of a
-hammer.</p>
-
-<figure id="fig_31" class="figright" style="max-width: 11em;">
- <img src="images/i_189.png" width="801" height="903" alt=" ">
- <figcaption class="caption"><span class="smcap">Fig. 31.</span>—Diagram of Microphone Transmitter.
-</figcaption></figure>
-
-<p>By means of this power of magnifying vibrations a
-microphone transmitter can be used on a line of tremendous
-length, where an ordinary Bell transmitter would be utterly
-useless. The general features of this transmitter, <a href="#fig_31">Fig. 31</a>,
-are a diaphragm and a block of carbon separated slightly
-from one another, the intervening space being filled with
-granules of carbon. These are enclosed in a case of
-ebonite having a mouthpiece in front and two terminals
-behind, one terminal being connected with the carbon block<span class="pagenum" id="Page_160">160</span>
-and the other with the diaphragm. From these terminals
-wires are led to a battery and to the receiver, which is of
-the Bell type. The current has to pass through the carbon
-granules, and the movements of the diaphragm when set in
-vibration by the voice vary the pressure upon the granules,
-and in this way set up variations in the current. Carbon
-dust also may be used instead of granular carbon, and then the
-instrument is called a “dust transmitter.”</p>
-
-<figure id="fig_32" class="figleft" style="max-width:7em;">
- <img src="images/i_190.png" width="528" height="1277" alt=" ">
- <figcaption class="caption hang"><span class="smcap">Fig. 32.</span>—Combined Telephone Transmitter
-and Receiver.
-</figcaption></figure>
-
-<p>It is usual to have a transmitter and
-a receiver on one handle for the greater
-convenience of the user. The arrangement
-is shown in <a href="#fig_32">Fig. 32</a>, and it will be
-seen that when the user places the receiver
-to his ear the transmitting mouthpiece is
-in position for speaking. The microphone
-with its carbon dust is placed at A, just
-below the mouthpiece, and the earpiece
-or receiver B contains a little magnet and
-coil with a diaphragm in front, so that it
-is really a Bell instrument. A little lever
-will be noticed at C. This is a switch
-which brings the transmitter into circuit
-on being pressed with the finger.</p>
-
-<p>It is now time to see something of the
-arrangement and working of telephone
-systems. As soon as the telephone became a commercially
-practicable instrument the necessity for some means of
-inter-communication became evident, and the telephone
-exchange was brought into being. The first exchange
-was started in 1877, in Boston, but this was a very
-small affair and it was run on very crude lines. When
-one subscriber wished to communicate with another he
-had to call up an operator, who received the message
-and repeated it to the person for whom it was intended;<span class="pagenum" id="Page_161">161</span>
-there was no direct communication between the various
-subscribers’ instruments. As the number of users increased
-it became necessary to devise some system
-whereby each subscriber could call the attention of an
-operator at the central station, and be put into direct
-communication with any other subscriber without delay;
-and the exchange system of to-day, which fulfils these
-requirements almost to perfection, is the result of gradual
-improvements in telephone methods extending over some
-thirty-five years.</p>
-
-<p>When a subscriber wishes to telephone, he first must
-call up the operator at the exchange. Until comparatively
-recently this was done by turning a handle placed at the
-side of the instrument. This handle operated a little
-dynamo, and the current produced caused a shutter at the
-exchange to drop and reveal a number, just as in the
-electric bell indicator, so that the operator knew which
-instrument was calling. As soon as the operator answered
-the call, the shutter replaced itself automatically. The
-signal to disconnect was given in the same way, but the
-indicator was of a different colour in order to prevent confusion
-with a call signal. These handle-operated telephones
-are still in common use, but they are being replaced by
-instruments which do away with handle-turning on the
-part of the subscriber, and with dropping shutters at the
-exchange. In this latest system all that the subscriber has
-to do is to lift his telephone from its rest, when a little
-electric lamp lights up at the exchange; and when he has
-finished his conversation he merely replaces the telephone,
-and again a little lamp glows.</p>
-
-<p>We must now see what happens at the exchange when
-a call is made. Each operator has control of a number of
-pairs of flexible cords terminating in plugs, the two cords
-of each pair being electrically connected. The plugs rest<span class="pagenum" id="Page_162">162</span>
-on a shelf in front of the operator, and the cords pass
-through the shelf and hang down below it. If a plug is
-lifted, the cord comes up through the shelf, and it is drawn
-back again by a weight when the plug is not in use. Two
-lamps are provided for each pair of cords, one being fixed
-close to each cord. The two wires leading from each
-subscriber’s instrument are connected to a little tube-shaped
-switch called a “jack,” and each jack has a lamp of its own.
-When a subscriber lifts his telephone from its rest a lamp
-glows, and the operator inserts one plug of a pair into the
-jack thus indicated, and the lamp goes out automatically.
-She then switches on her telephone to the caller and asks
-for the number of the subscriber to whom he wishes to
-speak; and as soon as she gets this she inserts the other
-plug of the pair into the jack belonging to this number.
-By a simple movement she then rings up the required
-person by switching on the current to his telephone bell.</p>
-
-<p>Here comes in the use of the two lamps connected with
-the cords. As long as the subscribers’ telephones are on
-their rests the lamps are lighted, but as soon as they are
-lifted off the lamps go out. The caller’s telephone is of
-course off its rest, and so the lamp connected with the first
-cord is not lit; but until the subscriber rung up lifts his
-instrument to answer the call, the lamp of the second cord
-remains lit, having first lighted up when the plug was
-inserted in the jack of his number. When the second
-lamp goes out the operator knows that the call has been
-responded to, and that the two subscribers are in communication
-with each other. Having finished their conversation,
-both subscribers replace their instruments on the
-rests, whereupon both lamps light up, informing the operator
-that she may disconnect by pulling out the plugs.</p>
-
-<p>It is manifestly impossible for one operator to attend
-to the calls of all the subscribers in the exchange, and so a<span class="pagenum" id="Page_163">163</span>
-number of operators are employed, each one having to
-attend to the calls of a certain number of subscribers. At
-the same time it is clear that each operator may be called
-upon to connect one of her subscribers to any other subscriber
-in the whole exchange. In order to make this
-possible the switchboard is divided into sections, each
-having as many jacks as there are lines in the exchange, so
-that in this respect all the sections are multiples of each
-other, and the whole arrangement is called a “multiple
-switchboard,” the repeated jacks being called “multiple
-jacks.” Then there are other jacks which it is not necessary
-to duplicate. We have seen that when a subscriber calls the
-exchange a lamp glows, and the operator inserts a plug into
-the jack beside the lamp, in order to answer the call and
-ascertain what number is required. These are called
-“answering jacks,” and the lamp is the line signal. It is
-usual to have three operators to each section of the switchboard,
-and each operator has charge of so many answering
-jacks, representing so many subscribers. At the same
-time she has access to the whole section, so that she can
-connect any of her subscribers to any other line in the
-exchange.</p>
-
-<p>When a number is called for, the operator must be able
-to tell at once whether the line is free or not. The jack
-in her section may be unoccupied, but she must know also
-whether all the multiple jacks belonging to that number
-are free, for an operator at another section may have
-connected the line to one of her subscribers. To enable
-an operator to ascertain this quickly an electrical test is
-provided. When two lines are connected, the whole of the
-multiple jacks belonging to each are charged with electricity,
-and if an operator at any section touches one of these jacks
-with a plug, a current through her receiver makes a click,
-and on hearing the click she knows that the line is engaged.<span class="pagenum" id="Page_164">164</span>
-The testing takes an extremely short time, and this is why a
-caller receives the reply, “Number engaged,” so promptly
-that he feels inclined to doubt whether the operator has
-made any attempt at all to connect him up to the number.</p>
-
-<p>In order that an operator may have both hands free to
-manipulate the plugs, her telephone receiver is fixed over
-one ear by a fastening passing over her head, and the
-transmitter is hung from her shoulders so as to be close to
-her mouth.</p>
-
-<p>In telegraphy it is the rule to employ the earth for the
-return part of the circuit, but this is not customary in
-telephony. The telephone is a much more sensitive
-instrument than the telegraph, and a telephone having an
-earth return is subject to all kinds of strange and weird
-noises which greatly interfere with conversation. These
-noises may be caused by natural electrical disturbances, or
-by the proximity of telegraph and other wires conveying
-electric currents. On this account telephone lines are
-made with a complete metallic circuit. As in telegraphy,
-protection from lightning flashes is afforded by lightning
-arresters. The current for the working of a telephone
-exchange is supplied from a central battery of accumulators,
-and also from dynamos.</p>
-
-<figure id="plate_XII" class="figcenter" style="max-width: 40em;">
- <p class="caption">PLATE XII.</p>
- <img src="images/i_195.jpg" width="3146" height="2012" alt=" ">
- <figcaption class="caption">
-
-<p class="floatl"><i>By permission of</i></p>
-<p class="floatr"><i>Craven Brothers Ltd.</i></p>
-
-<p class="floatc">LARGE ELECTRIC TRAVELLING CRANE AT A RAILWAY WORKS.</p>
-</figcaption></figure>
-
-<p>Although the manual exchange telephone system of
-to-day works with remarkable efficiency, it has certain
-weak points. For instance, if an operator cares to do so,
-she can listen to conversations between subscribers, so
-that privacy cannot be assured. As a matter of fact, the
-operators have little time for this kind of thing, at any rate
-during the busy hours of the day, and as a rule they are
-not sufficiently interested in other people’s affairs to make
-any attempt to listen to their remarks. The male operators
-who work through the slack hours of the night are
-occasionally guilty of listening. Some time ago the writer
-had to ring up a friend in the very early morning, and
-during the conversation this gentleman asked what time it
-was. Before the writer had time to get a word out, a deep
-bass voice from the exchange replied, “Half-past two.”
-Little incidents of this sort remind one that it is not wise
-to speak too freely by telephone. Then again operators are
-liable to make wrong connexions through faulty hearing
-of the number called for, and these are equally annoying to
-the caller and to the person rung up in mistake. Many
-other defects might be mentioned, but these are sufficient
-to show that the manual system is not perfect.</p>
-
-<p>For a long time inventors have been striving to do
-away with all such defects by abolishing the exchange
-operators, and substituting mechanism to work the
-exchanges automatically, and during the last few years the
-system of the Automatic Electric Company, of Chicago,
-has been brought to great perfection. This system is in
-extensive use in the United States, and is employed in
-two or three exchanges in this country. Unfortunately
-the mechanism of this system is extremely complicated, so
-that it is impossible to describe it fully in a book of this
-kind; but some idea of the method of working may be
-given without entering into technical details.</p>
-
-<p>Each subscriber’s telephone instrument is fitted with a
-dial which turns round on a pivot at its centre. This dial
-has a series of holes round its circumference, numbered
-consecutively from 1 to 9, and 0. Suppose now a
-subscriber wishes to speak to a friend whose telephone
-number is 2583. He removes the receiver from its hook,
-places his finger in the hole marked 2, and turns the dial
-round in a clockwise direction until his finger comes in
-contact with a stop. He then removes his finger, and the
-dial automatically returns to its original position. He then
-places his finger in the hole marked 5, and again turns the<span class="pagenum" id="Page_166">166</span>
-dial as far as the stop, and when the dial has returned to
-the normal position he repeats the process with his finger
-placed successively in the holes marked 8 and 3. He now
-places the receiver to his ear, and by the time he has done
-this the automatic mechanism at the exchange has made
-the necessary connexions, and has rung the bell of
-subscriber number 2583. On completing the conversation
-each subscriber returns his receiver to its hook, and the
-exchange mechanism returns to its normal position.</p>
-
-<p>The turning of the dial by the finger coils up a spring,
-and this spring, acting along with a speed governor, makes
-the dial return to its first position at a certain definite
-speed as soon as the finger is removed. During this
-retrograde movement a switch automatically sends out into
-the line a certain number of impulses, the number being
-determined by the hole in which the finger is placed. In
-the case supposed, groups of two, five, eight, and three
-impulses respectively would be sent out, each group
-separated from the next by an interval during which the
-subscriber is turning the dial.</p>
-
-<p>Now let us see what takes place at the exchange.
-The subscriber’s instrument is connected to a mechanical
-arrangement known as a “line switch.” This switch
-is brought into play by the act of removing the receiver
-from its hook, and it then automatically connects the
-subscriber’s line to what is called a “first selector” switch.
-The group of two impulses sent out by the first turning of
-the dial raises this first selector two steps, and it then
-sweeps along a row of contacts connected to “trunks”
-going to the 2000 section. Passing by occupied trunks, it
-finds an idle one, and so connects the line to an idle
-“second selector.” This selector is operated by the second
-group of impulses, five in number, and after being raised
-five steps it acts like the first selector, and finds an idle<span class="pagenum" id="Page_167">167</span>
-trunk leading to the 2500 section. This places the caller’s
-line in connexion with still another switch called a
-“connector,” and this switch, operated by the remaining
-groups of eight and three impulses, finds the required tens
-section, and selects the third member of that section. If
-the number 2583 is disengaged, the connector switch now
-sends current from the central battery to this instrument,
-thus ringing its bell, and it also supplies speaking current
-to the two lines during the conversation, restores the
-exchange mechanism to its original condition as soon as
-the conversation is ended and the subscribers have hung
-up their receivers, and registers the call on the calling
-subscriber’s meter. If the connector finds the number
-engaged, it sends out an intermittent buzzing sound, to
-inform the caller of the fact. All these operations take
-time to describe, even in outline, but in practice they are
-carried out with the utmost rapidity, each step in the connecting-up
-process taking only a small fraction of a second.</p>
-
-<p>For ordinary local calls the automatic system requires
-no operators at all, but for the convenience of users there
-are usually two clerks at the exchange, one to give
-any information required by subscribers, and the other to
-record complaints regarding faulty working. For trunk
-calls, the subscriber places his finger in the hole marked 0,
-and gives the dial one turn. This connects him to an
-operator at the trunk switchboard, who makes the required
-connexion and then calls him up in the usual way.</p>
-
-<p>It might be thought that the complex mechanism of an
-automatic exchange would constantly be getting out of
-order, but it is found to work with great smoothness.
-Each automatic switchboard has a skilled electrician in
-attendance, and he is informed instantly of any faulty
-working by means of supervisory lamps and other signals.
-Even without these signals the attendant would be quickly<span class="pagenum" id="Page_168">168</span>
-aware of any breakdown, for his ear becomes so accustomed
-to the sounds made by the apparatus during the connecting-up,
-that any abnormal sound due to faulty connecting
-attracts his attention at once. However detected, the
-faults are put right immediately, and it often happens that
-a defective line is noted and repaired before the subscriber
-knows that anything is wrong.</p>
-
-<p>On account of its high speed in making connexions
-and disconnexions, its absolute accuracy, and its privacy,
-the automatic telephone system has proved most popular
-wherever it has been given a fair trial. Its advantages are
-most obvious in large city exchanges where the traffic
-during business hours is tremendously heavy, and it is
-probable that before very long the automatic system will
-have replaced manual methods for all such exchanges.</p>
-
-<p>The telephone system is more highly developed in the
-United States than in this country, and some of the
-exchanges have been made to do a great deal more than
-simply transmit messages. For instance, in Chicago there
-is a system by which a subscriber, on connecting himself to
-a special circuit, is automatically informed of the correct
-time, by means of phonographs, between the hours of 8
-a.m. and 10 p.m. New York goes further than this however,
-and has a regular system of news circulation by telephone.
-According to <cite>Electricity</cite>, the daily programme is
-as follows: “8 a.m., exact astronomical time; 8 to 9 a.m.,
-weather reports, London Stock Exchange news, special
-news item; 9 to 9.30 a.m., sales, amusements, business
-events; 9.45 to 10 a.m., personal news, small notices; 10
-to 10.30 a.m., New York Stock Exchange and market
-news; 11.30 a.m. to 12 noon, local news, miscellaneous;
-12 noon, exact astronomical time, latest telegrams, military
-and parliamentary news; 2 to 2.15 p.m., European cables;
-1.15 to 2.30 p.m., Washington news; 2.30 to 2.45 p.m.,<span class="pagenum" id="Page_169">169</span>
-fashions, ladies’ news; 2.45 to 3.15 p.m., sporting and
-theatrical news; 3.15 to 3.30 p.m., closing news from Wall
-Street; 3.30 to 5 p.m., musical news, recitals, etc.; 5 to 6
-p.m., feuilleton sketches, literary news; 8 to 10.30 p.m.,
-selected evening performance—music, opera, recitations.”
-Considering the elaborate nature of this scheme one might
-imagine that the subscription would be high, but as a
-matter of fact it is only six shillings per month.</p>
-
-<p>The telephone has proved of great value in mine rescue
-work, in providing means of communication between the
-rescue party and those in the rear. This end is achieved
-by means of a portable telephone, but as the members of a
-rescue party often wear oxygen helmets, the ordinary telephone
-mouthpiece is of no use. To overcome this difficulty
-the transmitter is fastened round the throat. The vibrations
-of the vocal cords pass through the wall of the throat,
-and thus operate the transmitter. The receiver is fixed
-over one ear by means of suitable head-gear, and the connecting
-wire is laid by the advancing rescuers. A case
-containing some hundreds of feet of wire is strapped round
-the waist, and as the wearer walks forward this wire pays
-itself out automatically.</p>
-
-<p>By the time that the telephone came to be a really
-practical instrument, capable of communicating over long
-distances on land, the Atlantic telegraph cable was in
-operation, and an attempt was made to telephone from one
-continent to the other by means of it, but without success.
-In speaking of submarine telegraphy in <a href="#chapter_XVII">Chapter XVII</a>. we
-saw that the cable acts like a Leyden jar, and it was this
-fact that made it impossible to telephone through more than
-about 20 miles of cable, so that transatlantic telephony
-was quite out of the question. It was evident that little
-progress could be made in this direction unless some means
-could be devised for neutralizing this capacity effect, as it<span class="pagenum" id="Page_170">170</span>
-is called, of the cable, and finally it was discovered that
-this could be done by inserting at intervals along the cable
-a number of coils of wire. These coils are known as “loading
-coils,” and a cable provided with them is called a “loaded
-cable.” Such cables have been laid across various narrow
-seas, such as between England and France, and England
-and Ireland, and these have proved very successful for
-telephonic communication. The problem of transatlantic
-telephony however still remains to be solved. Experiments
-have been made in submarine telephony over a bare
-iron cable, instead of the usual insulated cable. Conversations
-have been carried on in this way without difficulty
-between Seattle, Washington, U.S.A., and Vashon
-Island, a total distance of about 11 miles, and it is
-possible that uninsulated cables may play an extremely
-important part in the development of submarine telephony.</p>
-<hr class="chap x-ebookmaker-drop">
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_171">171</span></p>
-
-<h2 class="nobreak" id="toclink_171"><a id="chapter_XIX"></a>CHAPTER XIX<br>
-
-<span class="subhead">SOME TELEGRAPHIC AND TELEPHONIC INVENTIONS</span></h2>
-</div>
-
-<p class="in0"><span class="firstword">In</span> telegraphy messages not only may be received, but also
-recorded, by the Morse printer or one of its modifications,
-but in ordinary telephony there is no mechanical method of
-recording messages. This means that we can communicate
-by telephone only when we can call up somebody to receive
-the message at the other end, and if no one happens to be
-within hearing of the telephone bell we are quite helpless.
-This is always annoying, and if the message is urgent the
-delay may be serious. Several arrangements for overcoming
-this difficulty by means of automatic recording
-mechanism have been invented, but the only really successful
-one is the telegraphone.</p>
-
-<p>This instrument is the invention of Waldemar Poulsen,
-whose apparatus for wireless telegraphy we shall speak of
-in the next chapter. The telegraphone performs at the
-same time the work of a telephone and of a phonograph.
-In the ordinary type of phonograph the record is made in
-the form of depressions or indentations on the surface of a
-cylinder of wax; these indentations being produced by a
-stylus actuated by vibrations set up in a diaphragm by the
-act of speaking. In the telegraphone the same result is
-obtained entirely by electro-magnetic action. The wax
-cylinder of the phonograph is replaced by a steel wire or
-ribbon, and the recording stylus by an electro-magnet.<span class="pagenum" id="Page_172">172</span>
-The steel ribbon is arranged to travel along over two
-cylinders or reels kept in constant rotation, and a small
-electro-magnet is fixed midway between the cylinders so
-that the ribbon passes close above it. This magnet is
-connected to the telephone line, so that its magnetism
-fluctuates in accordance with the variations in the current
-in the line. We have seen that steel retains magnetism
-imparted to it. In passing over the electro-magnet the
-steel ribbon is magnetized in constantly varying degrees,
-corresponding exactly with the variations in the line current
-set up by the speaker’s voice, and these magnetic impressions
-are retained by the ribbon. When the speaker has
-finished, the telephone line is disconnected, the ribbon is
-carried back to the point at which it started, and the
-apparatus is connected to the telephone receiver. The
-ribbon now moves forward again, and this time it acts like
-the speaker’s voice, the varying intensity of its magnetic
-record producing corresponding variations in the strength
-of the magnet, so causing the receiver diaphragm to reproduce
-the sounds in the ordinary way.</p>
-
-<p>The magnetic record made in this manner is fairly
-permanent, and if desired it may be reproduced over and
-over again. In most cases, however, a permanent record
-is of no value, and so the magnetic impressions are
-obliterated in order that the ribbon may be used to take
-a new record. This can be done by passing a permanent
-magnet along the ribbon, but it is more convenient to have
-an automatic obliterating arrangement. This consists of
-another electro-magnet fixed close to the recording magnet,
-so that the ribbon passes over it before reaching the latter.
-The obliterating magnet is connected with a battery, and
-its unvarying magnetism destroys all traces of the previous
-record, and the ribbon passes forward to the recording
-magnet ready to receive new impressions.</p>
-
-<p><span class="pagenum" id="Page_173">173</span></p>
-
-<p>For recording telephone messages the telegraphone is
-attached to the telephone instrument, and by automatically
-operated switches it is set working by a distant speaker.
-It records all messages received during the absence of its
-owner, who, on his return, connects it to his receiver, and
-thus hears a faithful reproduction of every word. By
-speaking into his instrument before going out, the owner
-can leave a message stating the time at which he expects
-to return, and this message will be repeated by the telegraphone
-to anybody ringing up in the meantime. The
-most recent forms of telegraphone are capable of recording
-speeches over an hour in length, and their reproduction is
-as clear as that of any phonograph, indeed in many respects
-it is considerably more perfect.</p>
-
-<p>Another electrical apparatus for recording speech may
-be mentioned. This rejoices in the uncouth name of the
-Photographophone, and it is the invention of Ernst
-Ruhmer, a German. Its working is based upon the fact
-that the intensity of the light of the electric arc may be
-varied by sound vibrations, each variation in the latter
-producing a corresponding variation in the amount of light.
-In the photographophone the light of an arc lamp is passed
-through a lens which focuses it upon a moving photographic
-film. By speaking or singing, the light is made to vary in
-brilliance, and proportionate effects are produced in the
-silver bromide of the film. On developing the film a
-permanent record of the changes in the light intensity is
-obtained, in the form of shadings of different degrees of
-darkness. The film is now moved forward from end to
-end in front of a fairly powerful lamp. The light passes
-through the film, and falls upon a sort of plate made of
-selenium. This is a non-metallic substance which possesses
-the curious property of altering its resistance to an electric
-current according to the amount of light falling upon it;<span class="pagenum" id="Page_174">174</span>
-the greater the amount of light, the more current will the
-selenium allow to pass. The selenium plate is connected
-with a telephone receiver and with a battery. As the film
-travels along, its varying shadings allow an ever-changing
-amount of light to pass through and fall upon the selenium,
-which varies its resistance accordingly. The resulting
-variations in the current make the receiver diaphragm give
-out a series of sounds, which are exact reproductions of the
-original sounds made by the voice. The reproduction of
-speech by the photographophone is quite good, but as a
-rule it is not so perfect as with the telegraphone.</p>
-
-<p>About ten years ago a German inventor, Professor A.
-Korn, brought out the first really practical method of
-telegraphing drawings or photographs. This invention is
-remarkable not only for what it accomplishes, but perhaps
-still more for the ingenuity with which the many peculiar
-difficulties of the process are overcome. Like the photographophone,
-Korn’s photo-telegraphic apparatus utilizes
-the power of selenium to alter its resistance with the amount
-of light reaching it.</p>
-
-<p>Almost everybody is familiar with the terms “positive”
-and “negative” as used in photography. The finished paper
-print is a positive, with light and shade in the correct
-positions; while the glass plate from which the print is made
-is a negative, with light and shade reversed. The lantern
-slide also is a positive, and it is exactly like the paper print,
-except that it has a base of glass instead of paper, so that
-it is transparent. Similarly, a positive may be made on a
-piece of celluloid, and this, besides being transparent, is
-flexible. The first step in transmitting on the Korn system
-is to make from the photograph to be telegraphed a positive
-of this kind, both transparent and flexible. This is bent
-round a glass drum or cylinder, and fixed so that it cannot
-possibly move. The cylinder is given a twofold movement.<span class="pagenum" id="Page_175">175</span>
-It is rotated by means of an electric motor, and at
-the same time it is made to travel slowly along in the
-direction of its length. In fact its movement is very
-similar to that of a screw, which turns round and moves
-forward at the same time. A powerful beam of light is
-concentrated upon the positive. This beam remains
-stationary, but owing to the dual movement of the cylinder
-it passes over every part of the positive, following a spiral
-path. Exactly the same effect would be produced by
-keeping the cylinder still and moving the beam spirally
-round it, but this arrangement would be more difficult to
-manipulate. The forward movement of the cylinder is
-extremely small, so that the spiral is as fine as it is possible
-to get it without having adjacent lines actually touching.
-The light passes through the positive into the cylinder, and
-is reflected towards a selenium cell; and as the positive
-has an almost infinite number of gradations of tone, or
-degrees of light and shade, the amount of light reaching
-the cell varies constantly all the time. The selenium
-therefore alters its resistance, and allows a constantly
-varying current to pass through it, and so to the transmission
-line.</p>
-
-<p>At the receiving end is another cylinder having the
-same rotating and forward movement, and round this is
-fixed a sensitive photographic film. This film is protected
-by a screen having a small opening, and no light can reach
-it except through this aperture. The incoming current is
-made to control a beam of light focused to fall upon the
-screen aperture, the amount of light varying according to
-the amount of current. In this way the beam of light, like
-the one at the transmitting end, traces a spiral from end to
-end of the film, and on developing the film a reproduction
-of the original photograph is obtained. The telegraphed
-photograph is thus made up of an enormous number<span class="pagenum" id="Page_176">176</span>
-of lines side by side, but these are so close to one
-another that they are scarcely noticed, and the effect is
-something like that of a rather coarse-grained ordinary
-photograph.</p>
-
-<p>It is obvious that the success of this method depends
-upon the maintaining of absolute uniformity in the motion
-of the two cylinders, and this is managed in a very ingenious
-way. It will be remembered that one method of securing
-uniformity in a number of sub-clocks under the control of
-a master-clock is that of adjusting the sub-clocks to go a
-little faster than the master-clock. Then, when the sub-clocks
-reach the hour, they are held back by electro-magnetic
-action until the master-clock arrives at the hour,
-when all proceed together.</p>
-
-<p>A similar method is employed for the cylinders. They
-are driven by electric motors, and the motor at the receiving
-end is adjusted so as to run very slightly faster
-than the motor at the sending end. The result is that
-the receiving cylinder completes one revolution a minute
-fraction of a second before the transmitting cylinder. It
-is then automatically held back until the sending cylinder
-completes its revolution, and then both commence the next
-revolution exactly together. The pause made by the
-receiving cylinder is of extremely short duration, but in
-order that there shall be no break in the spiral traced by
-light upon the film, the pause takes place at the point
-where the ends of the film come together. In actual
-practice certain other details of adjustment are required
-to ensure precision in working, but the main features of
-the process are as described.</p>
-
-<p>Although the above photo-telegraphic process is very
-satisfactory in working, it has been superseded to some
-extent by another process of a quite different nature. By
-copying the original photograph through a glass screen<span class="pagenum" id="Page_177">177</span>
-covered with a multitude of very fine parallel lines, a half-tone
-reproduction is made. This is formed of an immense
-number of light and dark lines of varying breadth, and it
-is printed in non-conducting ink on lead-foil, so that while
-the dark lines are bare foil, the light ones are covered with
-the ink. This half-tone is placed round a metal cylinder
-having the same movement as the cylinders in the previous
-processes, and a metal point, or “stylus” as it is called, is
-made to rest lightly upon the foil picture, so that it travels
-all over it, from one end to the other. An electrical circuit
-is arranged so that when the stylus touches a piece of the
-bare foil a current is sent out along the line wire. This
-current is therefore intermittent, being interrupted each
-time the stylus passes over a part of the half-tone picture
-covered with the non-conducting ink, the succeeding
-periods of current and no current varying with the breadth
-of the conducting and the non-conducting lines. This
-intermittent current goes to a similar arrangement of
-stylus and cylinder at the receiving end, this cylinder
-having round it a sheet of paper coated with a chemical
-preparation. The coating is white all over to begin with,
-but it turns black wherever the current passes through it.
-The final result is that the intermittent current builds up
-a reproduction in black-and-white of the original photograph.
-In this process also the cylinders have to be
-“synchronized,” or adjusted to run at the same speed.
-Both this process and the foregoing one have been used
-successfully for the transmission of press photographs,
-notably by the <cite>Daily Mirror</cite>.</p>
-
-<p>Professor Korn has carried out some interesting and
-fairly successful experiments in wireless transmission of
-photographs, but as yet the wireless results are considerably
-inferior to those obtained with a line conductor. For
-transmitting black-and-white pictures, line drawings, or<span class="pagenum" id="Page_178">178</span>
-autographs by wireless, a combination of the two methods
-just mentioned is employed; the second method being
-used for sending, and the first or selenium method for
-receiving. For true half-tone pictures the selenium method
-is used at each end.</p>
-<hr class="chap x-ebookmaker-drop">
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_179">179</span></p>
-
-<h2 class="nobreak" id="toclink_179"><a id="chapter_XX"></a>CHAPTER XX<br>
-
-<span class="subhead">WIRELESS TELEGRAPHY AND TELEPHONY—PRINCIPLES AND APPARATUS</span></h2>
-</div>
-
-<p class="in0"><span class="firstword">Wireless</span> telegraphy is probably the most remarkable and
-at the same time the most interesting of all the varied
-applications of electricity. The exceptional popular
-interest in wireless communication, as compared with most
-of the other daily tasks which electricity is called upon
-to perform, is easy to understand. The average man does
-not realize that although we are able to make electricity
-come and go at our bidding, we have little certain knowledge
-of its nature. He is so accustomed to hearing of
-the electric current, and of the work it is made to do, that
-he sees little to marvel at so long as there is a connecting
-wire. Electricity is produced by batteries or by a dynamo,
-sent along a wire, and made to drive the necessary
-machinery; apparently it is all quite simple. But take
-away the connecting wire, and the case is different. In
-wireless telegraphy electricity is produced as usual, but
-instantly it passes out into the unknown, and, as far as
-our senses can tell, it is lost for ever. Yet at some
-distant point, hundreds or even thousands of miles away,
-the electrical influence reappears, emerging from the
-unknown with its burden of words and sentences. There
-is something uncanny about this, something suggesting
-telepathy and the occult, and herein lies the fascination of
-wireless telegraphy.</p>
-
-<p><span class="pagenum" id="Page_180">180</span></p>
-
-<p>The idea of communicating without any connecting
-wires is an old one. About the year 1842, Morse, of telegraph
-fame, succeeded in transmitting telegraphic signals
-across rivers and canals without a connecting wire. His
-method was to stretch along each bank of the river a wire
-equal in length to three times the breadth of the river.
-One of these wires was connected with the transmitter and
-with a battery, and the other with a receiver, both wires
-terminating in copper plates sunk in the water. In this
-case the water took the place of a connecting wire, and
-acted as the conducting medium. A few years later
-another investigator, a Scotchman named Lindsay, succeeded
-in telegraphing across the river Tay, at a point
-where it is over a mile and a half wide, by similar methods.
-Lindsay appears to have been the first to suggest the possibility
-of telegraphing across the Atlantic, and although at
-that time, 1845, the idea must have seemed a wild one, he
-had the firmest faith in its ultimate accomplishment.</p>
-
-<p>Amongst those who followed Lindsay’s experiments
-with keen interest was the late Sir William, then Mr.
-Preece, but it was not until 1882, twenty years after
-Lindsay’s death, that he commenced experiments on his
-own account. In March of that year the cable across the
-Solent failed, and Preece took the opportunity of trying to
-signal across without a connecting wire. He used two
-overhead wires, each terminating in large copper plates
-sunk in the sea, one stretching from Southampton to
-Southsea Pier, and the other from Ryde Pier to Sconce
-Point. The experiment was successful, audible Morse
-signals being received on each side. In this experiment,
-as in those of Morse and Lindsay, the water acted as the
-conducting medium; but a year or two later, Preece
-turned his attention to a different method of wireless communication,
-by means of induction. This method was<span class="pagenum" id="Page_181">181</span>
-based upon the fact that at the instant of starting and
-stopping a current in one wire, another current is induced
-in a second wire placed parallel to it, even when the two
-wires are a considerable distance apart. Many successful
-experiments in this induction telegraphy were made, one
-of the most striking being that between the Island of Mull
-and the mainland, in 1895. The cable between the island
-and the mainland had broken, and by means of induction
-perfect telegraphic communication was maintained during
-the time that the cable was being repaired. Although this
-system of wireless telegraphy is quite successful for short
-distances, it becomes impracticable when the distance is
-increased, because the length of each of the two parallel
-wires must be roughly equal to the distance between them.
-These experiments of Preece are of great interest, but we
-must leave them because they have little connexion with
-present-day wireless telegraphy, in which utterly different
-methods are used.</p>
-
-<p>All the commercial wireless systems of to-day depend
-upon the production and transmission of electric waves.
-About the year 1837 it was discovered that the discharge
-of a Leyden jar did not consist of only one sudden rush of
-electricity, but of a series of electric oscillations, which
-surged backwards and forwards until electric equilibrium
-was restored. This discovery was verified by later
-experimenters, and it forms the foundation of our knowledge
-of electric waves. At this point many readers probably
-will ask, “What are electric waves?” It is impossible to
-answer this question fully, for we still have a great deal to
-learn about these waves, and we only can state the conclusions
-at which our greatest scientists have arrived after
-much thought and many experiments. It is believed that
-all space is filled with a medium to which the name
-“ether” has been given, and that this ether extends<span class="pagenum" id="Page_182">182</span>
-throughout the matter. We do not know what the ether
-is, but the important fact is that it can receive and transmit
-vibrations in the form of ether waves. There are different
-kinds of ether waves, and they produce entirely different
-effects. Some of them produce the effect which we call
-light, and these are called “light waves.” Others produce
-the effect known as heat, and they are called “heat waves”;
-and still others produce electricity, and these we call
-“electric waves.” These waves travel through the ether at
-the enormous speed of 186,000 miles per second, so that
-they would cross the Atlantic Ocean in about 1/80 second.
-The fact that light also travels at this speed suggested that
-there might be some connexion between the two sets of
-waves, and after much experiment it has been demonstrated
-that the waves of light and electricity are identical except
-in their length.</p>
-
-<p>Later on in this chapter we shall have occasion to refer
-frequently to wave-length, and we may take this opportunity
-of explaining what is understood by this term. Wave-length
-is the distance measured from the crest of one wave
-to the crest of the next, across the intervening trough or
-hollow. From this it will be seen that the greater the
-wave-length, the farther apart are the waves; and also that
-if we have two sets of waves of different wave-lengths but
-travelling at the same speed, then the number of waves
-arriving at any point in one second will be greater in the
-case of the shorter waves, because these are closer together.</p>
-
-<p>A tuning-fork in vibration disturbs the surrounding air,
-and sets up air waves which produce the effect called sound
-when they strike against the drums of our ears. In a
-similar way the discharge of a Leyden jar disturbs the
-surrounding ether, and sets up electric ether waves; but
-these waves produce no effect upon us in the shape of sight,
-sound, or feeling. There is however a very simple piece<span class="pagenum" id="Page_183">183</span>
-of apparatus which acts as a sort of electric eye or ear, and
-detects the waves for us. This consists of a glass tube
-loosely filled with metal filings, and having a cork at each
-end. A wire is passed through each cork so as to project
-well into the tube, but so that the two ends do not touch
-one another, and the outer ends of these wires are connected
-to a battery of one or two cells, and to some kind of
-electrically worked apparatus, such as an electric bell. So
-long as the filings lie quite loosely in the tube they offer
-a very high resistance, and no current passes. If now
-electric waves are set up by the discharge of a Leyden jar,
-these waves fall upon the tube and cause the resistance
-of the filings to decrease greatly. The filings now form a
-conducting path through which the current passes, and so
-the bell rings. If no further discharge takes place the
-electric waves cease, but the filings do not return to their
-original highly resistant condition, but retain their conductivity,
-and the current continues to pass, and the bell
-goes on ringing. To stop the bell it is only necessary
-to tap the tube gently, when the filings immediately fall
-back into their first state, so that the current cannot pass
-through them.</p>
-
-<p>Now let us see how the “coherer,” as the filings tube is
-called, is used in actual wireless telegraphy. <a href="#fig_33">Fig. 33<i>a</i></a>
-shows a simple arrangement for the purpose. A is an
-induction coil, and B the battery supplying the current.
-The coil is fitted with a spark gap, consisting of two
-highly polished brass balls CC, one of these balls being
-connected to a vertical wire supported by a pole, and the
-other to earth. D is a Morse key for starting and stopping
-the current. When the key is pressed down, current flows
-from the battery to the coil, and in passing through the
-coil it is raised to a very high voltage, as described in
-<a href="#chapter_VIII">Chapter VIII</a>. This high tension current is sent into the<span class="pagenum" id="Page_184">184</span>
-aerial wire, which quickly becomes charged up to its
-utmost limits. But more current continues to arrive, and
-so the electricity in the aerial, unable to bear any longer
-the enormous pressure, takes the only path of escape and
-bursts violently across the air gap separating the brass
-balls. Surging oscillations are then produced in the aerial,
-the ether is violently disturbed, and electric waves are
-set in motion. This is the transmitting part of the
-apparatus.</p>
-
-<figure id="fig_33" class="figcenter" style="max-width: 25em;">
- <img src="images/i_216.png" width="2000" height="1836" alt=" ">
- <figcaption class="caption"><p><i>a.</i> Transmitting.
- <span class="in4"><i>b.</i> Receiving.</span></p>
-
-<p><span class="smcap">Fig. 33.</span>—Diagram of simple Wireless Transmitting and Receiving Apparatus.</p>
-</figcaption></figure>
-
-<p>If a stone is dropped into a pond, little waves are set in<span class="pagenum" id="Page_185">185</span>
-motion, and these spread outwards in ever-widening rings.
-Electric waves also are propagated outwards in widening
-rings, but instead of travelling in one plane only, like the
-water waves, they proceed in every plane; and when they
-arrive at the receiving aerial they set up in it oscillations
-of the same nature as those which produced the waves.
-Let us suppose electric waves to reach the aerial wire of
-<a href="#fig_33">Fig. 33<i>b</i></a>. The resistance of the coherer H is at once lowered
-so that current from battery N flows and operates the relay
-F, which closes the circuit of battery M. This battery
-has a twofold task. It operates the sounder E, and it
-energizes the electro-magnet of the de-coherer K, as shown
-by the dotted lines. This de-coherer is simply an electric
-bell without the gong, arranged so that the hammer strikes
-the coherer tube; and its purpose is to tap the tube
-automatically and much more rapidly than is possible by
-hand. The sounder therefore gives a click, and the de-coherer
-taps the tube, restoring the resistance of the
-filings. The circuit of battery N is then broken, and the
-relay therefore interrupts the circuit of battery M. If
-waves continue to arrive, the circuits are again closed,
-another click is given, and again the hammer taps the
-tube. As long as waves are falling upon the aerial, the
-alternate makings and breakings of the circuits follow one
-another very rapidly and the sounder goes on working.
-When the waves cease, the hammer of the de-coherer has
-the last word, and the circuits of both batteries remain
-broken. To confine the electric waves to their proper
-sphere two coils of wire, LL, called choking coils, are
-inserted as shown.</p>
-
-<p>In this simple apparatus we have all the really essential
-features of a wireless installation for short distances. For
-long distance work various modifications are necessary,
-but the principle remains exactly the same. In land wireless<span class="pagenum" id="Page_186">186</span>
-stations the single vertical aerial wire becomes an
-elaborate arrangement of wires carried on huge masts and
-towers. The distance over which signals can be transmitted
-and received depends to a considerable extent upon
-the height of the aerial, and consequently land stations
-have the supporting masts or towers from one to several
-hundred feet in height, according to the range over which
-it is desired to work. As a rule the same aerial is used both
-for transmitting and receiving, but some stations have a
-separate aerial for each purpose. A good idea of the
-appearance of commercial aerials for long distance working
-may be obtained from the frontispiece, which shows the
-Marconi station at Glace Bay, Nova Scotia, from which
-wireless communication is held with the Marconi station at
-Clifden, in Galway, Ireland.</p>
-
-<p>In the first wireless stations what is called a “plain
-aerial” transmitter was used, and this was almost the same
-as the transmitting apparatus in <a href="#fig_33">Fig. 33<i>a</i></a>, except, of course,
-that it was on a larger scale. This arrangement had many
-serious drawbacks, including that of a very limited range,
-and it has been abandoned in favour of the “coupled”
-transmitter, a sketch of which is shown in <a href="#fig_34">Fig. 34</a>. In this
-transmitter there are two separate circuits, having the same
-rate of oscillation. A is an induction coil, supplied with
-current from the battery B, and C is a condenser. A
-condenser is simply an apparatus for storing up charges of
-electricity. It may take a variety of forms, but in every
-case it must consist of two conducting layers separated by
-a non-conducting layer, the latter being called the
-“dielectric.” The Leyden jar is a condenser, with conducting
-layers of tinfoil and a dielectric of glass, but the
-condensers used for wireless purposes generally consist of
-a number of parallel sheets of metal separated by glass or
-mica, or in some cases by air only. The induction coil<span class="pagenum" id="Page_187">187</span>
-charges up the condenser with high tension electricity, until
-the pressure becomes so great that the electricity is
-discharged in the form of a spark between the brass balls
-of the spark gap D. The accumulated electric energy in
-the condenser then surges violently backwards and forwards,
-and by induction corresponding surgings are produced in
-the aerial circuit, these latter surgings setting up electric
-waves in the ether.</p>
-
-<figure id="fig_34" class="figcenter" style="max-width: 20em;">
- <img src="images/i_219.png" width="1573" height="1941" alt=" ">
- <figcaption class="caption"><span class="smcap">Fig. 34.</span>—Wireless “Coupled” Transmitter.
-</figcaption></figure>
-
-<p>For the sake of simplicity we have represented the
-apparatus as using an induction coil, but in all stations of
-any size the coil is replaced by a step-up transformer, and<span class="pagenum" id="Page_188">188</span>
-the current is supplied either from an electric light power
-station at some town near by, or from a power house specially
-built for the purpose. Alternating current is generally used,
-and if the current supplied is continuous, it is converted into
-alternating current. This may be done by making the
-continuous current drive an electric motor, which in turn
-drives a dynamo generating alternating current. In any
-case, the original current is too low in voltage to be used
-directly, but in passing through the transformer it is raised
-to the required high pressure. The transmitting key,
-which is inserted between the dynamo and the transformer,
-is specially constructed to prevent the operator from receiving
-accidental shocks, and the spark gap is enclosed in a
-sort of sound-proof box, to deaden the miniature thunders
-of the discharge.</p>
-
-<p>During the time that signals are being transmitted,
-sparks follow one another across the spark gap in rapid
-succession, a thousand sparks per second being by no
-means an uncommon rate. The violence of these rapid
-discharges raises the brass balls of the gap to a great heat.
-This has the effect of making the sparking spasmodic and
-uncertain, with the result that the signals at the receiving
-station are unsatisfactory. To get over this difficulty
-Marconi introduced a rotary spark gap. This is a wheel
-with projecting knobs or studs, mounted on the shaft of the
-dynamo supplying the current, so that it rotates rapidly.
-Two stationary knobs are fixed so that the wheel rotates
-between them, and the sparks are produced between these
-fixed knobs and those of the wheel, a double spark gap
-thus being formed. Overheating is prevented by the
-currents of air set up by the rapid movement of the wheel,
-and the sparking is always regular.</p>
-
-<figure id="plate_XIIIa" class="figcenter" style="max-width: 26em;">
- <p class="caption">PLATE XIII.</p>
- <img src="images/i_221.jpg" width="2076" height="1403" alt=" ">
- <figcaption class="caption">
-
-<p class="floatl"><i>Photo by</i></p>
-<p class="floatr"><i>Daily Mirror</i>.</p>
-
-<p class="floatc">(<i>a</i>) MARCONI OPERATOR RECEIVING A MESSAGE.</p>
-</figcaption></figure>
-
-<figure id="plate_XIIIb" class="figcenter" style="max-width: 27em;">
- <img src="images/i_221b.jpg" width="2081" height="1202" alt=" ">
- <figcaption class="caption">
-
-<p class="floatl"><i>By permission of</i></p>
-<p class="floatr"><i>The Marconi Co. Ltd.</i></p>
-
-<p class="floatc">(<i>b</i>) MARCONI MAGNETIC DETECTOR.</p>
-</figcaption></figure>
-
-<p>In the receiving apparatus already described a filings
-coherer was used to detect the ether waves, and, by means
-of a local battery, to translate them into audible signals with
-a sounder, or printed signals with a Morse inker. This
-coherer however is unsuitable for commercial working.
-It is not sufficiently sensitive, and it can be used only for
-comparatively short distances; while its action is so slow
-that the maximum speed of signalling is not more than
-about seventeen or eighteen words a minute. A number
-of different detectors of much greater speed and sensitiveness
-have been devised. The most reliable of these,
-though not the most sensitive, is the Marconi magnetic detector,
-<a href="#plate_XIIIb">Plate XIII.<i>b</i></a>. This consists of a moving band made
-of several soft iron wires twisted together, and passing close
-to the poles of two horse-shoe magnets. As the band
-passes from the influence of one magnet to that of the other
-its magnetism becomes reversed, but the change takes a
-certain amount of time to complete owing to the fact that
-the iron has some magnetic retaining power, so that it
-resists slightly the efforts of one magnet to reverse the
-effect of the other. The moving band passes through two
-small coils of wire, one connected with the aerial, and the
-other with a specially sensitive telephone receiver. When
-the electric waves from the transmitting station fall upon
-the aerial of the receiving station, small, rapidly oscillating
-currents pass through the first coil, and these have the
-effect of making the band reverse its magnetism instantly.
-The sudden moving of the lines of magnetic force induces
-a current in the second coil, and produces a click in the
-telephone. As long as the waves continue, the clicks
-follow one another rapidly, and they are broken up into the
-long and short signals of the Morse code according to the
-manipulation of the Morse key at the sending station.
-Except for winding up at intervals the clockwork mechanism
-which drives the moving band, this detector requires no
-attention, and it is always ready for work.</p>
-
-<p><span class="pagenum" id="Page_190">190</span></p>
-
-<p>Another form of detector makes use of the peculiar
-power possessed by certain crystals to rectify the oscillatory
-currents received from the aerial, converting them into
-uni-directional currents. At every discharge of the condenser
-at the sending station a number of complete waves,
-forming what is called a “train” of waves, is set in motion.
-From each train of waves the crystal detector produces one
-uni-directional pulsation of current, and this causes a click
-in the telephone receiver. If these single pulsations follow
-one another rapidly and regularly, a musical note is heard
-in the receiver. Various combinations of crystals, and
-crystals and metal points, are used, but all work in the
-same way. Some combinations work without assistance,
-but others require to have a small current passed through
-them from a local battery. The crystals are held in small
-cups of brass or copper, mounted so that they can be
-adjusted by means of set-screws. Crystal detectors are
-extremely sensitive, but they require very accurate adjustment,
-and any vibration quickly throws them out of order.</p>
-
-<p>The “electrolytic” detector rectifies the oscillating
-currents in a different manner. One form consists of a thin
-platinum wire passing down into a vessel made of lead,
-and containing a weak solution of sulphuric acid. The
-two terminals of a battery are connected to the wire and
-the vessel respectively. As long as no oscillations are
-received from the aerial the current is unable to flow
-between the wire and the vessel, but when the oscillations
-reach the detector the current at once passes, and operates
-the telephone receiver. The action of this detector is not
-thoroughly understood, and the way in which the point of
-the platinum wire prevents the passing of the current until
-the oscillations arrive from the aerial is something of a
-mystery.</p>
-
-<p>The last detector that need be described is the Fleming<span class="pagenum" id="Page_191">191</span>
-valve receiver. This consists of an electric incandescent
-lamp, with either carbon or tungsten filament, into which
-is sealed a plate of platinum connected with a terminal outside
-the lamp. The plate and the filament do not touch
-one another, but when the lamp is lighted up a current can
-be passed from the plate to the filament, but not from filament
-to plate. This receiver acts in a similar way to the
-crystal detector, making the oscillating currents into uni-directional
-currents. It has proved a great success for
-transatlantic wireless communication between the Marconi
-stations at Clifden and Glace Bay, and is extensively used.</p>
-
-<p>The electric waves set in motion by the transmitting
-apparatus of a wireless station spread outwards through
-the ether in all directions, and so instead of reaching only
-the aerial of the particular station with which it is desired
-to communicate, they affect the aerials of all stations within
-a certain range. So long as only one station is sending
-messages this causes no trouble; but when, as is actually
-the case, large numbers of stations are hard at work transmitting
-different messages at the same time, it is evident
-that unless something can be done to prevent it, each of
-these messages will be received at the same moment by
-every station within range, thus producing a hopeless confusion
-of signals from which not a single message can be
-read. Fortunately this chaos can be avoided by what is
-called “tuning.”</p>
-
-<p>Wireless tuning consists in adjusting the aerial of the
-receiving station so that it has the same natural rate of
-oscillation as that of the transmitting station. A simple
-experiment will make clearer the meaning of this. If we
-strike a tuning-fork, so that it sounds its note, and while it
-is sounding strongly place near it another fork of the same
-pitch and one of a different pitch, we find that the fork of
-similar pitch also begins to sound faintly, whereas the third<span class="pagenum" id="Page_192">192</span>
-fork remains silent. The explanation is that the two forks
-of similar pitch have the same natural rate of vibration,
-while the other fork vibrates at a different rate. When
-the first fork is struck, it vibrates at a certain rate, and sets
-in motion air waves of a certain length. These waves
-reach both the other forks, but their effect is different in
-each case. On reaching the fork of similar pitch the first
-wave sets it vibrating, but not sufficiently to give out a
-sound. But following this wave come others, and as the
-fork has the same rate of vibration as the fork which
-produced the waves, each wave arrives just at the right
-moment to add its impulse to that of the preceding wave,
-so that the effect accumulates and the fork sounds. In the
-case of the third fork of different pitch, the first wave sets
-it also vibrating, but as this fork cannot vibrate at the same
-rate as the one producing the waves, the latter arrive at
-wrong intervals; and instead of adding together their
-impulses they interfere with one another, each upsetting
-the work of the one before it, and the fork does not sound.
-The same thing may be illustrated with a pendulum. If
-we give a pendulum a gentle push at intervals corresponding
-to its natural rate of swing, the effects of all these
-pushes are added together, and the pendulum is made to
-swing vigorously. If, on the other hand, we give the pushes
-at longer or shorter intervals, they will not correspond with
-the pendulum’s rate of swing, so that while some pushes
-will help the pendulum, others will hinder it, and the final
-result will be that the pendulum is brought almost to a
-standstill, instead of being made to swing strongly and
-regularly. The same principle holds good with wireless
-aerials. Any aerial will respond readily to all other aerials
-having the same rate of oscillation, because the waves in
-each case are of the same length; that is to say, they follow
-one another at the same intervals. On the other hand, an<span class="pagenum" id="Page_193">193</span>
-aerial will not respond readily to waves from another aerial
-having a different rate of oscillation, because these do not
-follow each other at intervals to suit it.</p>
-
-<p>If each station could receive signals only from stations
-having aerials similar to its own, its usefulness would be
-very limited, and so all stations are provided with means
-of altering the rate of oscillation of their aerials. The
-actual tuning apparatus by which this is accomplished need
-not be described, as it is complicated, but what happens in
-practice is this: The operator, wearing telephone receivers
-fixed over his ears by means of a head band, sits at a
-desk upon which are placed his various instruments. He
-adjusts the tuning apparatus to a position in which
-signals from stations of widely different wave-lengths are
-received fairly well, and keeps a general look out over
-passing signals. Presently he hears his own call-signal,
-and knows that some station wishes to communicate with
-him. Immediately he alters the adjustment of his tuner
-until his aerial responds freely to the waves from this
-station, but not to waves from other stations, and in this
-way he is able to cut out signals from other stations and to
-listen to the message without interruption.</p>
-
-<p>Unfortunately wireless tuning is yet far from perfect in
-certain respects. For instance, if two stations are transmitting
-at the same time on the same wave-length, it is
-clearly impossible for a receiving operator to cut one out
-by wave-tuning, and to listen to the other only. In such
-a case, however, it generally happens that although the
-wave-frequency is the same, the frequency of the wave
-groups or trains is different, so that there is a difference
-in the notes heard in the telephones; and a skilful operator
-can distinguish between the two sufficiently well to read
-whichever message is intended for him. The stations
-which produce a clear, medium-pitched note are the easiest<span class="pagenum" id="Page_194">194</span>
-to receive from, and in many cases it is possible to identify
-a station at once by its characteristic note. Tuning is also
-unable to prevent signals from a powerful station close at
-hand from swamping to some extent signals from another
-station at a great distance, the nearer station making the
-receiving aerial respond to it as it were by brute force,
-tuning or no tuning.</p>
-
-<p>Another source of trouble lies in interference by atmospheric
-electricity. Thunderstorms, especially in the
-tropics, interfere greatly with the reception of signals,
-the lightning discharges giving rise to violent, irregular
-groups of waves which produce loud noises in the telephones.
-There are also silent electrical disturbances in the
-atmosphere, and these too produce less strong but equally
-weird effects. Atmospheric discharges are very irregular,
-without any real wave-length, so that an operator cannot
-cut them out by wave-tuning pure and simple in the way
-just described, as they defy him by affecting equally all
-adjustments. Fortunately, the irregularity of the atmospherics
-produces correspondingly irregular sounds in the
-telephones, quite unlike the clear steady note of a wireless
-station; and unless the atmospherics are unusually strong
-this note pierces through them, so that the signals can be
-read. The effects of lightning discharges are too violent
-to be got rid of satisfactorily, and practically all that can
-be done is to reduce the loudness of the noises in the
-telephones, so that the operator is not temporarily deafened.
-During violent storms in the near neighbourhood of a
-station it is usual to connect the aerial directly to earth,
-so that in the event of its being struck by a flash the
-electricity passes harmlessly away, instead of injuring the
-instruments, and possibly also the operators. Marconi
-stations are always fitted with lightning-arresters.</p>
-
-<p>The methods and apparatus we have described so far<span class="pagenum" id="Page_195">195</span>
-are those of the Marconi system, and although in practice
-additional complicated and delicate pieces of apparatus are
-used, the description given represents the main features of
-the system. Although Marconi was not the discoverer of
-the principles of wireless telegraphy, he was the first to
-produce a practical working system. In 1896 Marconi came
-from Italy to England, bringing with him his apparatus,
-and after a number of successful demonstrations of its
-working, he succeeded in convincing even the most sceptical
-experts that his system was thoroughly sound. Commencing
-with a distance of about 100 yards, Marconi
-rapidly increased the range of his experiments, and by
-the end of 1897 he succeeded in transmitting signals
-from Alum Bay, in the Isle of Wight, to a steamer 18
-miles away. In 1899 messages were exchanged between
-British warships 85 miles apart, and the crowning achievement
-was reached in 1901, when Marconi received readable
-signals at St. John’s, Newfoundland, from Poldhu in Cornwall,
-a distance of about 1800 miles. In 1907 the Marconi
-stations at Clifden and Glace Bay were opened for public
-service, and by the following year transatlantic wireless
-communication was in full swing. The sending of wireless
-signals across the Atlantic was a remarkable accomplishment,
-but it did not represent by any means the limits
-of the system, as was shown in 1910. In that year
-Marconi sailed for Buenos Ayres, and wireless communication
-with Clifden was maintained up to the almost incredible
-distance of 4000 miles by day, and 6735 miles by night.
-The Marconi system has had many formidable rivals, but
-it still holds the proud position of the most successful commercial
-wireless system in the world.</p>
-
-<p>We have not space to give a description of the other
-commercial systems, but a few words on some of the chief
-points in which they differ from the Marconi system may<span class="pagenum" id="Page_196">196</span>
-be of interest. We have seen that an ordinary spark gap,
-formed by two metal balls a short distance apart, becomes
-overheated by the rapid succession of discharges, with the
-result that the sparking is irregular. What actually
-happens is that the violent discharge tears off and vaporizes
-minute particles of the metal. This intensely heated
-vapour forms a conducting path which the current is able
-to cross, so that an arc is produced just in the same way
-as in the arc lamp. This arc is liable to be formed by
-each discharge, and it lasts long enough to prevent the
-sparks from following one another promptly. In the
-Marconi system this trouble is avoided by means of a
-rotating spark gap, but in the German “Telefunken”
-system, so named from Greek <em>tele</em>, far off, and German
-<i lang="de">Funke</i>, a spark, a fixed compound spark gap is used
-for the same purpose. This consists of a row of metal
-discs about 1/100 inch apart, and the spark leaps these
-tiny gaps one after the other. The discs are about
-3 inches in diameter, and their effect is to conduct away
-quickly the heat of the discharge. By this means the
-formation of an arc is prevented, and the effect of each
-discharge is over immediately, the sparks being said to be
-“quenched.” The short discharges enable more energy to
-be radiated from the aerial into the ether, and very high
-rates of sparking are obtained, producing a high-pitched
-musical note.</p>
-
-<p>The “Lepel” system also uses a quenched spark.
-The gap consists of two metal discs clamped together and
-separated only by a sheet of paper. The paper has a hole
-through its centre, and through this hole the discharge
-takes place, the discs being kept cool by water in constant
-circulation. The discharge is much less noisy than in the
-Marconi and Telefunken systems, and the musical note
-produced is so sensitive that by varying the adjustments<span class="pagenum" id="Page_197">197</span>
-simple tunes can be played, and these can be heard quite
-distinctly in the telephone at the receiving stations.</p>
-
-<p>In the three systems already mentioned spark
-discharges are used to set up oscillatory currents in the
-aerial, which in turn set up waves in the ether. Each
-discharge sets in motion a certain number of waves,
-forming what is known as a train of waves. The discharges
-follow one another very rapidly, but still there is a
-minute interval between them, and consequently there is a
-corresponding interval between the wave-trains. In the
-“Goldschmidt” system the waves are not sent out in
-groups of this kind, but in one long continuous stream.
-The oscillatory currents which produce ether waves are
-really alternating currents which flow backwards and
-forwards at an enormous speed. The alternating current
-produced at an ordinary power station is of no use for
-wireless purposes, because its “frequency,” or rate of flow
-backwards and forwards, is far too low. It has been
-found possible however to construct special dynamos
-capable of producing alternating current of the necessary
-high frequency, and such dynamos are used in the
-Goldschmidt system. The dynamos are connected directly
-to the aerial, so that the oscillatory currents in the latter
-are continuous, and the ether waves produced are continuous
-also.</p>
-
-<p>The “Poulsen” system produces continuous waves in
-an altogether different manner, by means of the electric
-arc. The arc is formed between a fixed copper electrode
-and a carbon electrode kept in constant rotation, and it is
-enclosed in a kind of box filled with methylated spirit
-vapour, hydrogen, or coal gas. A powerful electro-magnet
-is placed close to the arc, so that the latter is surrounded
-by a strong magnetic field. Connected with the terminals
-of the arc is a circuit consisting of a condenser and a coil<span class="pagenum" id="Page_198">198</span>
-of wire, and the arc sets up in this circuit oscillatory
-currents which are communicated to the aerial. These
-currents are continuous, and so also are the resulting
-waves.</p>
-
-<p>The method of signalling employed in these two
-continuous-wave systems is quite different from that used
-in the Marconi and other spark systems. It is practically
-impossible to signal by starting and stopping the alternating-current
-dynamos or the arc at long or short intervals to
-represent dashes or dots. In one case the sudden changes
-from full load to zero would cause the dynamo to vary its
-speed, and consequently the wave-length would be
-irregular; besides which the dynamo would be injured by
-the sudden strains. In the other case it would be
-extremely difficult to persuade the arc to start promptly
-each time. On this account the dynamo and the arc are
-kept going continuously while a message is being transmitted,
-and the signals are given by altering the wave-length.
-In other words, the transmitting aerial is thrown
-in and out of tune alternately at the required long or short
-intervals, and the receiving aerial responds only during the
-“in-tune” intervals.</p>
-
-<p>The various receiving detectors previously described
-are arranged to work with dis-continuous waves, producing
-a separate current impulse from each group or train of
-waves. In continuous wave systems there are of course
-no separate groups, and for this reason these detectors are
-of no use, and a different arrangement is required. The
-oscillatory currents set up in the aerial are allowed to
-charge up a condenser, and this condenser is automatically
-disconnected from the aerial and connected to the operator’s
-telephones at regular intervals of about 1/1000 second.
-Each time the condenser is connected to the telephones
-it is discharged, and a click is produced. These clicks<span class="pagenum" id="Page_199">199</span>
-continue only as long as the waves are striking the aerial,
-and as the transmitting operator interrupts the waves at
-long or short intervals the clicks are split up into groups of
-corresponding length.</p>
-
-<p>Continuous waves have certain advantages over dis-continuous
-waves, particularly in the matter of sharp
-tuning, but these advantages are outweighed to a large
-extent by weak points in the transmitting apparatus. The
-dynamos used to produce the high-frequency currents in
-the Goldschmidt system are very expensive to construct
-and troublesome to keep in order; while in the Poulsen
-system the arc is difficult to keep going for long periods,
-and it is liable to fluctuations which greatly affect its
-working power. Although all the commercial Marconi
-installations make use of dis-continuous waves exclusively,
-Mr. Marconi is still carrying out experiments with continuous
-waves.</p>
-
-<p>There are many points in wireless telegraphy yet to be
-explained satisfactorily. Our knowledge of the electric
-ether waves is still limited, and we do not know for certain
-how these waves travel from place to place, or exactly
-what happens to them on their journeys. For this reason
-we are unable to give a satisfactory explanation of the
-curious fact that, generally speaking, it is easier to signal
-over long distances at night than during the day. Still
-more peculiar is the fact that it is easier to signal in a
-north and south direction than in an east and west
-direction. There are also remarkable variations in the
-strength of the signals at certain times, particularly about
-sunset and sunrise. Every station has a certain normal
-range which does not vary much as a rule, but at odd
-times astonishing “freak” distances are covered, stations
-having for a short time ranges far beyond their usual limits.
-These and other problems are being attacked by many<span class="pagenum" id="Page_200">200</span>
-investigators, and no doubt before very long they will be
-solved. Wireless telegraphy already has reached remarkable
-perfection, but it is still a young science, and we may
-confidently expect developments which will enable us to
-send messages with all speed across vast gulfs of distance
-at present unconquered.</p>
-
-<p>Wireless telephony, like wireless telegraphy, makes use
-of electric waves set up in, and transmitted through the
-ether. The apparatus is practically the same in each case,
-except in one or two important points. In wireless
-telegraphy either continuous or dis-continuous waves may
-be used, and in the latter case the spark-frequency may be
-as low as twenty-five per second. On the other hand,
-wireless telephony requires waves which are either
-continuous, or if dis-continuous, produced by a spark-frequency
-of not less than 20,000 per second. In other
-words, the frequency of the wave trains must be well above
-the limits of audibility. Although dis-continuous waves of
-a frequency of from 20,000 to 40,000 or more per second
-can be used, it has been found more convenient to use
-absolutely continuous waves for wireless telephony, and
-these may be produced by the Marconi disc generator, by
-the Goldschmidt alternator, or by the Poulsen arc, the last
-named being largely employed.</p>
-
-<p>In wireless telegraphy the wave trains are split up by
-a transmitting key so as to form groups of signals; but in
-telephony the waves are not interrupted at all, but are
-simply varied in intensity by means of the voice. All
-telephony, wireless or otherwise, depends upon the production
-of variations in the strength of a current of
-electricity, these variations being produced by air vibrations
-set up in speaking. In ordinary telephony with connecting
-wires the current variations are produced by means of a
-microphone in the transmitter, and in wireless telephony<span class="pagenum" id="Page_201">201</span>
-the same principle is adopted. Here comes in the outstanding
-difficulty in wireless transmission of speech. The
-currents used in ordinary telephony are small, and the
-microphone works with them quite satisfactorily; but in
-wireless telephony very heavy currents have to be employed,
-and so far no microphone has proved capable of
-dealing effectively with these currents. Countless devices
-to assist the microphone have been tried. It was found
-that one of the causes of trouble was the overheating of
-the carbon granules, which caused them to stick together,
-so becoming insensitive. To remedy this the granules
-have been cooled in various ways, by water, gas, or oil, but
-although the results have been improved, still the microphones
-worked far from perfectly. Improved results
-have been obtained also by connecting a number of
-microphones in parallel. The microphone difficulty is
-holding back the development of wireless telephony, and
-at present no satisfactory solution of the problem is in
-sight.</p>
-
-<p>The transmitting and receiving aerials are the same as
-in wireless telegraphy, and like them are tuned to the same
-frequency. The receiving apparatus too is of the ordinary
-wireless type, with telephones and electrolytic or other
-detectors.</p>
-
-<p>Wireless telephony has been used with considerable
-success in various German collieries, and at the Dinnington
-Main Colliery, Yorkshire. Early last year Marconi succeeded
-in establishing communication by wireless telephony
-between Bournemouth and Chelmsford, which are about
-100 miles apart; and about the same time a song sung
-at Laeken, in Belgium, was heard clearly at the Eiffel
-Tower, Paris, a distance of 225 miles. The German
-Telefunken Company have communicated by wireless
-telephony between Berlin and Vienna, 375 miles, and<span class="pagenum" id="Page_202">202</span>
-speech has been transmitted from Rome to Tripoli, a total
-distance of more than 600 miles. These distances are of
-course comparatively small, but if the microphone trouble
-can be overcome satisfactorily, transatlantic wireless
-telephony appears to be quite possible.</p>
-<hr class="chap x-ebookmaker-drop">
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_203">203</span></p>
-
-<h2 class="nobreak" id="toclink_203"><a id="chapter_XXI"></a>CHAPTER XXI<br>
-
-<span class="subhead">WIRELESS TELEGRAPHY—PRACTICAL APPLICATIONS</span></h2>
-</div>
-
-<p class="in0"><span class="firstword">A fairly</span> good idea of the principles and apparatus of wireless
-telegraphy should have been gained in reading <a href="#chapter_XX">Chapter
-XX</a>., but so far little has been said about its practical use.
-If we leave their power out of consideration, wireless
-stations may be divided into two classes: fixed stations on
-land, and moving stations, if the expression may be allowed,
-on ships. For moving stations wireless telegraphy has
-the field all to itself, but for communication between fixed
-stations it comes into conflict with ordinary telegraphy by
-wire or cable. As regards land messages over comparatively
-short distances, say throughout Great Britain, wireless
-telegraphy has no advantages over the older methods; and
-it is extremely unlikely that it ever will be substituted
-for the existing cable telegraphy. For long distances
-overland wireless has the great advantage of having all its
-apparatus concentrated at two points. A long land line
-passing through wild country, and exposed to all kinds of
-weather, requires constant labour to keep it in good repair,
-and when a breakdown occurs at any point, the repairing
-gang may be miles away, so that delay is caused. On the
-other hand, whatever may go wrong at a wireless station,
-no time is lost in effecting the necessary repairs, for everything
-is on the spot.</p>
-
-<p>At present there is no great competition between wireless<span class="pagenum" id="Page_204">204</span>
-and ordinary telegraphy for overland messages of any
-kind, but the case is different when we come to communication
-across seas and oceans. Already the cable companies have
-been affected considerably, and there is little doubt that
-they will feel the competition much more seriously before
-long. The general public, always conservative in such
-matters, have not yet grasped the fact that telegrams can
-be handed in at any telegraph office in the British Isles,
-and at most telegraph offices in the United States and
-Canada, for wireless transmission across the Atlantic, via
-the Marconi stations at Clifden and Glace Bay. The cost
-is remarkably small, being eightpence a word for ordinary
-messages.</p>
-
-<p>It is impossible to state with any accuracy how many
-land wireless stations there are in the world, but the list
-given in the <cite>Year-Book of Wireless Telegraphy</cite> for 1915
-enumerates about 700 stations. This list does not include
-private or experimental stations, and also many stations
-used exclusively for naval or military purposes are not
-given. The information available about these 700 stations
-is incomplete in many cases, but about 500 are controlled
-by various departments of the governments of the different
-states. Of the remainder, about 100 are controlled by the
-Marconi Company, the rest being in the hands of various
-wireless, commercial, or railway companies.</p>
-
-<p>Amongst the most important land stations are the
-Clifden and Glace Bay transatlantic stations. They are
-very similar in plan, and each has a separate aerial for
-sending and for receiving. Contrary to the usual practice,
-continuous current is used to charge the condensers. In
-<a href="#chapter_IV">Chapter IV</a>. we saw how a current of high voltage could
-be obtained by connecting a number of cells in series, and
-at these stations the necessary high voltage is produced by
-connecting a number of powerful dynamos in series, on the<span class="pagenum" id="Page_205">205</span>
-same principle. Along with the dynamos a huge battery
-of accumulators, consisting of about 6000 cells, is used as
-a sort of reservoir of current. These stations have a
-normal range of considerably over 3000 miles. Last year
-a large transmitting station was completed at Cefndu, near
-Carnarvon. This station, which is probably the most
-powerful in existence, is intended to communicate directly
-with New Jersey, United States, as an alternative to the
-Clifden-Glace Bay route.</p>
-
-<p>Other powerful stations are Poldhu, in Cornwall, of
-which we shall speak later; the French Eiffel Tower
-station; the German station at Nauen, near Berlin, which
-last year succeeded in exchanging messages with Windhoek,
-German South-West Africa, a distance of nearly 6000
-miles; and the extremely powerful station at Coltano, Italy.
-France has three stations in West Africa with a night
-range of 1600 miles; and Italy one in Somaliland with a
-normal range of about the same distance. The recently
-opened Chinese stations at Canton, Foochow, and Woosung
-have a range of 1300 miles by night, and 650 miles by day.
-With the fall of Tsingtau, China, Germany lost a wireless
-station capable of signalling over 1350 miles at night.
-Japan has six stations with a night range of over 1000
-miles. Massawa, on the Red Sea, has a range of 1600
-miles, and New Zealand has two stations with ranges of
-1200 miles by day, and 2500 miles by night. Australia
-has a large number of stations with a normal range of
-about 500 miles. In the United States, which has a very
-large number of stations, Arlington, Virginia, covers 1000
-miles, and Sayville from 600 to 2300 miles. South
-America has not many high-power stations, but Cerrito, in
-Uruguay, has a range of about 1000 miles.</p>
-
-<p>Until a thoroughly practical system of long-distance
-wireless telephony is developed, wireless telegraphy will<span class="pagenum" id="Page_206">206</span>
-remain the only possible means of communication between
-ships and shore, or between one ship and another, except
-where the distance is so small that some method of semaphore
-signalling can be used. In the days when wireless
-was unknown, a navigator was thrown entirely upon his
-own resources as soon as his vessel was out of sight of
-land, for no information of any kind could reach him.
-Even with a wireless installation on board, the captain of a
-vessel still needs the same skill and watchfulness as of old,
-but in the times of uncertainty and danger to which all
-ships are liable, he often is able to obtain information which
-may prevent disaster. In order to determine accurately
-his position, a navigator requires to know the exact Greenwich
-Mean Time, and he gets this time from his chronometers.
-These are wonderfully reliable instruments, but
-even they may err at times. To avoid the possibility of
-mistakes from this cause, wireless time signals are sent out
-at regular intervals by certain high-power stations, and as
-long as a vessel is within range of one of these stations the
-slightest variation in the chronometers may be detected
-immediately. Amongst these stations are the Eiffel Tower,
-giving time signals at 10 a.m. and at midnight; and Norddeich,
-Germany, giving signals at noon and midnight.
-These time signals have proved most useful also on land,
-more particularly for astronomers and for explorers engaged
-on surveying work.</p>
-
-<p>In addition to time signals, other valuable information
-is conveyed by wireless to ships at sea. A ship encountering
-ice, or a derelict, reports its discovery to other ships
-and to the shore stations, and in this way vessels coming
-along the same route are warned of the danger in time to
-take the necessary precautions. Weather reports are issued
-regularly from various shore stations in most parts of the
-world. The completeness of the information given varies<span class="pagenum" id="Page_207">207</span>
-a good deal with different stations, but in many cases it
-includes a report of the existing state of the weather at a
-number of different places, a forecast of the winds likely
-to be encountered at sea, say at a distance of 100 miles
-from land, and warnings of approaching storms, with
-remarks on any special atmospheric conditions at the time
-of sending. In Europe weather reports are issued daily
-from the Admiralty station at Cleethorpes, the Eiffel Tower,
-and Norddeich; and in the United States more than a
-dozen powerful stations are engaged in this work.</p>
-
-<p>Another valuable use of wireless is in carrying on the
-work of lighthouses and lightships during snowstorms or
-dense fogs, which the light cannot penetrate. So far not
-much has been done in this direction, but the French
-Government have decided to establish wireless lighthouses
-on the islands outside the port of Brest, and also at Havre.
-Automatic transmitting apparatus will be used, sending
-out signals every few seconds, and working for periods of
-about thirty hours without attention.</p>
-
-<p>The improvement in the conditions of ocean travel
-wrought by wireless telegraphy is very remarkable. The
-days when a vessel, on passing out of sight of land, entered
-upon a period of utter isolation, is gone for ever. Unless
-it strays far from all recognized trade routes, a ship fitted
-with a wireless installation is never isolated; and with the
-rapidly increasing number of high-power stations both on
-land and sea, it soon will be almost impossible for a vessel
-to find a stretch of ocean beyond the reach of wave-borne
-messages. The North Atlantic Ocean is specially remarkable
-for perfection of wireless communication. For the
-first 250 miles or so after leaving British shores, liners are
-within reach of various coast stations, and beyond this
-Poldhu takes up the work and maintains communication
-up to about mid-Atlantic. On passing beyond the reach<span class="pagenum" id="Page_208">208</span>
-of Poldhu, liners come within range of other Marconi
-stations at Cape Cod, Massachusetts, and Cape Race,
-Newfoundland, so that absolutely uninterrupted communication
-is maintained throughout the voyage. On many
-liners a small newspaper is published daily, in which are
-given brief accounts of the most striking events of the
-previous day, together with Stock Exchange quotations and
-market prices. This press news is sent out during the
-night from Poldhu and Cape Cod. During the whole
-voyage messages may be transmitted from ship to shore,
-or from shore to ship, with no more difficulty, as far as the
-public are concerned, than in sending an ordinary inland
-telegram.</p>
-
-<p>The transmitting ranges of ship installations vary
-greatly, the range of the average ocean liner being about
-250 miles. Small ships come as low as 50 miles, while a
-few exceptional vessels have night ranges up to 1200 or
-even 2500 miles. Although an outward-bound vessel is
-almost always within range of some high-power shore
-station, it is evident that it soon must reach a point beyond
-which it is unable to communicate directly with the shore.
-This difficulty is overcome by a system of relaying from
-ship to ship. The vessel wishing to speak with the shore
-hands on its message to some other vessel nearer to land
-or with a longer range, and this ship sends forward the
-message to a shore station if one is within its reach, and if
-not to a third vessel, which completes the transmission.</p>
-
-<p>The necessity for wireless installations on all sea-going
-vessels has been brought home to us in startling fashion on
-several occasions during the last few years. Time after
-time we have read thrilling accounts of ocean disasters in
-which wireless has come to the rescue in the most wonderful
-way. A magnificent liner, with its precious human
-freight, steams majestically out of harbour, and ploughs its<span class="pagenum" id="Page_209">209</span>
-way out into the waste of waters. In mid-ocean comes
-disaster, swift and awful, and the lives of all on board are
-in deadly peril. Agonized eyes sweep the horizon, but no
-sail is in sight, and succour seems hopeless. But on the
-deck of that vessel is a small, unpretentious cabin, and at a
-desk in that cabin sits a young fellow with strange-looking
-instruments before him. At the first tidings of disaster he
-presses a key, and out across the waters speed electric
-waves bearing the wireless cry for help, “S.O.S.,” incessantly
-repeated. Far away, on another liner, is a
-similar small cabin, and its occupant is busy with messages
-of everyday matters. Suddenly, in the midst of his work,
-comes the call from the stricken vessel, and instantly all
-else is forgotten. No matter what the message in hand, it
-must wait, for lives are in danger. Quickly the call is
-answered, the position of the doomed ship received, and
-the captain is informed. A few orders are hurriedly given,
-the ship’s course is changed, and away she steams to the
-rescue, urged on by the full power of her engines. In an
-hour or two she arrives alongside, boats are lowered, and
-passengers and crew are snatched from death and placed
-in safety. This scene, with variations, has been enacted
-many times, and never yet has wireless failed to play its
-part. It is only too true that in some instances many
-lives have been lost, but in these cases it is necessary to
-remember that without wireless every soul on board might
-have gone down. The total number of lives already saved
-by wireless is estimated at about 5000, and of these some
-3000 have been saved in the Atlantic.</p>
-
-<p>Ship aerials are carried from one mast to another, as
-high up as possible. The transmitting and receiving
-apparatus is much the same as in land stations, so that it
-need not be described. In addition, most liners carry a
-large induction coil and a suitable battery, so that distress<span class="pagenum" id="Page_210">210</span>
-signals can be transmitted even when the ordinary
-apparatus is rendered useless by the failure of the current
-supply. Most of the wireless systems are represented
-amongst ship installations, but the great majority of vessels
-have either Marconi or Telefunken apparatus.</p>
-
-<p>Every wireless station, whether on ship or on shore, has
-a separate call-signal, consisting of three letters. For
-instance, Clifden is MFT, Poldhu MPD, Norddeich KAV,
-s.s. <i>Lusitania</i> MFA, and H.M.S. <i>Dreadnought</i> BAU.
-Glace Bay, GB, and the Eiffel Tower, FL, have two
-letters only. In order to avoid confusion, different countries
-have different combinations of letters assigned to them
-exclusively, and these are allotted to the various ship
-and shore stations. For example, Great Britain has all
-combinations beginning with B, G, and M; France all
-combinations beginning with F, and also the combinations
-UAA to UMZ; while the United States is entitled to use
-all combinations beginning with N and W, and the combinations
-KIA to KZZ. There are also special signals to
-indicate nationality, for use by ships, British being indicated
-by - - — -, Japanese by — - — -, and so on.</p>
-
-<p>Wireless telegraphy apparently has a useful future in
-railway work. In spite of the great perfection of present-day
-railway signalling, no railway company is able to avoid
-occasional accidents. Some of these accidents are due to
-circumstances which no precautions can guard against
-entirely, such, for instance, as the sudden breakage of some
-portion of the mechanism of the train itself. In many cases,
-however, the accident is caused by some oversight on the
-part of the signalman or the engine-driver. Probably the
-great majority of such accidents are not due to real carelessness
-or inattention to duty, but to unaccountable freaks
-of the brain, through which some little detail, never before
-forgotten, is overlooked completely until too late. We all<span class="pagenum" id="Page_211">211</span>
-are liable to these curious mental lapses, but happily in most
-cases these do not lead to disaster of any kind. The ever-present
-possibility of accidents brought about in this way is
-recognized fully by railway authorities, and every effort is
-made to devise mechanism which will safeguard a train in
-case of failure of the human element. The great weakness
-of the ordinary railway system is that there is no reliable
-means of communicating with the driver of a train except
-by the fixed signals, so that when a train has passed one
-set of signals it is generally beyond the reach of a message
-until it arrives at the next set. On the enterprising
-Lackawanna Railroad, in the United States, an attempt has
-been made to remove this defect by means of wireless
-telegraphy, and the experiment has been remarkably
-successful. Wireless communication between moving
-passenger trains and certain stations along the route has
-been established, and the system is being rapidly developed.</p>
-
-<p>The wireless equipment of the stations is of the usual
-type, and does not call for comment, but the apparatus on
-the trains is worth mention. The aerial, which must be
-low on account of bridges and tunnels, consists of rectangles
-of wire fixed at a height of 18 inches above the roof of
-each car. These separate aerials are connected together
-by a wire running to a small operating room containing a
-set of Marconi apparatus, and situated at the end of one of
-the cars. The earth connexion is made to the track rails,
-and the current is taken from the dynamos used to supply
-the train with electric light. With this equipment messages
-have been transmitted and received while the train was
-running at the rate of 70 miles an hour, and distances
-up to 125 miles have been covered. During a severe
-storm in the early part of last year the telegraph and
-telephone lines along the railroad broke down, but
-uninterrupted communication was maintained by wireless,<span class="pagenum" id="Page_212">212</span>
-and the operations of the relief gangs and the snow-ploughs
-were directed by this means. For emergency signalling
-this system is likely to prove of enormous importance. If
-signals are set wrongly, through some misunderstanding,
-and a train which should have been held up is passed
-forward into danger, it can be stopped by a wireless message
-in time to prevent an accident. Again, if a train has a
-breakdown, or if it sticks fast in a snow-drift, its plight and
-its exact position can be signalled to the nearest station, so
-that help may be sent without delay. The possibilities of
-the system in fact are almost unlimited, and it seems not
-unlikely that wireless telegraphy will revolutionize the
-long-distance railway travelling of the future.</p>
-<hr class="chap x-ebookmaker-drop">
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_213">213</span></p>
-
-<h2 class="nobreak" id="toclink_213"><a id="chapter_XXII"></a>CHAPTER XXII<br>
-
-<span class="subhead">ELECTROPLATING AND ELECTROTYPING</span></h2>
-</div>
-
-<p class="in0"><span class="firstword">In</span> our chapter on the accumulator or storage cell we saw
-that a current of electricity has the power of decomposing
-certain liquids; that is to say, it is able to split them up
-into their component parts. This power has given rise
-to the important art of electroplating and electrotyping.
-Electroplating is the process of depositing a coating of a
-rarer metal, such as gold, silver, or nickel, upon the
-surface of baser or commoner metals; and electrotyping is
-the copying of casts, medals, types, and other similar
-objects. The fact that metals could be deposited by the
-decomposition of a solution by a current was known in the
-early days of the voltaic cell, but no one seems to have
-paid much attention to it. An Italian chemist published
-in 1805 an account of his success in coating two silver
-medals with gold, and some thirty years later Bessemer
-transformed lead castings into fairly presentable ornaments
-by coating them with copper, but commercial electroplating
-may be said to have begun about 1840, when an
-Englishman named Elkington took out a patent for the
-process. Since then the processes of electroplating and
-electrotyping have rapidly come more and more into use,
-until to-day they are practised on a vast scale, giving employment
-to thousands.</p>
-
-<p>Electroplating on a small scale is a very simple affair.
-A solution of the metal which it is desired to deposit is<span class="pagenum" id="Page_214">214</span>
-placed in a suitable vessel. Two metal rods are placed
-across the top of this vessel, and from one of these is
-suspended a plate of the same metal as that in the solution,
-and from the other is hung the article to receive the
-coating. The positive terminal of a voltaic battery is
-connected to the rod supporting the plate, and the negative
-terminal to the rod carrying the article to be plated. As
-the current passes through the solution from the plate to the
-article the solution is decomposed, and the article receives
-a coating of metal. The solution through which the
-current passes, and which is decomposed, is called the
-<em>electrolyte</em>, and the terminal points at which the current
-enters and leaves the solution are called <em>electrodes</em>. The
-electrode by which the current enters the electrolyte is
-called the <em>anode</em>, and the one by which it leaves is called
-the <em>cathode</em>.</p>
-
-<p>If we wish to deposit a coating of copper on, say, an
-old spoon which has been dismissed from household service,
-a solution of sulphate of copper must be made up and
-placed in a glass or stoneware jar. Two little rods of
-brass, copper, or any other good conductor are placed
-across the jar, one at each side, and by means of hooks of
-wire a plate of copper is hung from one rod and the spoon
-from the other. The positive terminal of a battery of
-Daniell cells is then connected to the anode rod which
-supports the copper plate, and the negative terminal to
-the cathode rod carrying the spoon. The current now
-commences its task of splitting up the copper-sulphate
-solution into pure copper and sulphuric acid, and depositing
-this copper upon the spoon. The latter is very quickly
-covered with a sort of “blush” copper, and the coating
-grows thicker and thicker as long as the current is kept
-at work. If there were no copper plate forming the anode
-the process would soon come to a standstill, on account of<span class="pagenum" id="Page_215">215</span>
-the copper in the electrolyte becoming used up; but as it
-is the sulphuric acid separated out of the electrolyte takes
-copper from the plate and combines with it to form a
-further supply of copper sulphate. In this way the strength
-of the solution is kept up, and the copper anode becomes
-smaller and smaller as the coating on the spoon increases
-in thickness. It is not necessary that the anode should
-consist of absolutely pure copper, because any impurities
-will be precipitated to the bottom or mixed with the
-solution, nothing but quite pure copper being deposited on
-the spoon. At the same time if the copper anode is very
-impure the electrolyte quickly becomes foul, and has to be
-purified or replaced by new solution.</p>
-
-<figure id="fig_35" class="figcenter" style="max-width: 21em;">
- <img src="images/i_249.jpg" width="1625" height="861" alt=" ">
- <figcaption class="caption">
-
-<p class="floatl"><i>By permission of</i>]</p>
-<p class="floatr">[<i>W. Canning & Co.</i></p>
-
-<p class="floatc"><span class="smcap">Fig. 35.</span>—Small Electroplating Outfit.</p>
-</figcaption></figure>
-
-<p>To nickel-plate the spoon we should require a nickel
-plate for the anode and a nickel solution; to silver-plate it,
-a silver anode and solution, and so on. <a href="#fig_35">Fig. 35</a> shows at
-simple but effective arrangement for amateur electroplating
-in a small way.</p>
-
-<p>Electroplating on a commercial scale is of course a
-much more elaborate process, but the principle remains
-exactly the same. <a href="#fig_36">Fig. 36</a> shows the general arrangement
-of a plating shop. It is obviously extremely important<span class="pagenum" id="Page_216">216</span>
-that the deposit on a plated article should be durable, and
-to ensure that the coating will adhere firmly the article
-must be cleaned thoroughly before being plated. Cleanliness
-in the ordinary domestic sense is not sufficient, for
-the article must be chemically clean. Some idea of the
-care required in this respect may be gained from the fact
-that if the cleaned surface is touched with the hand before
-being plated, the coating will strip off the parts that have
-been touched. The surfaces are first cleaned mechanically,
-and then chemically by immersion in solutions of acids or
-alkalies, the cleaning process varying to some extent with
-different metals. There is also a very interesting process
-of cleaning by electricity. The article is placed in a vat
-fitted with anode and cathode rods, just as in an ordinary
-plating vat, and containing a solution of hydrate of potash
-and cyanide of potassium. The anode consists of a carbon
-plate, and the article is hung from the cathode rod.
-Sufficient current is passed through the solution to cause
-gas to be given off rapidly at the cathode, and as this gas
-rises to the surface it carries with it the grease and dirt
-from the article, in the form of a dirty scum. After a
-short time the article becomes oxidized and discoloured,
-and the current is then reversed, so that the article becomes
-the anode, and the carbon plate the cathode. The
-current now removes the oxide from the surface of the
-article, which is left quite bright and chemically clean.</p>
-
-<figure id="fig_36" class="figcenter" style="max-width: 40em;">
- <img src="images/i_251.jpg" width="3127" height="1843" alt=" ">
- <figcaption class="caption">
-
-<p class="floatl"><i>By Permission of</i>]</p>
-<p class="floatr">[<i>W. Canning & Co.</i></p>
-
-<p class="floatc"><span class="smcap">Fig. 36.</span>—General Arrangement of an Electroplating Shop.</p>
-</figcaption></figure>
-
-<p>When thoroughly cleaned the articles are ready to be
-placed in the plating vats. These vats are usually made
-of wood lined with chemically pure lead, or of iron lined
-with enamel or cement. Anode and cathode rods made
-of brass are placed across the vats, and from these the
-anodes of the various metals and the articles to be plated
-are hung by hooks of nickel or brass. Any number of
-rods may be used, according to the size of the vat, so long<span class="pagenum" id="Page_218">218</span>
-as the articles have an anode on each side. If three rods
-are used the articles are hung from the centre one, and the
-anodes from the outside ones. If a number of small
-articles are to be plated together they are often suspended
-in perforated metal trays. Small articles are also plated
-by placing them in a perforated barrel of wood, or wood
-and celluloid, which revolves in the solution. While the
-articles are being plated the revolving of the barrel makes
-them rub one against the other, so that they are brightly
-burnished. Dog chains, cycle chain links, button-hooks,
-and harness fittings are amongst the articles plated by
-means of the revolving barrel.</p>
-
-<p>The strength of current required for different kinds of
-plating varies considerably, and if the work is to be of the
-best quality it is very important that the current should be
-exactly right for the particular process in hand. In order
-to adjust it accurately variable resistances of German silver
-wire are provided for each vat, the current having to pass
-through the resistance before reaching the solution. The
-volume and the pressure of the current are measured by
-amperemeters and voltmeters attached to the resistance
-boards. If the intensity of the current is too great the
-articles are liable to be “burnt,” when the deposit is dark
-coloured and often useless.</p>
-
-<p>When exceptionally irregular surfaces have to be plated
-it is sometimes necessary to employ an anode of special
-shape, in order to keep as uniform a distance as possible
-between the electrodes. If this is not done, those parts of
-the surface nearest the anode get more than their share of
-the current, and so they receive a thicker deposit than the
-parts farther away.</p>
-
-<p>An interesting process is that known as “parcel-plating,”
-by which decorative coatings of different coloured metals
-can be deposited on one article. For instance, if it is<span class="pagenum" id="Page_219">219</span>
-desired to have gold flowers on a silver brooch, the parts
-which are not to be gilded are painted over with a non-conducting
-varnish. When this varnish is quite dry the
-brooch is placed in the gilding vat and the current sent
-through in the usual way. The gold is then deposited only
-on the parts unprotected by varnish, and after the gilding
-the varnish is easily removed by softening it in turpentine
-and brushing with a bristle brush. More elaborate
-combinations of different coloured metals can be made in
-the same way.</p>
-
-<p>Sugar basins, cream jugs, ornamental bowls, cigarette
-cases, and other articles are often gilded only on the inside.
-The article is filled with gold solution and connected to the
-cathode rod. A piece of gold wrapped in calico is attached
-to the anode rod, suspended in the solution inside the
-article, and moved about quickly until the deposit is of the
-required thickness.</p>
-
-<p>The time occupied in plating is greatly shortened by
-stirring or agitating the solutions. This sets up a good
-circulation of the liquid, and a continual supply of fresh
-solution is brought to the cathode. At the same time the
-resistance to the current is decreased, and more current
-may be used without fear of burning. <a href="#fig_37">Fig. 37</a> shows an
-arrangement for this purpose. The solution is agitated by
-compressed air, and at the same time the cathode rods are
-moved backwards and forwards. Plating solutions are
-also frequently heated in order to hasten the rate of
-deposition.</p>
-
-<p>When the plating process is complete, the articles are
-removed from the vat, thoroughly swilled in water, and
-dried. They are then ready for finishing by polishing and
-burnishing, or they may be given a sort of frosted surface.
-During the finishing processes the appearance of the articles
-changes considerably, the rather dead-looking surface<span class="pagenum" id="Page_220">220</span>
-produced by the plating giving place to the bright lustre
-of the particular metal.</p>
-
-<figure id="fig_37" class="figcenter" style="max-width: 27em;">
- <img src="images/i_254.jpg" width="2159" height="2366" alt=" ">
- <figcaption class="caption">
-
-<p class="floatl"><i>By Permission of</i>]</p>
-<p class="floatr">[<i>W. Canning & Co.</i></p>
-
-<p class="floatc"><span class="smcap">Fig. 37.</span>—Method of agitating solution in Plating Vat.</p>
-</figcaption></figure>
-
-<p>It sometimes happens that an article which has been
-plated and polished shows little defects here and there in
-the deposit. In such a case it is not necessary to re-plate
-the whole article, for the defects can be made good by a<span class="pagenum" id="Page_221">221</span>
-process of “doctoring.” A piece of the same metal as that
-forming the deposit is placed between two pieces of wood,
-and a wire fastened to one end of it. At the other end
-several thicknesses of flannel are wrapped round and
-securely tied. This strip, which forms a miniature anode,
-is connected to the anode rod of the plating vat, and the
-article is connected to the cathode rod. The flannel is
-saturated with the plating solution, and the strip is rubbed
-gently over the defective places until the deposit formed is
-as thick as that on the rest of the article. If the work is
-done carefully the “doctored” portions cannot be distinguished
-from the rest of the surface.</p>
-
-<p>Electroplating may be employed to give ships’ plates
-a coating of copper to prevent barnacles from sticking to
-them. The work is done in sections by building up to the
-side of the vessel a sort of vat of which the plate to be
-coated forms one side. The plate is thus at the same time
-the cathode and part of the vat.</p>
-
-<p>So far we have spoken only of electroplating objects
-made of metal. If we tried to copperplate a plaster cast
-by simply suspending it as we did our spoon, we should
-get no result at all, because the plaster is a non-conductor.
-But if we sprinkle plumbago over the cast so as to give it
-a conducting surface, we can plate it quite well. Practically
-all materials can be electroplated, but if they are non-conductors
-they must be given a conducting surface in the
-way just described or by some similar means. Even
-flowers and insects may be plated, and by giving them first
-a coating of copper and then a coating of gold, delicately
-beautiful results are obtained.</p>
-
-<p>Electrotyping is practically the same as electroplating,
-except that the coating is removed from the support on
-which it is deposited. The process is largely used for
-copying engraved plates for printing purposes. The plate<span class="pagenum" id="Page_222">222</span>
-is first rubbed over with a very weak solution of beeswax
-in turpentine, to prevent the deposit from adhering to it,
-and it is then placed in a copperplating vat and given a
-good thick coating. The coating is then stripped off, and
-in this way a reversed copy of the plate is obtained. This
-copy is then replaced in the vat, and a coating of copper
-deposited upon it, and this coating, when stripped off, forms
-an exact reproduction of the original, with every detail
-faithfully preserved. An engraved plate may be copied
-also by making from it a mould of plaster or composition.
-The surface of this mould is then rendered conducting by
-sprinkling over it a quantity of plumbago, which is well
-brushed into all the recesses, and a coating of copper
-deposited on it. As the mould was a reversed copy of the
-original, the coating formed upon it is of course an exact
-copy of the plate. If the copy has to be made very quickly
-a preliminary deposit of copper is chemically formed on the
-mould before it is placed in the vat. This is done by
-pouring on to the mould a solution of sulphate of copper,
-and sprinkling iron filings over the surface. The filings
-are then brushed down on to the face of the mould with a
-fine brush, and a chemical reaction takes place, resulting in
-the precipitation of copper from the solution. After the
-filings have been washed away, the mould is placed in the
-vat, and the deposition of copper takes place very rapidly.</p>
-
-<p>Engraved copperplates are often nickel or steel-plated
-to give their surface greater hardness, so that the printer
-may obtain a larger number of sharp impressions from the
-same plate. Stereotypes also are coated with nickel for a
-similar reason.</p>
-
-<p>Before the dynamo came into general use all electroplating
-and electrotyping was done with current supplied
-by voltaic cells, and though the dynamo is now used exclusively
-in large plating works, voltaic cells are still<span class="pagenum" id="Page_223">223</span>
-employed for work on a very small scale. A cell which
-quickly polarizes is quite useless for plating purposes, and
-one giving a constant and ample supply of current is
-required. The Daniell cell, which was described in
-<a href="#chapter_IV">Chapter IV</a>., is used, and so also is the Bunsen cell, which
-consists of a porous pot containing strong nitric acid and a
-carbon rod, placed in an outer stoneware vessel containing
-dilute sulphuric acid and a zinc plate. The drawback to
-this cell is that it gives off very unpleasant fumes. The
-dynamos used for plating work are specially constructed to
-give a large amount of current at very low pressure.
-Continuous current only can be used, for alternating current
-would undo the work as fast as it was done, making the
-article alternately a cathode and an anode.</p>
-<hr class="chap x-ebookmaker-drop">
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_224">224</span></p>
-
-<h2 class="nobreak" id="toclink_224"><a id="chapter_XXIII"></a>CHAPTER XXIII<br>
-
-<span class="subhead">INDUSTRIAL ELECTROLYSIS</span></h2>
-</div>
-
-<p class="in0"><span class="firstword">The</span> metal copper, as obtained from copper ore, contains
-many impurities of various kinds. For most purposes
-these impurities greatly affect the value of the copper, and
-before the metal can be of much commercial use they must
-be got rid of in some way. In the previous chapter, in
-describing how to copperplate an old spoon, we saw that
-the anode need not consist of pure copper, because in any
-case nothing but the pure metal would be deposited upon
-the spoon. This fact forms the basis of the important
-industry of electrolytic copper refining. The process is
-exactly the same as ordinary copperplating, except that
-the cathode always consists of absolutely pure copper.
-This is generally in the form of a sheet no thicker than
-thin paper, but sometimes a number of suspended wires are
-used instead. A solution of copper sulphate is used as
-usual for the electrolyte, and the anode is a thick cast plate
-of the impure copper. The result of passing a current
-through the solution is that copper is taken from the anode
-and carried to the cathode, the impurities falling to the
-bottom of the vat and accumulating as a sort of slime. In
-this way thick slabs of pure copper are obtained, ready to
-be melted down and cast into ingots.</p>
-
-<p>The impurities in the raw copper vary according to the
-ore from which it is obtained, and sometimes gold and
-silver are found amongst them. When the copper is known<span class="pagenum" id="Page_225">225</span>
-to contain these metals the deposit at the bottom of the
-refining vats is carefully collected, and from it a considerable
-quantity of gold and silver is recovered. It is
-estimated that about half a million tons of copper are
-refined every year. An immense amount of this pure
-copper is used for electrical purposes, for making conducting
-wires and cables, and innumerable parts of electric
-appliances and machinery of all kinds; in fact it is calculated
-that more than half of the copper produced all over the
-world is used in this way.</p>
-
-<p>A similar method is employed to obtain the precious
-metals in a pure state, from the substance known as
-“bullion”; which consists usually of an intermingling of
-gold, silver, and copper, with perhaps also lead. Just as in
-copper refining, the raw material is used as the anode, and
-a strip of pure gold or silver, according to which metal is
-required, as the cathode. A silver solution is used if
-silver is wanted, and a gold solution if gold is to be
-deposited.</p>
-
-<p>The metal aluminium has come into general use with
-surprising rapidity, and during the last twenty-five or
-thirty years the amount of this metal produced annually
-has increased from two or three tons to many thousands
-of tons. Aluminium occurs naturally in large quantities,
-in the form of alumina, or oxide of aluminium, but for a
-long time experimenters despaired of ever obtaining the
-pure metal cheaply on a commercial scale. The oxides of
-most metals can be reduced, that is deprived of their oxygen,
-by heating them with carbon; but aluminium oxide holds
-on to its oxygen with extraordinary tenacity, and absolutely
-refuses to be parted from it in this way. One process
-after another was tried, without success, and cheap
-aluminium seemed to be an impossibility until about 1887,
-when two chemists, Hall, an American, and Héroult, a<span class="pagenum" id="Page_226">226</span>
-Frenchman, discovered a satisfactory solution of the
-problem. These chemists, who were then scarcely out of
-their student days, worked quite independently of one
-another, and it is a remarkable fact that their methods,
-which are practically alike, were discovered at almost the
-same time. The process is an interesting mixture of
-electrolysis and electric heating. An iron crucible containing
-a mixture of alumina, fluorspar, and cryolite is
-heated. The two last-named substances are quickly fused,
-and the alumina dissolves in the resulting fluid. When
-the mixture has reached the fluid state, electrodes made of
-carbon are dipped into it, and a current is passed through;
-with the result that oxygen is given off at the anode, and
-metallic aluminium is produced at the cathode, in molten
-drops. This molten metal is heavier than the rest of the
-fluid, and so it falls to the bottom. From here it is drawn
-off at intervals, while fresh alumina is added as required,
-so that the process goes on without interruption. After
-the first fusing of the mixture no further outside heat is
-required, for the heat produced by the passage of the
-current is sufficient to keep the materials in a fluid state.
-Vast quantities of aluminium are produced in this way at
-Niagara Falls, and in Scotland and Switzerland.</p>
-
-<p>Most of us are familiar with the substance known as
-caustic soda. The chemical name for this is sodium hydrate,
-and its preparation by electrolysis is interesting. Common
-salt is a chemical compound of the metal sodium and the
-greenish coloured, evil smelling gas chlorine, its proper
-name being sodium chloride. A solution of this in water
-is placed in a vat or cell, and a current is sent through it.
-The solution is then split up into chlorine, at the anode,
-and sodium at the cathode. Sodium has a remarkably
-strong liking for water, and as soon as it is set free from
-the chlorine it combines with the water of the solution, and<span class="pagenum" id="Page_227">227</span>
-a new solution of sodium hydrate is formed. The water
-in this is then got rid of, and solid caustic soda remains.</p>
-
-<p>Amongst the many purposes for which caustic soda is
-used is the preparation of oxygen and hydrogen. Water,
-to which a little sulphuric acid has been added, is split up
-by a current into oxygen and hydrogen, as we saw in
-<a href="#chapter_V">Chapter V</a>. This method may be used for the preparation
-of these two gases on a commercial scale, but more usually
-a solution of caustic soda is used as the electrolyte. If the
-oxygen and hydrogen are not to be used at the place where
-they are produced, they are forced under tremendous
-pressure into steel cylinders, and at a lantern lecture these
-cylinders may be seen supplying the gas for the lime-light.
-Although the cylinders are specially made and tested for
-strength, they are covered with a sort of rope netting; so
-that if by any chance one happened to burst, the shattered
-fragments of metal would be caught by the netting, instead
-of flying all over the room and possibly injuring a number
-of people. A large quantity of hydrogen is prepared by
-this process for filling balloons and military airships.</p>
-<hr class="chap x-ebookmaker-drop">
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_228">228</span></p>
-
-<h2 class="nobreak" id="toclink_228"><a id="chapter_XXIV"></a>CHAPTER XXIV<br>
-
-<span class="subhead">THE RÖNTGEN RAYS</span></h2>
-</div>
-
-<p class="in0"><span class="firstword">In</span> the chapter on electricity in the atmosphere we saw that
-whereas air at ordinary pressure is a bad conductor, its
-conducting power increases rapidly as the pressure is
-lowered. Roughly speaking, if we wish to obtain a spark
-across a gap of 1 inch in ordinary air, we must have an
-electric pressure of about 50,000 volts. The discharge
-which takes place under these conditions is very violent,
-and it is called a “disruptive” discharge. If however the
-air pressure is gradually lowered, the discharge loses its
-violent character, and the brilliant spark is replaced by a
-soft, luminous glow.</p>
-
-<p>The changes in the character of the discharge may be
-studied by means of an apparatus known as the “electric
-egg.” This consists of an egg-shaped bulb of glass, having
-its base connected with an air-pump. Two brass rods
-project into the bulb, one at each end; the lower rod being
-a fixture, while the upper one is arranged to slide in and
-out, so that the distance between the balls can be varied.
-The outer ends of the rods are connected to an induction
-coil or to a Wimshurst machine. If the distance between
-the balls has to be, say, half an inch, to produce a spark
-with the air at normal pressure, then on slightly reducing
-the pressure by means of the air-pump it is found that a
-spark will pass with the balls an inch or more apart. The
-brilliance of an electric spark is due to the resistance of the<span class="pagenum" id="Page_229">229</span>
-air, and as the pressure decreases the resistance becomes
-smaller, so that the light produced is much less brilliant.
-If the exhaustion is carried still further the discharge
-becomes redder in colour, and spreads out wider and wider
-until it loses all resemblance to a spark, and becomes a
-luminous glow of a purple or violet colour. At first this
-glow seems to fill the whole bulb, but at still higher vacua
-it contracts into layers of definite shape, these layers being
-alternately light and dark. Finally, when the pressure
-becomes equal to about one-millionth of an atmosphere, a
-luminous glow surrounds the cathode or negative rod,
-beyond this is dark space almost filling the bulb, and the
-walls of the bulb between the cathode and the anode
-glow with phosphorescent light. This phosphorescence is
-produced by rays coming from the cathode and passing
-through the dark space, and these rays have been given the
-name of “cathode rays.”</p>
-
-<p>Many interesting experiments with these rays may be
-performed with tubes permanently exhausted to the proper
-degree. The power of the rays to produce phosphorescence
-is shown in a most striking way with a tube fixed in a
-horizontal position upon a stand, and containing a light
-cross made of aluminium, placed in the path of the rays.
-This is hinged at the base, so that it can be stood up on end
-or thrown down by jerking the tube. Some of the rays
-streaming from the cathode are intercepted by the cross,
-while others pass by it and reach the other end of the tube.
-The result is that a black shadow of the cross is thrown on
-the glass, sharply contrasted with those parts of the tube
-reached by the rays, and which phosphoresce brilliantly.
-After a little while this brilliance decreases, for the glass
-becomes fatigued, and loses to a considerable extent its
-power of phosphorescing. If now the cross is jerked down,
-the rays reach the portions of the tube before protected by<span class="pagenum" id="Page_230">230</span>
-the cross, and this glass, being quite fresh, phosphoresces
-with full brilliance. The black cross now suddenly becomes
-brilliantly illuminated, while the tired glass is dark in
-comparison. If the tired glass is allowed to rest for a while
-it partly recovers its phosphorescing powers, but it never
-regains its first brilliance.</p>
-
-<p>An even more striking experiment may be made with a
-horizontal tube containing a tiny wheel with vanes of mica,
-something like a miniature water-wheel, mounted on glass
-rails. When the discharges are sent through the tube, the
-cathode rays strike against the vanes and cause the little
-wheel to move forward in the direction of the anode. Other
-experiments show that the cathode rays have great heating
-power, and that they are deflected by a magnet held close
-to the tube.</p>
-
-<p>For a long time the nature of these cathode rays was in
-dispute. German physicists held that they were of the same
-character as ordinary light, while English scientists, headed
-by Sir William Crookes, maintained that they were streams
-of extremely minute particles of matter in a peculiar fourth
-state. That is to say, the matter was not liquid, or solid, or
-gaseous in the ordinary sense, but was <em>ultra-gaseous</em>, and
-Crookes gave it the name of <em>radiant matter</em>. Most of us
-have been taught to look upon the atom as the smallest
-possible division of matter, but recent researches have made
-it clear that the atom itself is divisible. It is believed that
-an atom is made up of very much more minute particles
-called <em>electrons</em>, which are moving about or revolving all the
-time with incredible rapidity. According to Sir Oliver
-Lodge, if we imagine an atom of hydrogen to be as big as
-an ordinary church, then the electrons which constitute it
-will be represented by about 700 grains of sand, 350 being
-positively electrified and 350 negatively electrified. It is not
-yet definitely determined whether these electrons are minute<span class="pagenum" id="Page_231">231</span>
-particles of matter charged with electricity, or whether they
-are actually atoms of electricity. The majority of scientists
-now believe that the cathode rays consist of a stream of
-negative electrons repelled from the cathode at a speed of
-124 miles per second, or not quite 1/1000 of the velocity of
-light.</p>
-
-<p>In November 1895, Professor Röntgen, a German
-physicist, announced his discovery of certain invisible rays
-which were produced at the same time as the cathode rays,
-and which could penetrate easily solids quite opaque to
-ordinary light. He was experimenting with vacuum tubes,
-and he found that certain rays emerged from the tube. These
-were not cathode rays, because they were able to pass
-through the glass, and were not deflected by a magnet. To
-these strange rays he gave the name of the “<em>X</em>,” or unknown
-rays, but they are very frequently referred to by the name
-of their discoverer.</p>
-
-<p>It was soon found that the Röntgen rays affected an
-ordinary photographic plate wrapped up in black paper so
-as to exclude all ordinary light, and that they passed
-through flesh much more easily than through bone. This
-fact makes it possible to obtain what we may call “shadow-graphs”
-of the bones through the flesh, and the value of
-this to the medical profession was realized at once. The
-rays also were found to cause certain chemical compounds
-to become luminous. A cardboard screen covered with
-one of these compounds is quite opaque to ordinary light,
-but if it is examined when the Röntgen rays are falling
-upon it, it is seen to be brightly illuminated, and if the
-hand is held between the screen and the rays the bones
-become clearly visible.</p>
-
-<figure id="fig_38" class="figcenter" style="max-width: 19em;">
- <img src="images/i_266.png" width="1518" height="1065" alt=" ">
- <figcaption class="caption"><span class="smcap">Fig. 38.</span>—X-Ray Tube, showing paths of Cathode and X-Rays.
-</figcaption></figure>
-
-<p>Röntgen rays are produced when the cathode rays fall
-upon, and as it were bombard, an obstacle of some kind.
-Almost any tube producing cathode rays will produce also<span class="pagenum" id="Page_232">232</span>
-Röntgen rays, but special forms of tube are used when the
-main object is to obtain these latter rays. <a href="#fig_38">Fig. 38</a> shows
-a typical form of simple X-ray tube. This, like all other
-tubes for X-ray work, is exhausted to a rather higher
-vacuum than tubes intended for the production of cathode
-rays only. The cathode C is made of aluminium, and is
-shaped like a saucer, its curvature being arranged so that
-the cathode rays are focused on to the anti-cathode A.
-The focusing as a rule is not done very accurately, for
-although sharper radiographs are obtained when the cathode
-rays converge exactly to a point on the anti-cathode, the
-heating effect at this point is so great that a hole is quickly
-burned. The target, or surface of the anti-cathode, is
-made of some metal having an extremely high melting-point,
-such as platinum, iridium, or tungsten. It has a flat
-surface inclined at an angle of about 45°, so that the rays
-emanating from it proceed in the direction shown by the
-dotted lines in the figure. The continuous lines show the
-direction of the cathode rays. The anode is made of
-aluminium, and it is shown at N. It is not necessary to<span class="pagenum" id="Page_233">233</span>
-have a separate anode, and the anti-cathode may be used
-as the anode. In the tube shown in <a href="#fig_38">Fig. 38</a> the anode
-and the anti-cathode are joined by an insulated wire, so
-that they both act as anodes. The tube is made of soda-glass,
-as the X-rays do not pass at all readily through lead-glass.</p>
-
-<figure id="fig_39" class="figcenter" style="max-width: 18em;">
- <img src="images/i_267.png" width="1372" height="759" alt=" ">
- <figcaption class="caption">
-
-<p class="floatl"><i>By permission of</i>]</p>
-<p class="floatr">[<i>C. H. F. Muller.</i></p>
-
-<p class="floatc"><span class="smcap">Fig. 39.</span>—Diagram of Mica Vacuum Regulator
-for X-Ray Tubes.</p>
-</figcaption></figure>
-
-<p>The penetrating power of the X-rays varies with the
-vacuum of the tube, a low vacuum giving rays of small
-penetration, and a high vacuum rays of great penetration.
-Tubes are called hard or soft according to the degree of
-the vacuum, a hard
-tube having a high
-vacuum and a soft
-tube a low one. It
-should be remembered
-that the
-terms high and
-low, as applied to
-the vacuum of X-ray
-tubes, are only
-relative, because
-the vacuum must
-be very high to admit of the production of X-rays at all.
-The vacuum becomes higher as the tube is used, and after a
-while it becomes so high that the tube is practically useless,
-for the penetrating power of the rays is then so great that
-sharp contrasts between different substances, such as flesh
-and bone, cannot be obtained, and the resulting radiographs
-are flat and poor. The vacuum of a hard tube may be
-lowered temporarily by gently heating the tube, but this is
-not a very convenient or satisfactory process, and tubes are
-now made with special arrangements for lowering the
-vacuum when necessary. There are several vacuum-regulating
-devices, and <a href="#fig_39">Fig. 39</a> is a diagram of the<span class="pagenum" id="Page_234">234</span>
-“Standard” mica regulator used in most of the well-known
-“Muller” X-ray tubes. This consists of a small additional
-bulb containing an electrode D carrying a series of mica
-discs. A wire DF is attached to D by means of a hinged
-cap. The vacuum is lowered while the discharges are
-passing through the tube. The wire DF is moved towards
-the cathode terminal B, and kept there for a few seconds.
-Sparks pass between F and B, and the current is now
-passing through the electrode D in the regulator chamber.
-This causes the mica to become heated, so that it gives off
-a small quantity of gas, which passes into the main tube
-and so lowers the vacuum. The wire DF is then moved
-well away from B, and after a few hours’ rest the tube, now
-of normal hardness, is ready for further use.</p>
-
-<p>We have already referred to the heating of the anti-cathode
-caused by the bombardment of the cathode rays.
-Even if these rays are not focused very sharply, the anti-cathode
-of an ordinary tube becomes dangerously hot if
-the tube is run continuously for a fairly long period, and
-for hospital and other medical work on an extensive scale
-special tubes with water-cooled anti-cathodes are used.
-These tubes have a small bulb blown in the anti-cathode
-neck. This bulb is filled with water, which passes down a
-tube to the back of the target of the anti-cathode. By this
-arrangement the heat generated in the target is absorbed
-by the water, so that the temperature of the target can
-become only very slightly higher then 212° F., which
-is the temperature of boiling water, and quite a safe
-temperature for the anti-cathode. In some tubes the rise
-in temperature is made slower by the use of broken bits of
-ice in place of water. <a href="#fig_40">Fig. 40</a> shows a Muller water-cooled
-tube, and <a href="#fig_41">Fig. 41</a> explains clearly the parts of an X-ray
-tube and their names.</p>
-
-<figure id="fig_40" class="figcenter" style="max-width: 22em;">
- <img src="images/i_269.png" width="1732" height="905" alt=" ">
- <figcaption class="caption"><span class="smcap">Fig. 40.</span>—Muller Water-cooled X-Ray Tube.
-</figcaption></figure>
-
-<figure id="fig_41" class="figcenter" style="max-width: 26em;">
- <img src="images/i_269b.png" width="2031" height="2225" alt=" ">
- <figcaption class="caption">
-
-<p class="floatl"><i>By permission of</i>]</p>
-<p class="floatr">[<i>C. H. F. Muller.</i></p>
-
-<p class="floatc"><span class="smcap">Fig. 41.</span>—Diagram showing parts of X-Ray Tube.</p>
-</figcaption></figure>
-
-<p>An induction coil is generally used to supply the high-tension<span class="pagenum" id="Page_236">236</span><span class="pagenum" id="Page_235">235</span>
-electricity required for the production of the Röntgen
-rays. For amateur or experimental purposes a coil
-giving continuous 4-inch or even 3-inch sparks will
-do, but for medical work, in which it is necessary to take
-radiographs with very short exposures, coils giving sparks
-of 10, 12, or more inches in length are employed. An
-electrical influence machine, such as the Wimshurst, may
-be used instead of an induction coil. Very powerful
-machines with several pairs of plates of large diameter,
-and driven by an electric motor, are in regular use for
-X-ray work in the United States, but in this country they
-are used only to a very small extent. A Wimshurst
-machine is particularly suitable for amateur work. If a
-screen is to be used for viewing bones through the flesh a
-fairly large machine is required, but for screen examination
-of such objects as coins in a box, or spectacles in a case,
-and for taking radiographs of these and other similar
-objects, a machine giving a fairly rapid succession of sparks
-as short as 2 inches can be used. Of course the exposure
-required for taking radiographs with a machine as small as
-this are very long, but as the objects are inanimate this
-does not matter very much.</p>
-
-<p>For amateur X-ray work the arrangement of the
-apparatus is simple. The tube is held in the required
-position by means of a wooden clamp attached to a stand
-in such a way that it is easily adjustable. Insulated wires
-are led from the coil or from the Wimshurst machine to the
-tube, the positive wire being connected to the anode, and
-the negative wire to the cathode. With a small Wimshurst
-machine light brass chains may be used instead of wires,
-and these have the advantage of being easier to manipulate.
-For medical purposes the arrangements are more complicated,
-and generally a special room is set apart for X-ray
-work.</p>
-
-<p><span class="pagenum" id="Page_237">237</span></p>
-
-<p>If the connexions have been made correctly, then on
-starting the coil or the machine the tube lights up. The
-bulb appears to be sharply divided into two parts, the
-part in front of the anti-cathode glowing with a beautiful
-greenish-yellow light, while the part behind the anti-cathode
-is dark, except for lighter patches close to the
-anode. The Röntgen rays are now being produced. The
-illumination is not steady like that of an electric lamp, but
-it consists of a series of flickers, which, with powerful
-apparatus, follow one another so rapidly as to give the
-impression of continuity. If the connexions are wrong, so
-that the negative wire goes to the anode instead of to the
-cathode, the bulb is not divided in this way, but has
-patches of light almost all over. As soon as this appearance
-is seen the apparatus must be stopped and the connexions
-reversed, for the tube is quickly damaged by passing the
-discharge through it in the wrong direction.</p>
-
-<p>Having produced the X-rays, we will suppose that it
-is desired to examine the bones of the hand. For this
-purpose a fluorescent screen is required. This consists of
-a sheet of white cardboard coated usually with crystals of
-barium platino-cyanide. In order to shut out all light but
-that produced by the rays, the cardboard is placed at the
-larger end of a box or bellows shaped like a pyramid.
-This pyramid is brought close to the X-ray tube, with its
-smaller end held close to the eyes, and the hand is placed
-against the outer side of the cardboard sheet. The outline
-of the hand is then seen as a light shadow, and the very
-much blacker shadow of the bones is clearly visible. For
-screen work it is necessary to darken the room almost
-entirely, on account of the feebleness of the illumination of
-the screen.</p>
-
-<p>If a radiograph of the bones of the hand is to be taken,
-a very sensitive photographic plate is necessary. An<span class="pagenum" id="Page_238">238</span>
-ordinary extra-rapid plate will do fairly well, but for the
-best work plates made specially for the purpose are used.
-The emulsion of an ordinary photographic plate is only
-partially opaque to the X-rays, so that while some of the
-rays are stopped by it, others pass straight through. The
-silver bromide in the emulsion is affected only by those
-rays which are stopped, so that the energy of the rays
-which pass through the emulsion is wasted. If a plate is
-coated with a very thick film, a larger proportion of the
-rays can be stopped, and many X-ray plates differ from
-photographic plates only in the thickness of the emulsion.
-A thick film however is undesirable because it makes the
-after processes of developing, fixing, and washing very
-prolonged. In the “Wratten” X-ray plate the emulsion is
-made highly opaque to the rays in a different and ingenious
-manner. Salts of certain metals have the power of
-stopping the X-rays, and in this plate a metallic salt of this
-kind is contained in the emulsion. The film produced in
-this way stops a far larger proportion of the rays than any
-ordinary film, and consequently the plate is more sensitive
-to the rays, so that shorter exposures can be given.</p>
-
-<p>X-ray plates are sold usually wrapped up separately in
-light-tight envelopes of black paper, upon which the film
-side of the plate is marked. If there is no such wrapping
-the plate must be placed in a light-tight envelope, with
-its film facing that side of the envelope which has no folds.
-The ordinary photographic double envelopes, the inner one
-of yellow paper and the outer one of black paper, are very
-convenient for this purpose. The plate in its envelope is
-then laid flat on the table, film side upwards, and the
-X-ray tube is clamped in a horizontal position so that the
-anti-cathode is over and pointing towards the plate. The
-hand is laid flat on the envelope, and the coil or machine is
-set working. The exposure required varies so much with<span class="pagenum" id="Page_239">239</span>
-the size of the machine or coil, the distance between the
-tube and the plate, the condition of the tube, and the nature
-of the object, that it is impossible to give any definite
-times, and these have to be found by experiment. The
-hand requires a shorter exposure than any other part of the
-body. If we call the correct exposure for the hand 1, then
-the exposures for other parts of the body would be
-approximately 3 for the foot and the elbow, 6 for the
-shoulder, 8 for the thorax, 10 for the spine and the hip,
-and about 12 for the head. The exposures for such objects
-as coins in a box are much less than for the hand. After
-exposure, the plate is developed, fixed, and washed just as
-in ordinary photography. <a href="#plate_XIV">Plate XIV</a>. shows a Röntgen
-ray photograph of a number of fountain pens, British and
-foreign.</p>
-
-<p>Prolonged exposure to the X-rays gives rise to a
-painful and serious disease known as X-ray dermatitis.
-This danger was not realized by the early experimenters,
-and many of them contracted the disease, with fatal results
-in one or two cases. Operators now take ample precautions
-to protect themselves from the rays. The tubes
-are screened by substances opaque to the rays, so that
-these emerge only where they are required, and
-impenetrable gloves or hand-shields, aprons, and face-masks
-made of rubber impregnated with lead-salts are
-worn.</p>
-
-<p>X-ray work is a most fascinating pursuit, and it can be
-recommended strongly to amateurs interested in electricity.
-There is nothing particularly difficult about it, and complete
-outfits can be obtained at extremely low prices, although it
-is best to get the most powerful Wimshurst machine or
-induction coil that can be afforded. As radiography is
-most likely to be taken up by photographers, it may be
-well to state here that any photographic plates or papers<span class="pagenum" id="Page_240">240</span>
-left in their usual wrappings in the room in which X-rays
-are being produced are almost certain to be spoiled, and
-they should be placed in a tightly fitting metal box or be
-taken into the next room. It is not necessary for the
-amateur doing only occasional X-ray work with small
-apparatus to take any of the precautions mentioned in the
-previous paragraph, for there is not the slightest danger in
-such work.</p>
-
-<figure id="plate_XIV" class="figcenter" style="max-width: 35em;">
- <p class="caption">PLATE XIV.</p>
- <img src="images/i_275.jpg" width="3106" height="1886" alt=" ">
- <figcaption class="caption">
-
-<p class="floatl"><i>By permission of</i></p>
-<p class="floatr"><i>Kodak Ltd.</i></p>
-
-<p class="floatc">RÖNTGEN RAY PHOTOGRAPH OF BRITISH AND FOREIGN FOUNTAIN PENS. TAKEN ON WRATTEN X-RAY PLATE.</p>
-</figcaption></figure>
-
-<hr class="chap x-ebookmaker-drop">
-
-<div class="chapter">
-<h2 class="nobreak" id="toclink_241"><a id="chapter_XXV"></a>CHAPTER XXV<br>
-
-<span class="subhead">ELECTRICITY IN MEDICINE</span></h2>
-</div>
-
-<p class="in0"><span class="firstword">One</span> of the most remarkable things about electricity is the
-immense number of different purposes for which it may be
-used. We have already seen it driving trams and trains,
-lighting and heating our houses, and carrying our messages
-thousands of miles over land and sea, and now we come to its
-use in medical work. In the minds of many people medical
-electricity is associated with absolute quackery. Advertisements
-of electric belts, rings, and other similar appliances
-have appeared regularly for many years in our newspapers
-and magazines, and constant exposures of the utter worthlessness
-of almost all these appliances have produced the
-impression that medical electricity is nothing but a bare-faced
-fraud, while the disgusting exhibitions of so-called
-electric healing which have been given on the music-hall
-stage have greatly deepened this impression. This state
-of things is very unfortunate, because electricity, in the
-hands of competent medical men, is a healing agent of
-wonderful potency. Still another source of prejudice
-against electricity may be found in the fact that electric
-healing is popularly associated with more or less violent
-shocks. On this account nervously-sensitive people shrink
-from the idea of any kind of electrical treatment. As a
-matter of fact electric shocks have no healing value, but on
-the contrary they are frequently harmful, and a very severe
-shock to a sensitive person may cause permanent injury.<span class="pagenum" id="Page_242">242</span>
-No shocks whatever are given in electric treatment by
-medical men, and indeed in the majority of cases the treatment
-is unaccompanied by unpleasant sensations of any kind.</p>
-
-<p>In the previous chapter we spoke of the use of the
-Röntgen or X-rays in examining the various bones of the
-body. By means of the fluorescent screen it is quite easy
-to find and examine fractures and dislocations, and many
-of the diseases of the bones and joints can be seen and
-recognized. Metals are opaque to the X-rays, and so the
-screen shows plainly such objects as needles or bullets
-embedded in the flesh. Sometimes people, especially
-young children, swallow coins and other small metal articles,
-and here again the X-rays will show the exact position of
-the intruder. A particularly valuable application of the
-rays is in the discovering and locating of tiny fragments of
-metal in the eye, for very often it is quite impossible to do
-this by ordinary observation. Most of these fragments are
-of steel or iron, and they are most easily removed by means
-of an electro-magnet. If the fragment is very small a
-powerful magnet is used, one capable of supporting 500
-or 600 lb.; but if it is fairly large a weaker magnet,
-supporting perhaps 30 lb., must be employed, because
-the forceful and rapid dragging out of a large body might
-seriously damage the eye.</p>
-
-<p>If the chest is examined by the Röntgen rays the lungs
-are seen as light spaces between the clearly marked ribs,
-and any spot of congestion appears as a darker patch. In
-this way the early stages of consumption may be revealed,
-and in pneumonia and other similar complaints valuable
-information regarding the condition of the lungs can be
-obtained. It is possible also to follow to a considerable extent
-the processes of digestion. X-rays easily pass through ordinary
-food, but if bismuth oxychloride, which is quite harmless,
-is mixed with the food, the mixture becomes opaque<span class="pagenum" id="Page_243">243</span>
-to the rays, and so its course may be followed on the screen.
-The normal movements of the food are well known, and an
-abnormal halt is probably caused by an obstruction of some
-kind, and thus the X-rays enable the physician to locate
-the obstruction, and to form an opinion of its nature.</p>
-
-<p>In our chapter on wireless telegraphy we saw that the
-discharge of a Leyden jar takes the form of a number of
-rapid oscillations backwards and forwards. These oscillations
-take place at a rate of more than half a million per
-second, but by the use of an apparatus called a “high frequency
-transformer” the rate is increased to more than a
-million per second. Electricity in this state of rapid oscillation
-is known as high frequency electricity, and high frequency
-currents are very valuable for some kinds of medical
-work. The application of these currents is quite painless, and
-but for the strange-looking apparatus the patient probably
-would not know that anything unusual was taking place.
-To some extent the effect maybe said to be not unlike that
-of a powerful tonic. Insomnia and other troubles due to
-disordered nerves are quickly relieved, and even such
-obstinate complaints as neuritis and crippling rheumatism
-have been cured. The treatment is also of great value in
-certain forms of heart trouble. By increasing the strength
-of the high frequency currents the tissues actually may
-be destroyed, and this power is utilized for exterminating
-malignant growths, such as lupus or cancer.</p>
-
-<p>The heat produced by a current of electricity is made
-use of in cauterizing. The burner is a loop of platinum
-wire, shaped according to the purpose for which it is
-intended, and it is used at a dull red heat. Very tiny
-electric incandescent lamps, fitted in long holders of special
-shape, are largely used for examining the throat and the
-various cavities of the body.</p>
-
-<p>In the Finsen light treatment electric light is used for<span class="pagenum" id="Page_244">244</span>
-a very different purpose. The spectrum of white light consists
-of the colours red, orange, yellow, green, blue, indigo,
-and violet. Just beyond the violet end of the spectrum are
-the ultra-violet rays. Ultra-violet light consists of waves
-of light which are so short as to be quite invisible to the
-eye, and Dr. N. R. Finsen, a Danish physician, made the
-discovery that this light is capable of destroying bacterial
-germs. In the application of ultra-violet rays to medical
-work, artificial light is used in preference to sunlight; for
-though the latter contains ultra-violet light, a great deal of
-it is absorbed in passing through the atmosphere. Besides
-this, the sun sends out an immense amount of radiant heat,
-and this has to be filtered out before the light can be used.
-The usual source of light is the electric arc, and the arc is
-much richer in ultra-violet rays if it is formed between
-electrodes of iron, instead of the usual carbon rods. The
-light, which, in addition to the ultra-violet rays, includes
-the blue, indigo, and violet parts of the spectrum, is passed
-along a tube something like that of a telescope, and is
-focused by means of a double lens, consisting of two
-separate plates of quartz. Glass cannot be used for the
-lens, because it is opaque to the extreme ultra-violet rays.
-A constant stream of water is passed between the two
-plates forming the lens, and this filters out the heat rays,
-which are not wanted. In some forms of Finsen lamp an
-electric spark is used as the source of light, in place of the
-arc.</p>
-
-<p>The most important application of the Finsen light is
-in the cure of the terribly disfiguring disease called lupus.
-This is a form of tuberculosis of the skin, and it is produced
-by the same deadly microbe which, when it attacks
-the lungs, causes consumption. In all but extreme cases
-the Finsen light effects a remarkable cure. A number of
-applications are necessary, each of half an hour or more;<span class="pagenum" id="Page_245">245</span>
-and after a time the disease begins to disappear, leaving
-soft, normal skin. The exact action of the light rays is a
-disputed point. Finsen himself believed that the ultra-violet
-rays attacked and exterminated the microbe, but a
-later theory is that the rays stimulate the tissues to such
-an extent that they are enabled to cure themselves. As
-early as the year 1899 Finsen had employed his light
-treatment in 350 cases of lupus, and out of this number
-only five cases were unsuccessful.</p>
-
-<p>The ultra-violet rays are said to have a very beneficial
-effect upon the teeth. Experiments carried out in Paris,
-using a mercury vapour lamp as the source of light, show
-that discoloured teeth are whitened and given a pearly
-lustre by these rays, at the same time being sterilized so
-that they do not easily decay. The Röntgen rays are
-used for the treatment of lupus, and more particularly for
-deeper growths, such as tumours and cancers, for which
-the Finsen rays are useless, owing to their lack of penetrating
-power. The action of these two kinds of rays appears
-to be similar, but the X-rays are much the more active of
-the two.</p>
-
-<p>Electricity is often applied to the body through water,
-in the form of the hydro-electric bath, and such baths
-are used in the treatment of different kinds of paralysis.
-Electric currents are used too for conveying drugs into the
-tissues of the body. This is done when it is desired to
-concentrate the drug at some particular point, and it has
-been found that chemicals can be forced into the tissues for
-a considerable distance.</p>
-
-<p>Dr. Nagelschmidt, a great authority on medical electricity,
-has suggested the use of electricity for weight reducing.
-In the ordinary way superfluous flesh is got rid
-of by a starvation diet coupled with exercise, but in many
-cases excessively stout people are troubled with heart<span class="pagenum" id="Page_246">246</span>
-disorders and asthma, so that it is almost impossible for
-them to undergo the necessary muscular exertion. By the
-application of electric currents, however, the beneficial
-effects of the gentle exercise may be produced without any
-exertion on the part of the patient, and an hour’s treatment
-is said to result in a decrease in weight of from 200 to 800
-grammes, or roughly 7 to 27 ounces.</p>
-<hr class="chap x-ebookmaker-drop">
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_247">247</span></p>
-
-<h2 class="nobreak" id="toclink_247"><a id="chapter_XXVI"></a>CHAPTER XXVI<br>
-
-<span class="subhead">OZONE</span></h2>
-</div>
-
-<p class="in0"><span class="firstword">The</span> great difference between the atmospheric conditions
-before and after a thunderstorm must have been noticed
-by everybody. Before the storm the air feels lifeless. It
-does not satisfy us as we draw it into our lungs, and however
-deeply we breathe, we feel that something is lacking.
-After the storm the air is delightful to inhale, and it refreshes
-us with every breath. This remarkable transformation
-is brought about to a very large extent by ozone
-produced by the lightning discharges.</p>
-
-<p>As far back as 1785 it was noticed that oxygen became
-changed in some way when an electric spark was passed
-through it, and that it acquired a peculiar odour. No
-particular attention was paid to the matter however until
-about 1840, when Schönbein, a famous German chemist, and
-the discoverer of gun-cotton and collodion, became interested
-in it. He gave this strange smelling substance the name
-of “ozone,” and he published the results of his experiments
-with it in a treatise entitled, “On the Generation of
-Ozone.” Schönbein showed that ozone could be produced
-by various methods, chemical as well as electrical. For
-instance, if a piece of phosphorus is suspended in a jar of
-air containing also a little water, in such a manner that it
-is partly in the water and partly out of it, the air acquires
-the characteristic smell of ozone, and it is found to have
-gained increased chemical energy, so that it is a more<span class="pagenum" id="Page_248">248</span>
-powerful oxidizing agent. For a long time the exact
-chemical nature of ozone could not be determined, mainly
-because it was impossible to obtain the substance in
-quantities sufficiently large for extensive experimental
-research, but also on account of its extremely energetic
-properties, which made it very troublesome to examine.
-These difficulties were so great that investigators were in
-doubt as to whether ozone was an element or a compound
-of two or more elements; but finally it was proved that it
-was simply oxygen in a condensed or concentrated state.</p>
-
-<p>Apparently ozone is formed by the contraction of
-oxygen, so that from three volumes of oxygen two volumes
-of ozone are produced. In other words, ozone has one and
-a half times the density of oxygen. Ozone has far greater
-oxidizing power than oxygen itself; in fact it is probably
-the most powerful of all oxidizing agents, and herein lies
-its great value. It acts as nature’s disinfectant or sterilizer,
-and plays a very important part in keeping the air pure,
-by destroying injurious organic matter. Bacteria apparently
-have a most decided objection to dying; at any
-rate they take an extraordinary amount of killing. Ozone
-is more than a match for them however, and under its
-influence they have a short life and probably not a merry
-one.</p>
-
-<p>Ozone exists naturally in the atmosphere in the open
-country, and more especially at the seaside. It is produced
-by lightning discharges, by silent electrical discharges
-in the atmosphere, by the evaporation of water,
-particularly salt water, by the action of sunlight, and also
-by the action of certain vegetable products upon the air.
-The quantity of ozone in the air is always small, and even
-pure country or sea air contains only one volume of ozone
-in about 700,000 volumes of air. No ozone can be detected
-in the air of large towns, or over unhealthy swamps or<span class="pagenum" id="Page_249">249</span>
-marshes. The exhilarating effects of country and sea air,
-and the depressing effects of town air, are due to a very
-large extent to the presence or absence of ozone.</p>
-
-<p>A great proportion of our common ailments are caused
-directly or indirectly by a sort of slow poisoning, produced
-by the impure air in which we live and work. It is popularly
-supposed that the tainting of the air of rooms in
-which large numbers of people are crowded together is due
-to an excessive amount of carbonic acid gas. This is a
-mistake, for besides being tasteless and odourless, carbonic
-acid gas is practically harmless, except in quantities far
-greater than ever exist even in the worst ventilated rooms.
-The real source of the tainted air is the great amount of
-animal matter thrown off as waste products from the skin
-and lungs, and this tainting is further intensified by the
-absence of motion in the air. Even in an over-crowded
-room the conditions are made much more bearable if the
-air is kept in motion, and in a close room ladies obtain
-relief by the use of their fans. What we require, therefore,
-in order to maintain an agreeable atmosphere under
-all conditions, is some means of keeping the air in gentle
-motion, and at the same time destroying as much as possible
-of the animal matter contained in it. Perhaps the
-most interesting and at the same time the most scientific
-method of doing this is by ozone ventilation.</p>
-
-<p>In the well-known “Ozonair” system of ventilation,
-ozone is generated by high-tension current. Low-tension
-current is taken from the public mains or from accumulators,
-and raised to a very high voltage by passing it through a
-step-up transformer. The secondary terminals of the
-transformer are connected to a special form of condenser,
-consisting of layers of fine metal gauze separated by an
-insulating substance called “micanite.” The high tension
-between the gauze layers produces a silent electrical discharge<span class="pagenum" id="Page_250">250</span>
-or glow. A small fan worked by an electric motor
-draws the air over the condenser plates, and so a certain
-proportion of the oxygen is ozonized, and is driven out of
-the other side of the apparatus into the room. The amount
-of ozone generated and the amount of air drawn over the
-condenser are regulated carefully, so that the ozonized air
-contains rather less than one volume of ozone in one
-million volumes of air, experiment having shown that this
-is the most suitable strength for breathing. Ozone diluted
-to this degree has a slight odour which is very refreshing,
-and besides diminishing the number of organic germs in
-the air, it neutralizes unpleasant smells, such as arise from
-cooking or stale tobacco smoke. Ozone ventilation is now
-employed successfully in many hotels, steamships, theatres
-and other places of entertainment, municipal and public
-buildings, and factories.</p>
-
-<figure id="fig_42" class="figcenter" style="max-width: 38em;">
- <img src="images/i_287.jpg" width="2999" height="2089" alt=" ">
- <figcaption class="caption">
-
-<p class="floatl"><i>By permission of</i>]</p>
-<p class="floatr">[<i>Ozonair, Ltd.</i></p>
-
-<p class="floatc"><span class="smcap">Fig. 42.</span>—Diagram of Ozonizing Plant, Central London Tube Electric Railway.</p>
-</figcaption></figure>
-
-<p>One of the most interesting examples of ozone ventilation
-is that of the Central London tube electric railway.
-The installation consists of a separate ozonizing plant at
-every station, except Shepherd’s Bush, which is close to
-the open end of the tunnel. <a href="#fig_42">Fig. 42</a> is a diagram of the
-general arrangement of one of these plants, and it shows
-how the air is purified, ozonized, and sent into the tunnel.
-The generating plant is seen at the top left-hand corner of
-the figure. Air is drawn in as shown by the arrows, and
-by passing through the filter screen F it is freed from dirt
-and smuts, and from most of the injurious gases which
-always are present in town air. The filter screen is kept
-moist by a continual flow of water from jets above it, the
-waste water falling into the trough W. The ozone
-generator is shown at O. Continuous current at about
-500 volts, from the power station, is passed through a
-rotary converter, which turns it into alternating current at
-380 volts. This current goes to the transformer T, from<span class="pagenum" id="Page_252">252</span>
-which it emerges at a pressure of 5000 volts, and is supplied
-to the ozone generator. From the generator the strongly
-ozonized air is taken by way of the ozone pipe P, to the
-mixing chamber of the large ventilating fan M, where it is
-mixed with the main air current and then blown down the
-main air trunk. From this trunk it is distributed to various
-conduits, and delivered at the air outlets marked A.
-Altogether the various plants pump more than eighty
-million cubic feet of ozonized air into the tunnels every
-working day.</p>
-
-<p>In many industries pure air is very essential, especially
-during certain processes. This is the case in brewing, in
-cold storage, and in the manufacture and canning of food
-products; and in these industries ozone is employed as an
-air purifier, with excellent results. Other industries cannot
-be carried on without the production of very unpleasant
-fumes and smells, which are a nuisance to the workers and
-often also to the people living round about; and here
-again ozone is used to destroy and remove the offending
-odours. It is employed also in the purification of sewage
-and polluted water; in bleaching delicate fabrics; in drying
-and seasoning timber; in maturing tobacco, wines and
-spirits, and in many other processes too numerous to
-mention.</p>
-<hr class="chap x-ebookmaker-drop">
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_253">253</span></p>
-
-<h2 class="nobreak" id="toclink_253"><a id="chapter_XXVII"></a>CHAPTER XXVII<br>
-
-<span class="subhead">ELECTRIC IGNITION</span></h2>
-</div>
-
-<p class="in0"><span class="firstword">The</span> petrol motor, which to-day is busily engaged all over
-the world in driving thousands upon thousands of self-propelled
-vehicles or automobiles, belongs to the important
-class of internal-combustion engines. Combustion means
-the operation of burning, and an internal-combustion engine
-is one in which the motive power is produced by the combustion
-of a highly explosive mixture of gases. In the
-ordinary petrol motor this mixture consists of petrol and air,
-and it is made by means of a device called a “carburetter.”
-By suction, a quantity of petrol is forced through a jet with
-a very fine nozzle, so that it is reduced to an extremely fine
-spray. A certain proportion of air is allowed to enter, and
-the mixture passes into the cylinder. Here it is compressed
-by the rising piston so that it becomes more and more
-heated, and at the right point it is ignited. Combustion
-takes place with such rapidity that it takes the form of an
-explosion, and the energy produced in this way drives
-forward the piston, which turns the crank-shaft and so
-communicates motion to the driving-wheels.</p>
-
-<p>The part played by electricity in this process is confined
-to the ignition of the compressed charge of petrol and air.
-This may be done in two ways; by means of an accumulator
-and a small induction coil, or by means of a dynamo driven
-by the engine. At one time the first method was employed
-exclusively, but to-day it is used as a rule only for starting<span class="pagenum" id="Page_254">254</span>
-the car engine, the second or magneto method being used
-when the engine has started up.</p>
-
-<p>In accumulator ignition the low-tension current from
-the accumulator passes through an induction coil, and is
-thus transformed to high-tension current. This current
-goes through a sparking plug, which is fixed in the head
-of the cylinder. The sparking plug contains two metal
-points separated by a tiny air gap of from about 1/30 to 1/50
-inch. This gap provides the only possible path for the
-high-tension current, so that the latter leaps across it in
-the form of a spark. The spark is arranged to take place
-when the piston is at the top of its stroke, that is, when the
-explosive mixture is at its maximum compression, and the
-heat of the spark ignites the mixture, the resulting explosion
-forcing down the piston with great power. In practice it
-is found better as a rule to cause the spark to pass very
-slightly before the piston reaches the extreme limit of its
-stroke. The reason of this is that the process of igniting
-and exploding the charge occupies an appreciable, though
-of course exceedingly small amount of time. Immediately
-on reaching the top of its stroke the piston begins to
-descend again, and if the spark and the top of the stroke
-coincide in time the explosion does not take place until the
-piston has moved some little distance down the cylinder,
-and so a certain amount of power is lost. By having the
-spark a little in advance of the piston, the explosion occurs
-at the instant when the piston begins to return, and so the
-full force of the explosion is utilized.</p>
-
-<p>In magneto ignition the current is supplied by a small
-dynamo. This generates alternating current, and it is
-driven by the car engine. The current is at first at low
-pressure, and it has to be transformed to high-tension
-current in order to produce the spark. There are two
-methods of effecting this transformation. One is by turning<span class="pagenum" id="Page_255">255</span>
-the armature of the dynamo into a sort of induction coil, by
-giving it two separate windings, primary and secondary;
-so that the dynamo delivers high-tension current directly.
-The other method is to send the low-tension current
-through one or more transformer coils, just as in accumulator
-ignition. Accumulators can give current only for a
-certain limited period, and they are liable consequently to
-run down at inconvenient times and places. They also
-have the defect of undergoing a slight leakage of current
-even when they are not in use. Magneto ignition has
-neither of these drawbacks, and on account of its superior
-reliability it has come into universal use.</p>
-
-<p>In the working of quarries and mines of various kinds,
-and also in large engineering undertakings, blasting plays
-a prominent part. Under all conditions blasting is a more
-or less dangerous business, and it has been the cause of
-very many serious accidents to the men engaged in carrying
-it out. Many of these accidents are due to the carelessness
-resulting from long familiarity with the work, but apart
-from this the danger lies principally in uncertainty in
-exploding the charge. Sometimes the explosion occurs
-sooner than expected, so that the men have not time to get
-away to a safe distance. Still more deadly is the delayed
-explosion. After making the necessary arrangements the
-men retire out of danger, and await the explosion. This
-does not take place at the expected time, and after waiting
-a little longer the men conclude that the ignition has failed,
-and return to put matters right. Then the explosion takes
-place, and the men are killed instantly or at least seriously
-injured. Although it is impossible to avoid altogether
-dangers of this nature, the risk can be reduced to the
-minimum by igniting the explosives by electricity.</p>
-
-<p>Electrical shot firing may be carried out in different
-ways, according to circumstances. The current is supplied<span class="pagenum" id="Page_256">256</span>
-either by a dynamo or by a battery, and the firing is controlled
-from a switchboard placed at a safe distance from the point
-at which the charge is to be exploded, the connexions being
-made by long insulated wires. The actual ignition is
-effected by a hot spark, as in automobile ignition, or by an
-electric detonator or fuse. Explosives such as dynamite
-cannot be fired by simple ignition, but require to be
-detonated. This is effected by a detonator consisting of a
-small cup-shaped tube, made of ebonite or other similar
-material. The wires conveying the current project into this
-tube, and are connected by a short piece of very fine wire
-having a high resistance. Round this wire is packed a
-small quantity of gun-cotton, and beyond, in a sort of continuation
-of the tube, is placed an extremely explosive
-substance called “fulminate of mercury,” the whole arrangement
-being surrounded by the dynamite to be fired. When
-all is ready the man at the switchboard manipulates a
-switch, and the current passes to the detonator and forces
-its way through the resistance of the thin connecting wire.
-This wire becomes sufficiently hot to ignite the gun-cotton,
-and so explode the fulminate of mercury. The explosion
-is so violent that the dynamite charge is detonated, and
-the required blasting carried out. Gunpowder and similar
-explosives do not need to be detonated, and so a simple
-fuse is used. Electric fuses are much the same as detonators,
-except that the tube contains gunpowder instead of
-fulminate of mercury, this powder being ignited through an
-electrically heated wire in the same way. These electrical
-methods do away with the uncertainty of the slow-burning
-fuses formerly employed, which never could be relied upon
-with confidence.</p>
-
-<p>Enormous quantities of explosives are now used in
-blasting on a large scale, where many tons of hard rock
-have to be removed. One of the most striking blasting<span class="pagenum" id="Page_257">257</span>
-feats was the blowing up of Flood Island, better known as
-Hell Gate. This was a rocky islet, about 9 acres in
-extent, situated in the East River, New York. It was a
-continual menace to shipping, and after many fine vessels
-had been wrecked upon it the authorities decided that it
-should be removed. The rock was bored and drilled in all
-directions, the work taking more than a year to complete;
-and over 126 tons of explosives were filled into the borings.
-The exploding was carried out by electricity, and the
-mighty force generated shattered nearly 300,000 cubic
-yards of solid rock.</p>
-<hr class="chap x-ebookmaker-drop">
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_258">258</span></p>
-
-<h2 class="nobreak" id="toclink_258"><a id="chapter_XXVIII"></a>CHAPTER XXVIII<br>
-
-<span class="subhead">ELECTRO-CULTURE</span></h2>
-</div>
-
-<p class="in0"><span class="firstword">About</span> thirty years ago a Swedish scientist, Professor
-Lemström, travelled extensively in the Polar regions, and
-he was greatly struck by the development of the Polar
-vegetation. In spite of the lack of good soil, heat, and
-light, he observed that this vegetation came to maturity
-quicker than that of regions having much more favourable
-climates, and that the colours of the flowers were remarkably
-fresh and clear, and their perfumes exceptionally
-strong. This was a surprising state of things, and
-Lemström naturally sought a clue to the mystery. He
-knew that peculiar electrical conditions prevailed in these
-high latitudes, as was shown by the wonderful displays of
-the Aurora Borealis, and he came to the conclusion that
-the development of the vegetation was due to small currents
-of electricity continually passing backwards and forwards
-between the atmosphere and the Earth. On his return to
-civilization Lemström at once began a series of experiments
-to determine the effect of electricity upon the growth of
-plants, and he succeeded in proving beyond all doubt that
-plants grown under electrical influence flourished more
-abundantly than those grown in the ordinary way.
-Lemström’s experiments have been continued by other
-investigators, and striking and conclusive results have been
-obtained.</p>
-
-<p>The air surrounding the Earth is always charged to<span class="pagenum" id="Page_259">259</span>
-some extent with electricity, which in fine weather is
-usually positive, but changes to negative on the approach
-of wet weather. This electricity is always leaking away to
-the earth more or less rapidly, and on its way it passes
-through the tissues of the vegetation. An exceedingly
-slow but constant discharge therefore is probably taking
-place in the tissues of all plants. Experiments appear to
-indicate that the upper part of a growing plant is negative,
-and the lower part positive, and at any rate it is certain that
-the leaves of a plant give off negative electricity. In dull
-weather this discharge is at its minimum, but under the
-influence of bright sunshine it goes on with full vigour. It
-is not known exactly how this discharge affects the plant,
-but apparently it assists its development in some way, and
-there is no doubt that when the discharge is at its maximum
-the flow of sap is most vigorous. Possibly the electricity
-helps the plant to assimilate its food, by making this more
-readily soluble.</p>
-
-<p>This being so, a plant requires a regular daily supply of
-uninterrupted sunshine in order to arrive at its highest
-possible state of maturity. In our notoriously variable
-climate there are many days with only short intermittent
-periods of bright sunshine, and many other days without
-any sunshine at all. Now if, on these dull days, we can
-perform at least a part of the work of the sunshine, and
-strengthen to some extent the minute currents passing
-through the tissues of a plant, the development of this
-plant should be accelerated, and this is found to be the
-case. Under electrical influence plants not only arrive at
-maturity quicker, but also in most cases their yield is
-larger and of finer quality.</p>
-
-<p>Lemström used a large influence machine as the source
-of electricity in his experiments in electro-culture. Such
-machines are very suitable for experimental work on a<span class="pagenum" id="Page_260">260</span>
-small scale, and much valuable work has been done with
-them by Professor Priestly and others; but they have the
-great drawback of being uncertain in working. They are
-quite satisfactory so long as the atmosphere remains dry,
-but in damp weather they are often very erratic, and may
-require hours of patient labour to coax them to start. For
-this reason an induction coil is more suitable for continuous
-work on an extensive scale.</p>
-
-<p>The most satisfactory apparatus for electro-culture is
-that used in the Lodge-Newman method, designed by Sir
-Oliver Lodge and his son, working in conjunction with
-Mr. Newman. This consists of a large induction coil
-supplied with current from a dynamo driven by a small
-engine, or from the public mains if available. This coil
-is fitted with a spark gap, and the high-tension current goes
-through four or five vacuum valve globes, the invention
-of Sir Oliver Lodge, which permit the current to pass
-through them in one direction only. This is necessary
-because, as we saw in <a href="#chapter_VIII">Chapter VIII</a>., two opposite currents
-are induced in the secondary winding of the coil, one at the
-make and the other at the break of the primary circuit.
-Although the condenser fitted in the base of the coil
-suppresses to a great extent the current induced on making
-the circuit, still the current from the coil is not quite
-uni-directional, but it is made so by the vacuum rectifying
-valves. These are arranged to pass only the positive
-current, and this current is led to overhead wires out in the
-field to be electrified. Lemström used wires at a height of
-18 inches from the ground, but these were very much in
-the way, and in the Lodge-Newman system the main wires
-are carried on large porcelain insulators fixed at the top of
-poles at a height of about 15 feet. This arrangement
-allows carting and all other agricultural operations to be
-carried on as usual. The poles are set round the field,<span class="pagenum" id="Page_261">261</span>
-about one to the acre, and from these main wires finer
-ones are carried across the field. These wires are placed
-about 30 feet apart, so that the whole field is covered by a
-network of wires. The electricity supplied to the wires is
-at a pressure of about 100,000 volts, and this is constantly
-being discharged into the air above the plants. It then
-passes through the plants, and so reaches the earth. This
-system may be applied also to plants growing in greenhouses,
-but owing to the confined space, and to the amount
-of metal about, in the shape of hot-water pipes and wires
-for supporting plants such as vines and cucumbers, it is
-difficult to make satisfactory arrangements to produce the
-discharge.</p>
-
-<p>The results obtained with this apparatus at Evesham,
-in Gloucestershire, by Mr. Newman, have been most
-striking. With wheat, increases of from 20 per cent. to
-nearly 40 per cent. have been obtained, and the electrified
-wheat is of better quality than unelectrified wheat grown at
-the same place, and, apart from electrification, under exactly
-the same conditions. In some instances the electrified
-wheat was as much as 8 inches higher than the
-unelectrified wheat. Mr. Newman believes that by
-electrification land yielding normally from 30 to 40 bushels
-of wheat per acre can be made to yield 50 or even 60
-bushels per acre. With cucumbers under glass increases
-of 17 per cent. have been obtained, and in the case of
-strawberries, increases of 36 per cent. with old plants, and
-80 per cent. with one-year-old plants. In almost every
-case electrification has produced a marked increase in the
-crop, and in the few cases where there has been a decrease
-the crops were ready earlier than the normal. For instance,
-in one experiment with broad beans a decrease of 15 per
-cent. resulted, but the beans were ready for picking five
-days earlier. In another case a decrease of 11½ per cent.<span class="pagenum" id="Page_262">262</span>
-occurred with strawberries, but the fruit was ready for
-picking some days before the unelectrified fruit, and also
-was much sweeter. In some of the experiments resulting
-in a decrease in the yield it is probable that the electrification
-was overdone, so that the plants were over-stimulated.
-It seems likely that the best results will be obtained only
-by adjusting the intensity and the duration of the electrification
-in accordance with the atmospheric conditions, and
-also with the nature of the crop, for there is no doubt that
-plants vary considerably in their electrical requirements.
-A great deal more experiment is required however to
-enable this to be done with anything like certainty.</p>
-
-<p>Unlike the farmer, the market gardener has to produce
-one crop after another throughout the year. To make up
-for the absence of sufficient sunshine he has to resort to
-“forcing” in many cases, but unfortunately this process,
-besides being costly, generally results in the production of
-a crop of inferior quality. Evidently the work of the
-market gardener would be greatly facilitated by some
-artificial substitute for sunshine, to keep his plants growing
-properly in dull weather. In 1880, Sir William Siemens,
-knowing that the composition of the light of the electric
-arc was closely similar to that of sunlight, commenced
-experiments with an arc lamp in a large greenhouse. His
-idea was to add to the effects of the solar light by using
-the arc lamp throughout the night. His first efforts were
-unsuccessful, and he discovered that this was due to the
-use of the naked light, which apparently contained rays
-too powerful for the plants. He then passed the light
-through glass, which filtered out the more powerful rays,
-and this arrangement was most successful, the plants
-responding readily to the artificial light. More scientifically
-planned experiments were carried out at the London
-Royal Botanic Gardens in 1907, by Mr. B. H. Thwaite,<span class="pagenum" id="Page_263">263</span>
-and these showed that by using the arc lamp for about five
-hours every night, a great difference between the treated
-plants and other similar plants grown normally could be
-produced in less than a month. Other experiments made
-in the United States with the arc lamp, and also with
-ordinary electric incandescent lamps, gave similar results,
-and it was noticed that the improvement was specially
-marked with cress, lettuce, spinach, and other plants of this
-nature.</p>
-
-<p>In 1910, Miss E. C. Dudgeon, of Dumfries, commenced
-a series of experiments with the Cooper-Hewitt mercury
-vapour lamp. Two greenhouses were employed, one of
-which was fitted with this lamp. Seeds of various plants
-were sown in small pots, one pot of each kind being placed
-in each house. The temperature and other conditions
-were kept as nearly alike as possible in both houses, and
-in the experimental house the lamp was kept going for
-about five hours every night. In every case the seeds in
-the experimental house germinated several days before
-those in the other house, and the resulting plants were
-healthy and robust. Later experiments carried out by
-Miss Dudgeon with plants were equally successful.</p>
-
-<p>From these experiments it appears that the electric arc,
-and still more the mercury vapour lamp, are likely to prove
-of great value to the market gardener. As compared with
-the arc lamp, the mercury vapour lamp has the great
-advantage of requiring scarcely any attention, and also it
-uses less current. Unlike the products of ordinary forcing
-by heat, the plants grown under the influence of the
-mercury vapour light are quite sturdy, so that they can be
-planted out with scarcely any “hardening off.” The crop
-yields too are larger, and of better quality. The wonderful
-effects produced by the Cooper-Hewitt lamp are
-certainly not due to heat, for this lamp emits few heat rays.<span class="pagenum" id="Page_264">264</span>
-The results may be due partly to longer hours worked by
-the plants, but this does not explain the greater accumulation
-of chlorophyll and stronger development of fibre.</p>
-
-<p>Most of us are familiar with the yarn about the poultry
-keeper who fitted all his nests with trap-doors, so that when
-a hen laid an egg, the trap-door opened under the weight
-and allowed the egg to fall through into a box lined with
-hay. The hen then looked round, and finding no egg, at
-once set to work to lay another. This in turn dropped,
-another egg was laid, and so on. It is slightly doubtful
-whether the modern hen could be swindled in this bare-faced
-manner, but it is certain that she can be deluded into
-working overtime. The scheme is absurdly simple.
-Electric lamps are fitted in the fowl-house, and at sunset
-the light is switched on. The unsuspecting hens, who are
-just thinking about retiring for the night, come to the conclusion
-that the day is not yet over, and so they continue
-to lay. This is not a yarn, but solid fact, and the increase
-in the egg yield obtained in this way by different poultry
-keepers ranges from 10 per cent. upwards. Indeed, one
-poultry expert claims to have obtained an increase of about
-40 per cent.</p>
-
-<p>The ease with which a uniform temperature can be
-maintained by electric heating has been utilized in incubator
-hatching of chickens. By means of a specially designed
-electric radiator the incubator is kept at the right temperature
-throughout the hatching period. When the chickens
-emerge from the eggs they are transferred to another
-contrivance called a “brooder,” which also is electrically
-heated, the heat being decreased gradually day by day until
-the chicks are sturdy enough to do without it. Even at
-this stage however the chickens do not always escape
-from the clutches of electricity. Some rearers have
-adopted the electric light swindle for the youngsters,<span class="pagenum" id="Page_265">265</span>
-switching on the light after the chickens have had a fair
-amount of slumber, so that they start feeding again. In
-this way the chickens are persuaded to consume more food
-in the twenty-four hours, and the resulting gain in weight
-is said to be considerable. More interesting than this
-scheme is the method of rearing chickens under the
-influence of an electric discharge from wires supplied with
-high-tension current. Comparative tests show that electrified
-chickens have a smaller mortality and a much greater
-rate of growth than chickens brought up in the ordinary
-way. It even is said that the electrified chickens have
-more kindly dispositions than their unelectrified relatives!</p>
-
-<p>Possibly the high-tension discharge may turn out to be
-as beneficial to animals as it has been proved to be for
-plants, but so far there is little reliable evidence on this
-point, owing to lack of experimenters. A test carried out
-in the United States with a flock of sheep is worth
-mention. The flock was divided into two parts, one-half
-being placed in a field under ordinary conditions, and the
-other in a field having a system of overhead discharge
-wires, similar to those used in the Lodge-Newman system.
-The final result was that the electrified sheep produced
-more than twice as many lambs as the unelectrified sheep,
-and also a much greater weight of wool. If further experiments
-confirm this result, the British farmer will do well to
-consider the advisability of electrifying his live-stock.</p>
-<hr class="chap x-ebookmaker-drop">
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_266">266</span></p>
-
-<h2 class="nobreak" id="toclink_266"><a id="chapter_XXIX"></a>CHAPTER XXIX<br>
-
-<span class="subhead">SOME RECENT APPLICATIONS OF ELECTRICITY—AN ELECTRIC PIPE LOCATOR</span></h2>
-</div>
-
-<p class="in0"><span class="firstword">One</span> of the great advantages of living in a town is the
-abundant supply of gas and water. These necessary
-substances are conveyed to us along underground pipes,
-and a large town has miles upon miles of such pipes,
-extending in all directions and forming a most complex
-network. Gas and water companies keep a record of these
-pipes, with the object of finding any pipe quickly when the
-necessity arises; but in spite of such records pipes are
-often lost, especially where the whole face of the neighbourhood
-has changed since the pipes were laid. The finding
-of a lost pipe by digging is a very troublesome process, and
-even when the pipe is known to be close at hand, it is quite
-surprising how many attempts are frequently necessary
-before it can be located, and its course traced. As may be
-imagined, this is an expensive business, and often it has been
-found cheaper to lay a new length of pipe than to find the
-old one. There is now an electrical method by which pipe
-locating is made comparatively simple, and unless it is very
-exceptionally deep down, a pipe never need be abandoned
-on account of difficulty in tracing it.</p>
-
-<p>The mechanism of an electric pipe locator is not at all
-complicated, consisting only of an induction coil with
-battery, and a telephone receiver connected to a coil of a
-large number of turns of thin copper wire. If a certain<span class="pagenum" id="Page_267">267</span>
-section of a pipe is lost, and has to be located, operations
-are commenced from some fitting known to be connected
-with it, and from some other fitting which may or may not
-be connected with the pipe, but which is believed to be so
-connected. The induction coil is set working, and its
-secondary terminals are connected one to each of these
-fittings. If the second fitting is connected with the pipe,
-then the whole length of the pipe between these two points
-is traversed by the high-frequency current. The searcher,
-wearing the head telephone receiver, with the coil hanging
-down from it so as to be close to the ground, walks to and
-fro over the ground beneath which the pipe must lie.
-When he approaches the pipe the current passing through
-the latter induces a similar current in the suspended coil,
-and this produces a sort of buzzing or humming sound in
-the telephone. The nearer he approaches to the pipe the
-louder is the humming, and it reaches its maximum when
-he is standing directly over the pipe. In this way the
-whole course of the pipe can be traced without any digging,
-even when the pipe is 15 or 20 feet down. The absence
-of any sounds in the receiver indicates that the second
-fitting is not on the required pipe line, and other fittings
-have to be tried until one on this line is found.</p>
-
-<h3><span class="smcap">An Electric Iceberg Detector</span></h3>
-
-<p>Amongst the many dangers to which ships crossing the
-Atlantic are exposed is that of collision with icebergs.
-These are large masses of ice which have become detached
-from the mighty ice-fields of the north, and which travel
-slowly and majestically southwards, growing smaller and
-smaller as they pass into warmer seas. Icebergs give no
-warning of their coming, and in foggy weather, which is very
-prevalent in the regions where they are encountered, they<span class="pagenum" id="Page_268">268</span>
-are extremely difficult to see until they are at dangerously
-close quarters.</p>
-
-<p>Attempts have been made to detect the proximity of
-icebergs by noting the variations in the temperature of the
-water. We naturally should expect the temperature of the
-water to become lower as we approach a large berg, and
-this is usually the case. On the other hand, it has been
-found that in many instances the temperature near an
-iceberg is quite as high as, and sometimes higher than the
-average temperature of the ocean. For this reason the
-temperature test, taken by itself, is not at all reliable. A
-much more certain test is that of the salinity or saltness of
-the water. Icebergs are formed from fresh water, and as
-they gradually melt during their southward journey the
-fresh water mixes with the sea water. Consequently the
-water around an iceberg is less salt than the water of the
-open ocean. The saltness of water may be determined by
-taking its specific gravity, or by various chemical processes;
-but while these tests are quite satisfactory when performed
-under laboratory conditions, they cannot be carried out at sea
-with any approach to accuracy. There is however an electrical
-test which can be applied accurately and continuously.
-The electrical conducting power of water varies greatly with
-the proportion of salt present. If the conductivity of normal
-Atlantic water be taken as 1000, then the conductivity of
-Thames water is 8, and that of distilled water about 1/22.
-The difference in conductivity between normal ocean water
-and water in the vicinity of an iceberg is therefore very great.</p>
-
-<figure id="fig_43" class="figcenter" style="max-width: 37em;">
- <img src="images/i_305.jpg" width="2901" height="1623" alt=" ">
- <figcaption class="caption">
-
-<p class="floatl"><i>By permission of</i>]</p>
-<p class="floatr">[<i>Dr. Myer Coplans.</i></p>
-
-<p class="floatc"><span class="smcap">Fig. 43.</span>—Diagram of Heat-compensated Salinometer.</p>
-</figcaption></figure>
-
-<p>The apparatus for detecting differences in salinity by
-measuring the conductivity of the water is called a “salinometer,”
-and its most perfect form, known as the heat-compensated
-conductivity salinometer, is due to Dr. Myer
-Coplans. <a href="#fig_43">Fig. 43</a> shows a diagram of this interesting
-piece of apparatus, which is most ingeniously devised. Two<span class="pagenum" id="Page_270">270</span>
-insulated electrodes of copper, with platinum points, are
-suspended in a <span class="sans bold">U</span>-tube through which the sea water passes
-continuously, as indicated in the diagram. A steady current
-is passed through the column of water between the two
-platinum points, and the conductivity of this column is
-measured continuously by very accurate instruments.
-Variations in the conductivity, indicating corresponding
-variations in the saltness of the water, are thus shown
-immediately; but before these indications can be relied
-upon the instrument must be compensated for temperature,
-because the conductivity of the water increases with a rise,
-and decreases with a fall in temperature. This compensation
-is effected by the compound bars of brass and steel
-shown in the vessel at the right of the figure. These bars
-are connected with the wheel and disc from which the
-electrodes are suspended. When the temperature of the
-water rises, the bars contract, and exert a pull upon the
-wheel and disc, so that the electrodes are raised slightly in
-the <span class="sans bold">U</span>-tube. This increases the length of the column of
-water between the platinum points, and so increases the
-resistance, or, what amounts to the same thing, lowers the
-conductivity, in exact proportion to the rise in temperature.
-Similarly, a fall in temperature lowers the electrodes, and
-decreases the resistance by shortening the column of water.
-In this way the conductivity of the water remains constant
-so far as temperature is concerned, and it varies only with
-the saltness of the water. Under ordinary conditions a
-considerable decrease in the salinity of the water indicates
-the existence of ice in the near neighbourhood, but the
-geographical position of the ship has to be taken into
-account. Rivers such as the St. Lawrence pour vast
-quantities of fresh water into the ocean, and the resulting
-decrease in the saltness of the water within a considerable
-radius of the mouth of the river must be allowed for.</p>
-
-<p><span class="pagenum" id="Page_271">271</span></p>
-
-<h3><span class="smcap">A “Flying Train”</span></h3>
-
-<p>Considerable interest was aroused last year by a model
-of a railway working upon a very remarkable system. This
-was the invention of Mr. Emile Bachelet, and the model
-was brought to London from the United States. The main
-principle upon which the system is based is interesting.
-About 1884, Professor Elihu Thompson, a famous American
-scientist, made the discovery that a plate of copper could be
-attracted or repelled by an electro-magnet. The effects
-took place at the moment when the magnetism was varied
-by suddenly switching the current on or off; the copper
-being repelled when the current was switched on, and
-attracted when it was switched off. Copper is a non-magnetic
-substance, and the attraction and repulsion are
-not ordinary magnetic effects, but are due to currents
-induced in the copper plate at the instant of producing or
-destroying the magnetism. The plate is attracted or
-repelled according to whether these induced currents flow in
-the same direction as, or in the opposite direction to, the
-current in the magnet coil. Brass and aluminium plates
-act in the same way as the copper plate, and the effects are
-produced equally well by exciting the magnet with alternating
-current, which, by changing its direction, changes the
-magnetism also. Of the two effects, the repulsion is
-much the stronger, especially if the variations in the
-magnetism take place very rapidly; and if a powerful
-and rapidly alternating current is used, the plate is repelled
-so strongly that it remains supported in mid-air above the
-magnet.</p>
-
-<p>This repulsive effect is utilized in the Bachelet system
-(<a href="#plate_XV">Plate XV</a>.). There are no rails in the ordinary sense, and
-the track is made up of a continuous series of electro-magnets.<span class="pagenum" id="Page_272">272</span>
-The car, which is shaped something like a cigar,
-has a floor of aluminium, and contains an iron cylinder,
-and it runs above the line of magnets. Along each side
-of the track is a channel guide rail, and underneath the car
-at each end are fixed two brushes with guide pieces, which
-run in the guide rails. Above the car is a third guide rail,
-and two brushes with guide pieces fixed on the top of the
-car, one at each end, run in this overhead rail. These
-guide rails keep the car in position, and also act as conductors
-for the current. The repulsive action of the
-electro-magnets upon the aluminium floor raises the car
-clear of the track, and keeps it suspended; and while
-remaining in this mid-air position it is driven, or rather
-pulled forward, by powerful solenoids, which are supplied
-with continuous current. We have referred previously to
-the way in which a solenoid draws into it a core of iron.
-When the car enters a solenoid, the latter exerts a pulling
-influence upon the iron cylinder inside the car, and so the
-car is given a forward movement. This is sufficient to
-carry it along to the next solenoid, which gives it another
-pull, and so the car is drawn forward from one solenoid
-to another to the end of the line. The model referred
-to has only a short track of about 30 feet, with one
-solenoid at each end; but its working shows that the
-pulling power of the solenoids is sufficient to propel the
-car.</p>
-
-<figure id="plate_XV" class="figcenter" style="max-width: 40em;">
- <p class="caption">PLATE XV.</p>
- <img src="images/i_309.jpg" width="3141" height="2019" alt=" ">
- <figcaption class="caption">
-
-<p class="floatl"><i>Photo by</i></p>
-<p class="floatr"><i>Record Press.</i></p>
-
-<p class="floatc">BACHELET “FLYING TRAIN” AND ITS INVENTOR.</p>
-</figcaption></figure>
-
-<p>To avoid the necessity of keeping the whole of the
-electro-magnets energized all the time, these are arranged
-in sections, which are energized separately. By means of
-the lower set of brushes working in the track guides, each
-of these sections has alternating current supplied to it as
-the car approaches, and switched off from it when the car
-has passed. The brushes working in the overhead guide
-supply continuous current to each solenoid as the car enters
-it, and switch off the current when the car has passed
-through. The speed at which the model car travels is
-quite extraordinary, and the inventor believes that in actual
-practice speeds of more than 300 miles an hour are attainable
-on his system.</p>
-<hr class="chap x-ebookmaker-drop">
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_274">274</span></p>
-
-<h2 class="nobreak" id="toclink_274"><a id="chapter_XXX"></a>CHAPTER XXX<br>
-
-<span class="subhead">ELECTRICITY IN WAR</span></h2>
-</div>
-
-<p class="in0"><span class="firstword">One</span> of the most striking features of modern naval warfare
-is the absolute revolution in methods of communication
-brought about by wireless telegraphy. To-day every
-warship has its wireless installation. Our cruiser squadrons
-and destroyer flotillas, ceaselessly patrolling the waters of
-the North Sea, are always in touch with the Admiral of
-the Fleet, and with the Admiralty at Whitehall. In the
-Atlantic, and in the Pacific too, our cruisers, whether
-engaged in hunting down the marauding cruisers of the
-enemy or in searching for merchant ships laden with contraband,
-have their comings and goings directed by wireless.
-Even before the actual declaration of war between
-Great Britain and Germany wireless telegraphy began its
-work. At the conclusion of the great naval review of
-July 1914, the Fleet left Portland to disperse as customary
-for manœuvre leave, but a wireless message was dispatched
-ordering the Fleet not to disperse. As no state of war
-then existed, this was a precautionary measure, but subsequent
-events quickly proved how urgently necessary it
-had been to keep the Fleet in battle array. Immediately
-war was declared Great Britain was able to put into the
-North Sea a fleet which hopelessly outnumbered and outclassed
-the German battle fleet.</p>
-
-<p>At the outset Germany had a number of cruisers in the
-Atlantic and the Pacific Oceans. Owing to the vigilance<span class="pagenum" id="Page_275">275</span>
-of our warships these vessels were unable to join the
-German Home Fleet, and they immediately adopted the
-rôle of commerce destroyers. In this work they made
-extensive use of wireless telegraphy to ascertain the whereabouts
-of British merchant ships, and for a short time they
-played quite a merry game. Prominent among these
-raiders was the <i>Emden</i>. It was really astonishing how
-this cruiser obtained information regarding the sailings of
-British ships. It is said that on one occasion she called up
-by wireless a merchant ship, and inquired if the latter had
-seen anything of a German cruiser. The unsuspecting
-merchantman replied that there was no such thing as a
-German warship in the vicinity. “Oh yes, there is,”
-returned the <i>Emden</i>; “I’m it!” and shortly afterwards she
-appeared on the horizon, to the great discomfiture of the
-British skipper. An interesting account of the escape of a
-British liner from another notorious raider, the <i>Karlsruhe</i>,
-has been given in the <cite>Nautical Magazine</cite>. The writer
-says:</p>
-
-<p>“I have just returned home after a voyage to South
-America in one of the Pacific Steam Navigation Company’s
-cargo boats. When we left Montevideo we heard that
-France and Germany were at war, and that there was
-every possibility of Great Britain sending an ultimatum to
-Germany. We saw several steamers after leaving the
-port, but could get no information, as few of them were
-fitted with wireless and passed at some distance off. When
-about 200 miles east of Rio, our wireless operator overheard
-some conversation between the German cruiser
-<i>Karlsruhe</i> and a German merchant ship at anchor in Rio.
-It was clearly evident that the German merchant ship had
-no special code, as the conversation was carried on in plain
-German language, and our operator, who, by the way, was
-master of several languages, was able to interpret these<span class="pagenum" id="Page_276">276</span>
-messages without the slightest difficulty. It was then that
-we learned that Great Britain was at war. The German
-cruiser was inquiring from the German merchant ship what
-British vessels were leaving Rio, and asking for any information
-which might be of use. We also picked up some
-news of German victories in Belgium, which were given
-out by the German merchant ship. It was clearly evident
-that the <i>Karlsruhe</i> had information about our ship, and
-expected us to be in the position she anticipated, for she
-sent out a signal to us in English, asking us for our latitude
-and longitude. This our operator, under the instructions
-of the captain, declined to give. The German operator
-evidently got furious, as he called us an English ‘swine-hound,’
-and said, ‘This is a German warship, <i>Karlsruhe</i>;
-we will you find.’ Undoubtedly he thought he was going
-to strike terror to our hearts, but he made a mistake.</p>
-
-<p>“That night we steamed along without lights, and we
-knew from the sound of the wireless signals that were
-being flashed out from the German ship that we were
-getting nearer and nearer to her. Fortunately for us,
-about midnight a thick misty rain set in and we passed the
-German steamer, and so escaped. Our operator said that
-we could not have been more than 8 or 10 miles away
-when we passed abeam. Undoubtedly our wireless on
-this occasion saved us from the danger from which we
-escaped.”</p>
-
-<p>Apparently little is known of the end of the <i>Karlsruhe</i>,
-but the <i>Emden</i> met with the fate she richly deserved; and
-fittingly enough, wireless telegraphy, which had enabled
-her to carry out her marauding exploits, was the means of
-bringing her to her doom. On 9th November 1914 the
-<i>Emden</i> anchored off the Cocos-Keeling Islands, a group
-of coral islets in the Indian Ocean, and landed a party of
-three officers and forty men to cut the cable and destroy<span class="pagenum" id="Page_277">277</span>
-the wireless station. Before the Germans could get to the
-station, a wireless message was sent out stating the
-presence of the enemy warship, and this call was received
-by the Australian cruisers <i>Melbourne</i> and <i>Sydney</i>. These
-vessels, which were then only some 50 miles away, were
-engaged, along with a Japanese cruiser, in escorting transports.
-The <i>Sydney</i> at once went off at full speed, caught
-the <i>Emden</i>, and sent her to the bottom after a short but
-sharp engagement. As the <i>Emden</i> fled at sight of the
-Australian warship, the landing party had not time to get
-aboard, and consequently were left behind. They seized
-an old schooner, provisioned her, and set sail, but what
-became of them is not known.</p>
-
-<p>In land warfare field telegraphs play a very important
-part; indeed it is certain that without them the vast military
-operations of the present war could not be carried on.
-The General Headquarters of our army in France is in
-telegraphic communication not only with neighbouring
-French towns, but also with Paris and London. From
-Headquarters also run wires to every point of the firing-line,
-so that the Headquarters Staff, and through them the
-War Office in London, know exactly what is taking place
-along the whole front. The following extract from a letter
-from an officer, published by <cite>The Times</cite>, gives a remarkably
-good idea of the work of the signal companies of the
-Royal Engineers.</p>
-
-<p>“As the tide of battle turns this way and the other, and
-headquarters are constantly moving, some means have to be
-provided to keep in constant touch with General Headquarters
-during the movement. This emergency is met
-by cable detachments. Each detachment consists of two
-cable waggons, which usually work in conjunction with one
-another, one section laying the line whilst the other remains
-behind to reel up when the line is finished with. A<span class="pagenum" id="Page_278">278</span>
-division is ordered to move quickly to a more tactical
-position. The end of the cable is connected with the
-permanent line, which communicates to Army Headquarters,
-and the cable detachment moves off at the trot; across
-country, along roads, through villages, and past columns of
-troops, the white and blue badge of the signal service
-clears the way. Behind the waggon rides a horseman, who
-deftly lays the cable in the ditches and hedges out of danger
-from heavy transport and the feet of tramping infantry,
-with the aid of a crookstick. Other horsemen are in the
-rear tying back and making the line safe. On the box of
-the waggon sits a telegraphist, who is constantly in touch
-with headquarters as the cable runs swiftly out. An
-orderly dashes up with an important message; the waggon
-is stopped, the message dispatched, and on they go again.”</p>
-
-<p>Wireless telegraphy too has its part to play in land
-war, and for field purposes it has certain advantages over
-telegraphy with wires. Ordinary telegraphic communication
-is liable to be interrupted by the cutting of the wire by
-the enemy, or, in spite of every care in laying, by the
-breaking of the wire by passing cavalry or artillery. No
-such trouble can occur with wireless telegraphy, and if it
-becomes necessary to move a wireless station with great
-rapidity, as for instance on an unexpected advance of the
-enemy, it is an advantage to have no wire to bother about.
-The Marconi portable wireless sets for military purposes
-are marvels of compactness and lightness, combined with
-simplicity. They are of two kinds, pack-saddle sets and
-cart sets. The former weigh about 360 lb., this being
-divided amongst four horses. They can be set up in ten
-minutes by five or six men, and require only two men to
-work them. Their guaranteed range is 40 miles, but
-they are capable of transmitting twice this distance or even
-more under favourable conditions. The cart sets can be<span class="pagenum" id="Page_279">279</span>
-set up in twenty minutes by seven or eight men, and they
-have a guaranteed range of from 150 to 200 miles.</p>
-
-<p>It is obviously very important that wireless military
-messages should not be intercepted and read by the enemy,
-and the method of avoiding danger of this kind adopted
-with the Marconi field stations is ingenious and effective.
-The transmitter and the receiver are arranged to work on
-three different fixed wave-lengths, the change from one to
-another being effected quickly by the movement of a three-position
-switch. By this means the transmitting operator
-sends three or four words on one wave-length, then changes
-to another, transmits a few words on this, changes the wave-length
-again, and so on. Each change is accompanied by
-the sending of a code letter which informs the receiving
-operator to which wave-length the transmitter is passing.
-The receiving operator adjusts his switch accordingly, and
-so he hears the whole message without interruption, the
-change from one wave-length to another taking only a small
-fraction of a second. An enemy operator might manage
-to adjust his wave-length so as to hear two or three words,
-but the sudden change of wave-length would throw him out
-of tune, and by the time he had found the new wave-length
-this would have changed again. Thus he would hear at
-most only a few disconnected words at intervals, and he
-would not be able to make head or tail of the message.
-To provide against the possibility of the three wave-lengths
-being measured and prepared for, these fixed lengths themselves
-can be changed, if necessary, many times a day, so
-that the enemy operators would never know beforehand
-which three were to be used.</p>
-
-<p>Wireless telegraphy was systematically employed in
-land warfare for the first time in the Balkan War, during
-which it proved most useful both to the Allies and to the
-Turks. One of the most interesting features of the war<span class="pagenum" id="Page_280">280</span>
-was the way in which wireless communication was kept up
-between the beleaguered city of Adrianople and the
-Turkish capital. Some time before war broke out the
-Turkish Government sent a portable Marconi wireless set
-to Adrianople, and this was set up at a little distance from
-the city. When war was declared the apparatus was
-brought inside the city walls and erected upon a small hill.
-Then came the siege. For 153 days Shukri Pasha kept
-the Turkish flag flying, but the stubborn defence was
-broken down in the end through hunger and disease. All
-through these weary days the little wireless set did its duty
-unfalteringly, and by its aid regular communication was
-maintained with the Government station at Ok Meidan,
-just outside Constantinople, 130 miles away. Altogether
-about half a million words were transmitted from Adrianople
-to the Turkish capital.</p>
-
-<figure id="plate_XVIa" class="figcenter" style="max-width: 26em;">
- <p class="caption">PLATE XVI.</p>
- <img src="images/i_319.jpg" width="2068" height="1406" alt=" ">
- <figcaption class="caption">
-
-<p>(<i>a</i>) CAVALRY PORTABLE WIRELESS CART SET.</p>
-</figcaption></figure>
-
-<figure id="plate_XVIb" class="figcenter" style="max-width: 26em;">
- <img src="images/i_319b.jpg" width="2053" height="1471" alt=" ">
- <figcaption class="caption">
-
-<p class="floatl"><i>By permission of</i></p>
-<p class="floatr"><i>Marconi Co. Ltd.</i></p>
-
-<p class="floatc">(<i>b</i>) AEROPLANE FITTED WITH WIRELESS TELEGRAPHY.</p>
-</figcaption></figure>
-
-<p>The rapid development of aviation during the past few
-years has drawn attention to the necessity for some means
-of communication between the land and airships and
-aeroplanes in flight. At first sight it might appear that
-wireless telegraphy could be used for this purpose without
-any trouble, but experience has shown that there are
-certain difficulties in the way, especially with regard to
-aeroplanes. The chief difficulty with aeroplanes lies in the
-aerial. This must take the form either of a long trailing
-wire or of fixed wires running between the planes and the
-tail. A trailing wire is open to the objection that it is
-liable to get mixed up with the propeller, besides which it
-appears likely to hamper to some slight extent the movements
-of a small and light machine. A fixed aerial between
-planes and tail avoids these difficulties, but on the other
-hand its wave-length is bound to be inconveniently small.
-The heavy and powerful British military aeroplanes
-apparently use a trailing wire of moderate length, carried
-in a special manner so as to clear the propeller, but few
-details are available at present. A further trouble with
-aeroplanes lies in the tremendous noise made by the
-engine, which frequently makes it quite impossible to hear
-incoming signals; and the only way of getting over this
-difficulty appears to be for the operator to wear some sort
-of sound-proof head-gear. Signals have been transmitted
-from an aeroplane in flight up to distances of 40 or 50 miles
-quite successfully, but the reception of signals by aeroplanes
-is not so satisfactory, except for comparatively short distances.
-Although few particulars have been published
-regarding the work of the British aeroplanes in France, it
-seems evident that wireless telegraphy is in regular use.
-In addition to their value as scouts, our aeroplanes appear
-to be extremely useful for the direction of heavy artillery
-fire, using wireless to tell the gunners where each shell falls,
-until the exact range is obtained. In the case of airships
-the problem of wireless communication is much simpler.
-A trailing wire presents no difficulties, and on account of
-their great size much more powerful sets of apparatus can
-be carried. The huge German Zeppelin airships have a
-long freely-floating aerial consisting of a wire which can be
-wound in or let out as required, its full length being about
-750 feet. The total weight of the apparatus is nearly
-300 lb., and the transmitting range is said to be from
-about 120 to 200 miles.</p>
-
-<p>Electricity is used in the navy for a great variety of
-purposes besides telegraphy. Our battleships are lighted
-by electricity, which is generated at a standard pressure of
-220 volts. This current is transformed down for the
-searchlights, and also for the intricate systems of telephone,
-alarm, and firing circuits. The magazines containing the
-deadly cordite are maintained at a constant temperature of
-70° F. by special refrigerating machinery driven by electricity,<span class="pagenum" id="Page_282">282</span>
-and the numerous fans for ventilating the different parts of
-the ship are also electrically driven. Electric power is used
-for capstans, coaling winches, sounding machines, lifts,
-pumps, whether for drainage, fire extinction, or raising
-fresh water from the tanks, and for the mechanism for
-operating boats and torpedo nets. The mechanism for
-manipulating the great guns and their ammunition is
-hydraulic. Electricity was tried for this purpose on the
-battle cruiser <i>Invincible</i>, but was abandoned in favour of
-hydraulic power. But though electricity is apparently out of
-favour in this department, it takes an extremely important
-share in the work of controlling and firing the guns; its duties
-being such as could not be carried out by hydraulic power.</p>
-
-<p>The guns are controlled and fired from what is known
-as the fire-control room, which is situated in the interior of
-the ship, quite away from the guns themselves. The
-range-finder, from his perch up in the gigantic mast,
-watches an enemy warship as she looms on the horizon,
-and when she comes within range he estimates her distance
-by means of instruments of wonderful precision. He then
-telephones to the fire-control room, giving this distance,
-and also the enemy’s speed and course. The officer in
-charge of the fire-control room calculates the elevation of
-the gun required for this distance, and decides upon the
-instant at which the gun must be fired. A telephoned
-order goes to the gun-turret, and the guns are brought to
-bear upon the enemy, laid at the required elevation, and
-sighted. At the correct instant the fire-control officer
-switches on an electric current to the gun, which fires a
-small quantity of highly explosive material, and this in
-turn fires the main charge of cordite. The effect of the
-shell is watched intently from the fire-control top, up above
-the range-finder, and if, as is very likely, this first shell
-falls short of, or overshoots the mark, an estimate of the<span class="pagenum" id="Page_283">283</span>
-amount of error is communicated to the fire-control room.
-Due corrections are then made, the gun is laid at a slightly
-different elevation, and this time the shell finds its mark
-with unerring accuracy.</p>
-
-<p>The range of movement, horizontal and vertical, of
-modern naval guns is so great that it is possible for two
-guns to be in such relative positions that the firing of one
-would damage the other. To guard against a disaster of
-this kind fixed stops are used, supplemented by ingenious
-automatic alarms. The alarm begins to sound as soon as
-any gun passes into a position in which it could damage
-another gun, and it goes on sounding until the latter gun
-is moved out of the danger line.</p>
-
-<p>Since the outbreak of war the subject of submarine
-mines has been brought to our notice in very forcible
-fashion. Contrary to the general impression, the explosive
-submarine mine is not a recent introduction. It is difficult
-to say exactly when mines were first brought into use, but
-at any rate we know that they were employed by Russia
-during the Crimean War, apparently with little success.
-The first really successful use of mines occurred in the
-American Civil War, when the Confederates sank a number
-of vessels by means of them. This practical demonstration
-of their possibilities did not pass unnoticed by European
-nations, and in the Franco-German War we find that mines
-were used for harbour defence by both belligerents. It is
-doubtful whether either nation derived much benefit from
-its mines, and indeed as the war progressed Germany
-found that the principal result of her mining operations was
-to render her harbours difficult and dangerous to her own
-shipping. Much greater success attended the use of mines
-in the Russo-Japanese War, but all previous records shrink
-into insignificance when compared with the destruction
-wrought by mines in the present great conflict.</p>
-
-<p><span class="pagenum" id="Page_284">284</span></p>
-
-<p>Submarine mines may be divided into two classes;
-those for harbour defence, and those for use in the open
-sea. Harbour defence mines are almost invariably electrically
-controlled; that is, they are connected with the shore
-by means of a cable, and fired by an electric impulse sent
-along that cable. In one system of control the moment of
-firing is determined entirely by observers on shore, who,
-aided by special optical instruments, are able to tell exactly
-when a vessel is above any particular mine. The actual
-firing is carried out by depressing a key which completes
-an electric circuit, thus sending a current along the cable
-to actuate the exploding mechanism inside the mine. A
-hostile ship therefore would be blown up on arriving at the
-critical position, while a friendly vessel would be allowed
-to pass on in safety. In this system of control there is no
-contact between the vessel and the mine, the latter being
-well submerged or resting on the sea floor, so that the
-harbour is not obstructed in any way. This is a great
-advantage, but against it must be set possible failure of the
-defence at a critical moment owing to thick weather, which
-of course interferes seriously with the careful observation
-of the mine field necessary for accurate timing of the explosions.
-This difficulty may be surmounted by a contact
-system of firing. In this case the mines are placed so near
-the surface as to make contact with vessels passing over
-them. The observers on shore are informed of the contact
-by means of an electric impulse automatically transmitted
-along the cable, so that they are independent of continuous
-visual observation of the mined area. As in the previous
-system, the observers give the actual firing impulse. The
-drawback to this method is the necessity for special pilotage
-arrangements for friendly ships in order to avoid unnecessary
-striking of the mines, which are liable to have their
-mechanism deranged by constant blows. If the harbour or<span class="pagenum" id="Page_285">285</span>
-channel can be closed entirely to friendly shipping, the
-observers may be dispensed with, their place being taken
-by automatic electric apparatus which fires at once any mine
-struck by a vessel.</p>
-
-<p>Shore-controlled mines are excellent for harbour
-defence, and a carefully distributed mine-field, backed by
-heavy fort guns, presents to hostile vessels a barrier which
-may be regarded as almost impenetrable. A strong fleet
-might conceivably force its way through, but in so doing it
-would sustain tremendous losses; and as these losses would
-be quite out of proportion to any probable gains, such an
-attempt is not likely to be made except as a last resort.</p>
-
-<p>For use in the open sea a different type of mine is
-required. This must be quite self-contained and automatic
-in action, exploding when struck by a passing vessel. The
-exploding mechanism may take different forms. The blow
-given by a ship may be made to withdraw a pin, thus
-releasing a sort of plunger, which, actuated by a powerful
-spring, detonates the charge. A similar result is obtained
-by the use of a suspended weight, in place of plunger and
-spring. Still another form of mine is fired electrically by
-means of a battery, the circuit of which is closed automatically
-by the percussion. Deep-sea mines may be anchored
-or floating free. Free mines are particularly dangerous on
-account of the impossibility of knowing where they may be
-at any given moment. They are liable to drift for considerable
-distances, and to pass into neutral seas; and to
-safeguard neutral shipping international rules require them
-to have some sort of clockwork mechanism which renders
-them harmless after a period of one hour. It is quite
-certain that some, at least, of the German free mines have
-no such mechanism, so that neutral shipping is greatly
-endangered.</p>
-
-<p>Submarine mines are known as <em>ground</em> mines, or<span class="pagenum" id="Page_286">286</span>
-<em>buoyant</em> mines, according to whether they rest on the sea
-bottom or float below the surface. Ground mines are
-generally made in the form of a cylinder, buoyant mines
-being usually spherical. The cases are made of steel, and
-buoyancy is given when required by enclosing air spaces.
-Open-sea mines are laid by special vessels, mostly old
-cruisers. The stern of these ships is partly cut away, and
-the mines are run along rails to the stern, and so overboard.
-The explosive employed is generally gun-cotton, fired by
-a detonator, charges up to 500 lb. or more being used,
-according to the depth of submersion and the horizontal
-distance at which the mine is desired to be effective.
-Ground mines can be used only in shallow water, and even
-then they require a heavier charge than mines floating near
-the surface. Mines must not be laid too close together, as
-the explosion of one might damage others. The distance
-apart at which they are placed depends upon the amount
-of charge, 500-lb. mines requiring to be about 300 feet apart
-for safety.</p>
-<hr class="chap x-ebookmaker-drop">
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_287">287</span></p>
-
-<h2 class="nobreak" id="toclink_287"><a id="chapter_XXXI"></a>CHAPTER XXXI<br>
-
-<span class="subhead">WHAT IS ELECTRICITY?</span></h2>
-</div>
-
-<p class="in0"><span class="firstword">The</span> question which heads this, our final chapter, is one
-which must occur to every one who takes even the most
-casual interest in matters scientific, and it would be very
-satisfactory if we could bring this volume to a conclusion
-by providing a full and complete answer. Unfortunately
-this is impossible. In years to come the tireless labours of
-scientific investigators may lead to a solution of the problem;
-but, as Professor Fleming puts it: “The question—What
-is electricity?—no more admits of a complete and final
-answer to-day than does the question—What is life?”</p>
-
-<p>From the earliest days of electrical science theories of
-electricity have been put forward. The gradual extension
-and development of these theories, and the constant substitution
-of one idea for another as experimental data
-increased, provide a fascinating subject for study. To
-cover this ground however, even in outline, would necessitate
-many chapters, and so it will be better to consider
-only the theory which, with certain reservations in some
-cases, is held by the scientific world of to-day. This is
-known as the <em>electron</em> theory of electricity.</p>
-
-<p>We have referred already, in <a href="#chapter_XXIV">Chapter XXIV</a>., to atoms
-and electrons. All matter is believed to be constituted
-of minute particles called “atoms.” These atoms are so
-extremely small that they are quite invisible, being far
-beyond the range of the most powerful microscope; and
-their diameter has been estimated at somewhere about one<span class="pagenum" id="Page_288">288</span>
-millionth of a millimetre. Up to a few years ago the atom
-was believed to be quite indivisible, but it has been proved
-beyond doubt that this is not the case. An atom may be
-said to consist of two parts, one much larger than the
-other. The smaller part is negatively electrified, and is
-the same in all atoms; while the larger part is positively
-electrified, and varies according to the nature of the atom.
-The small negatively electrified portion of the atom consists of
-particles called “electrons,” and these electrons are believed
-to be indivisible units or atoms of negative electricity. To
-quote Professor Fleming: “An atom of matter in its neutral
-condition has been assumed to consist of an outer shell or
-envelope of negative electrons associated with some core or
-matrix which has an opposite electrical quality, such that if
-an electron is withdrawn from the atom the latter is left
-positively electrified.”</p>
-
-<p>The electrons in an atom are not fixed, but move with
-great velocity, in definite orbits. They repel one another,
-and are constantly endeavouring to fly away from the atom,
-but they are held in by the attraction of the positive core.
-So long as nothing occurs to upset the constitution of the
-atom, a state of equilibrium is maintained and the atom is
-electrically neutral; but immediately the atom is broken up
-by the action of an external force of some kind, one or
-more electrons break their bonds and fly away to join some
-other atom. An atom which has lost some of its electrons
-is no longer neutral, but is electro-positive; and similarly,
-an atom which has gained additional electrons is electro-negative.
-Electrons, or atoms of negative electricity, can
-be isolated from atoms of matter, as in the case of the
-stream of electrons proceeding from the cathode of a vacuum
-tube. So far, however, it has been found impossible to
-isolate corresponding atoms of positive electricity.</p>
-
-<p>From these facts it appears that we must regard a<span class="pagenum" id="Page_289">289</span>
-positively charged body as possessing a deficiency of
-electrons, and a negatively charged body as possessing an
-excess of electrons. In <a href="#chapter_I">Chapter I</a>. we spoke of the
-electrification of sealing-wax or glass rods by friction, and
-we saw that according to the nature of the substance used
-as the rubber, the rods were either positively or negatively
-electrified. Apparently, when we rub a glass rod with a
-piece of silk, the surface atoms of each substance are
-disturbed, and a certain number of electrons leave the glass
-atoms, and join the silk atoms. The surface atoms of the
-glass, previously neutral, are now electro-positive through
-the loss of electrons; and the surface atoms of the silk,
-also previously neutral, are now electro-negative through
-the additional electrons received from the glass atoms.
-As the result we find the glass to be positively, and silk to
-be negatively electrified. On the other hand, if we rub the
-glass with fur, a similar atomic disturbance and consequent
-migration of electrons takes place, but this time the glass
-receives electrons instead of parting with them. In this case
-the glass becomes negatively, and the fur positively electrified.
-The question now arises, why is the movement of the electrons
-away from the glass in the first instance, and toward it in
-the second? To understand this we may make use of a
-simple analogy. If we place in contact two bodies, one hot
-and the other cold, the hot body gives up some of its heat
-to the cold body; but if we place in contact with the hot
-body another body which is still hotter, then the hot body
-receives heat instead of parting with it. In a somewhat
-similar manner an atom is able to give some of its electrons
-to another atom which, in comparison with it, is deficient in
-electrons; and at the same time it is able to receive electrons
-from another atom which, compared with it, has an
-excess of electrons. Thus we may assume that the glass
-atoms have an excess of electrons as compared with<span class="pagenum" id="Page_290">290</span>
-silk atoms, and a deficiency in electrons as compared with
-fur atoms.</p>
-
-<p>A current of electricity is believed to be nothing more
-or less than a stream of electrons, set in motion by the
-application of an electro-motive force. We have seen that
-some substances are good conductors of electricity, while
-others are bad conductors or non-conductors. In order to
-produce an electric current, that is a current of electrons, it
-is evidently necessary that the electrons should be free to
-move. In good conductors, which are mostly metals, it is
-believed that the electrons are able to move from atom to
-atom without much hindrance, while in a non-conductor
-their movements are hampered to such an extent that inter-atomic
-exchange of electrons is almost impossible. Speaking on
-this point, Professor Fleming says: “There may be (in
-a good conductor) a constant decomposition and recomposition
-of atoms taking place, and any given electron so to
-speak flits about, now forming part of one atom and now of
-another, and anon enjoying a free existence. It resembles
-a person visiting from house to house, forming a unit in
-different households, and, in between, being a solitary
-person in the street. In non-conductors, on the other hand,
-the electrons are much restricted in their movements, and
-can be displaced a little way but are pulled back again
-when released.”</p>
-
-<p>Let us try to see now how an electric current is set up
-in a simple voltaic cell, consisting of a zinc plate and a
-copper plate immersed in dilute acid. First we must
-understand the meaning of the word <em>ion</em>. If we place a
-small quantity of salt in a vessel containing water, the salt
-dissolves, and the water becomes salt, not only at the
-bottom where the salt was placed, but throughout the
-whole vessel. This means that the particles of salt must be
-able to move through the water. Salt is a chemical<span class="pagenum" id="Page_291">291</span>
-compound of sodium and chlorine, and its molecules
-consist of atoms of both these substances. It is supposed
-that each salt molecule breaks up into two parts, one part
-being a sodium atom, and the other a chlorine atom; and
-further, that the sodium atom loses an electron, while the
-chlorine atom gains one. These atoms have the power of
-travelling about through the solution, and they are called
-<em>ions</em>, which means “wanderers.” An ordinary atom is unable
-to wander about in this way, but it gains travelling power
-as soon as it is converted into an ion, by losing electrons if
-it be an atom of a metal, and by gaining electrons if it be
-an atom of a non-metal.</p>
-
-<p>Returning to the voltaic cell, we may imagine that the
-atoms of the zinc which are immersed in the acid are trying
-to turn themselves into ions, so that they can travel through
-the solution. In order to do this each atom parts with two
-electrons, and these electrons try to attach themselves to
-the next atom. This atom however already has two
-electrons, and so in order to accept the newcomers it must
-pass on its own two. In this way electrons are passed on
-from atom to atom of the zinc, then along the connecting
-wire, and so to the copper plate. The atoms of zinc which
-have lost their electrons thus become ions, with power of
-movement. They leave the zinc plate immediately, and so
-the plate wastes away or dissolves. So we get a constant
-stream of electrons travelling along the wire connecting the
-two plates, and this constitutes an electric current.</p>
-
-<p>The electron theory gives us also a clear conception of
-magnetism. An electric current flowing along a wire
-produces magnetic effects; that is, it sets up a field of
-magnetic force. Such a current is a stream of electrons,
-and therefore we conclude that a magnetic field is produced
-by electrons in motion. This being so, we are led to
-suppose that there must be a stream of electrons in a steel<span class="pagenum" id="Page_292">292</span>
-magnet, and this stream must be constant because the
-magnetic field of such a magnet is permanent. The
-electron stream in a permanent magnet however is not
-quite the same as the electron stream in a wire conveying a
-current. We have stated that the electrons constituting an
-atom move in definite orbits, so that we may picture them
-travelling round the core of the atom as the planets travel
-round the Sun. This movement is continuous in every
-atom of every substance. Apparently we have here the
-necessary conditions for the production of a magnetic field,
-that is, a constant stream of electrons; but one important
-thing is still lacking. In an unmagnetized piece of steel the
-atoms are not arranged symmetrically, so that the orbits of
-their electrons lie some in one plane and some in another.
-Consequently, although the electron stream of each atom
-undoubtedly produces an infinitesimally small magnetic
-field, no magnetic effect that we can detect is produced,
-because the different streams are not working in unison and
-adding together their forces. In fact they are upsetting
-and neutralizing each other’s efforts. By stroking the piece
-of steel with a magnet, or by surrounding it by a coil of
-wire conveying a current, the atoms are turned so that their
-electron orbits all lie in the same plane. The electron
-streams now all work in unison, their magnetic effects are
-added together, and we get a strong magnetic field as the
-result of their combined efforts. Any piece of steel or iron
-may be regarded as a potential magnet, requiring only a
-rearrangement of its atoms in order to become an active
-magnet. In <a href="#chapter_VI">Chapter VI</a>. it was stated that other substances
-besides iron and steel show magnetic effects, and this is what
-we should expect, as the electron movement is common to
-all atoms. None of these substances is equal to iron
-and steel in magnetic power, but why this is so is not
-understood.</p>
-
-<p><span class="pagenum" id="Page_293">293</span></p>
-
-<p>This brings us to the production of an electric current by
-the dynamo. Here we have a coil of wire moving across a
-magnetic field, alternately passing into this field and out of
-it. A magnetic field is produced, as we have just seen, by
-the steady movement of electrons, and we may picture it
-as being a region of the ether disturbed or strained by the
-effect of the moving electrons. When the coil of wire
-passes into the magnetic field, the electrons of its atoms are
-influenced powerfully and set in motion in one direction, so
-producing a current in the coil. As the coil passes away
-from the field, its electrons receive a second impetus, which
-checks their movement and starts them travelling in the
-opposite direction, and another current is produced. The
-coil moves continuously and regularly, passing into and out
-of the magnetic field without interruption; and so we get a
-current which reverses its direction at regular intervals, that
-is, an alternating current. This current may be made continuous
-if desired, as explained in <a href="#chapter_IX">Chapter IX</a>.</p>
-
-<p>Such, stated briefly and in outline, is the electron theory
-of electricity. It opens up possibilities of the most fascinating
-nature; it gives us a wonderfully clear conception of
-what might be called the inner mechanism of electricity; and
-it even introduces us to the very atoms of electricity.
-Beyond this, at present, it cannot take us, and the actual
-nature of electricity itself remains an enigma.</p>
-<hr class="chap x-ebookmaker-drop">
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_295">295</span></p>
-
-<h2 class="nobreak" id="toclink_295">INDEX</h2>
-
-<div class="index">
-<ul class="index">
-<li class="ifrst">Accumulators, <a href="#Page_38">38</a>, <a href="#Page_90">90</a>.</li>
-
-<li class="indx">Alarms, electric, <a href="#Page_120">120</a>.</li>
-
-<li class="indx">Alternating currents, <a href="#Page_70">71</a>, <a href="#Page_75">75</a>.</li>
-
-<li class="indx">Amber, discovery of, <a href="#Page_2">2</a>.</li>
-
-<li class="indx">Ampère, <a href="#Page_33">33</a>.</li>
-
-<li class="indx">Arc lamp, <a href="#Page_93">93</a>.</li>
-
-<li class="indx">Armature, <a href="#Page_68">68</a>.</li>
-
-<li class="indx">Atlantic cable, <a href="#Page_145">145</a>.</li>
-
-<li class="indx">Atom, <a href="#Page_287">287</a>.</li>
-
-<li class="indx">Aurora borealis, <a href="#Page_25">25</a>.</li>
-
-<li class="indx">Automatic telephone exchange, <a href="#Page_164">165</a>.</li>
-
-<li class="indx">Aviation and “wireless,” <a href="#Page_280">280</a>.</li>
-
-<li class="ifrst">Bachelet “flying” train, <a href="#Page_271">271</a>.</li>
-
-<li class="indx">Bastian heater, the, <a href="#Page_110">110</a>.</li>
-
-<li class="indx">Battery, voltaic, <a href="#Page_33">33</a>.</li>
-
-<li class="indx">Bell telephone, the, <a href="#Page_156">156</a>.</li>
-
-<li class="indx">Bells and alarms, electric, <a href="#Page_116">116</a>.</li>
-
-<li class="indx">Blasting, <a href="#Page_256">256</a>.</li>
-
-<li class="indx">Bunsen cell, <a href="#Page_223">223</a>.</li>
-
-<li class="ifrst">Cable-laying, <a href="#Page_150">150</a>.</li>
-
-<li class="indx">Cables, telegraph, <a href="#Page_144">144</a>.</li>
-
-<li class="indx">Cell, voltaic, <a href="#Page_29">29</a>.</li>
-
-<li class="indx">Clocks, electric, <a href="#Page_124">124</a>.</li>
-
-<li class="indx">Coherer, the, <a href="#Page_183">183</a>.</li>
-
-<li class="indx">Commutator, <a href="#Page_70">70</a>.</li>
-
-<li class="indx">Compass, magnetic, <a href="#Page_52">52</a>.</li>
-
-<li class="indx">Condenser, <a href="#Page_63">63</a>.</li>
-
-<li class="indx">Conductors, <a href="#Page_6">6</a>.</li>
-
-<li class="indx">Conduit system, <a href="#Page_83">83</a>.</li>
-
-<li class="indx">Convectors, <a href="#Page_109">109</a>.</li>
-
-<li class="indx">Cookers, electric, <a href="#Page_110">110</a>.</li>
-
-<li class="indx">Creed telegraph, <a href="#Page_137">137</a>.</li>
-
-<li class="indx">Crookes, Sir W., <a href="#Page_230">230</a>.</li>
-
-<li class="indx">Current, electric, <a href="#Page_30">30</a>.</li>
-
-<li class="ifrst">Daniell cell, <a href="#Page_30">31</a>, <a href="#Page_223">223</a>.</li>
-
-<li class="indx">Davy, Sir Humphry, <a href="#Page_93">93</a>.</li>
-
-<li class="indx">Detector, in wireless telegraphy, <a href="#Page_188">188</a>, <a href="#Page_198">198</a>.</li>
-
-<li class="indx">Diamond-making, <a href="#Page_113">113</a>.</li>
-
-<li class="indx">Duplex telegraphy, <a href="#Page_139">139</a>.</li>
-
-<li class="indx">Dussaud cold light, <a href="#Page_106">106</a>.</li>
-
-<li class="indx">Dynamo, <a href="#Page_66">66</a>.</li>
-
-<li class="ifrst">Edison, Thomas A., <a href="#Page_103">103</a>.</li>
-
-<li class="indx">Electric cookers, <a href="#Page_110">110</a>.</li>
-
-<li class="indx">Electric heating, <a href="#Page_109">109</a>.</li>
-
-<li class="indx">Electric motor, <a href="#Page_66">66</a>.</li>
-
-<li class="indx">Electric lighting, <a href="#Page_70">70</a>, <a href="#Page_75">75</a>, <a href="#Page_93">93</a>.</li>
-
-<li class="indx">Electricity, early discoveries, <a href="#Page_1">1</a>;</li>
-<li class="isub1">nature of, <a href="#Page_287">287</a>.</li>
-
-<li class="indx">Electro-culture, <a href="#Page_258">258</a>.</li>
-
-<li class="indx">Electrolysis, <a href="#Page_224">224</a>.</li>
-
-<li class="indx">Electro-magnets, <a href="#Page_58">58</a>.</li>
-
-<li class="indx">Electron, <a href="#Page_287">287</a>.</li>
-
-<li class="indx">Electroplating, <a href="#Page_213">213</a>.</li>
-
-<li class="indx">Electrophorus, the, <a href="#Page_11">11</a>.</li>
-
-<li class="indx">Electrotyping, <a href="#Page_213">213</a>.</li>
-
-<li class="ifrst">Faraday, <a href="#Page_66">66</a>.</li>
-
-<li class="indx">Finsen light treatment, <a href="#Page_243">243</a>.</li>
-
-<li class="indx">Franklin, Benjamin, <a href="#Page_19">19</a>.</li>
-
-<li class="indx">Frictional electricity, <a href="#Page_2">2</a>.</li>
-
-<li class="indx">Furnace, electric, <a href="#Page_111">111</a>.</li>
-
-<li class="ifrst">Galvani, <a href="#Page_27">27</a>.</li>
-
-<li class="indx">Galvanometer, <a href="#Page_59">59</a>.</li>
-
-<li class="indx">Glass, <a href="#Page_4">4</a>.</li>
-
-<li class="indx">Goldschmidt system, <a href="#Page_197">197</a>.</li>
-
-<li class="indx"><i>Great Eastern</i>, the, <a href="#Page_148">148</a>.</li>
-
-<li class="ifrst">Half-watt lamp, <a href="#Page_105">105</a>.</li>
-
-<li class="indx">Heating by electricity, <a href="#Page_109">109</a>.</li>
-
-<li class="indx">Hughes printing telegraph, <a href="#Page_136">136</a>.</li>
-
-<li class="ifrst">Iceberg detector, <a href="#Page_267">267</a>.</li>
-
-<li class="indx">Ignition, electric, <a href="#Page_253">253</a>.</li>
-
-<li class="indx">Incandescent lamps, <a href="#Page_103">103</a>.</li>
-
-<li class="indx">Induction, <a href="#Page_9">9</a>.</li>
-
-<li class="indx">Induction coil, <a href="#Page_61">61</a>.</li>
-
-<li class="indx">Ion, <a href="#Page_291">291</a>.</li>
-
-<li class="ifrst">Kelvin, Lord, <a href="#Page_152">152</a>.</li>
-
-<li class="indx">Korn’s photo-telegraph, <a href="#Page_174">174</a>.</li>
-
-<li class="ifrst">Lamps, electric, <a href="#Page_93">93</a>.</li>
-
-<li class="indx">Leclanché cell, <a href="#Page_32">32</a>, <a href="#Page_116">116</a>.</li>
-
-<li class="indx">Lemström’s experiments in electro-culture, <a href="#Page_258">258</a>.</li>
-
-<li class="indx">Lepel system, <a href="#Page_196">196</a>.</li>
-
-<li class="indx">Leyden jar, <a href="#Page_15">15</a>, <a href="#Page_181">181</a>.</li>
-
-<li class="indx">Light, <a href="#Page_23">23</a>.</li>
-
-<li class="indx"><span class="pagenum" id="Page_296">296</span>Lighting, electric, <a href="#Page_75">75</a>, <a href="#Page_93">93</a>.</li>
-
-<li class="indx">Lightning, <a href="#Page_1">1</a>, <a href="#Page_19">19</a>, <a href="#Page_23">23</a>.</li>
-
-<li class="indx">Lightning conductors, <a href="#Page_25">25</a>.</li>
-
-<li class="indx">Lindsay, wireless experiments, <a href="#Page_180">180</a>.</li>
-
-<li class="indx">Lodge, Sir Oliver, <a href="#Page_260">260</a>.</li>
-
-<li class="ifrst">Machines for producing static electricity, <a href="#Page_9">9</a>.</li>
-
-<li class="indx">Magnetic poles, <a href="#Page_50">50</a>.</li>
-
-<li class="indx">Magnetism, <a href="#Page_44">44</a>, <a href="#Page_56">56</a>, <a href="#Page_291">291</a>.</li>
-
-<li class="indx">Marconi, <a href="#Page_186">186</a>, <a href="#Page_195">195</a>.</li>
-
-<li class="indx">Medicine, electricity in, <a href="#Page_240">241</a>.</li>
-
-<li class="indx">Mercury-vapour lamp, <a href="#Page_99">99</a>.</li>
-
-<li class="indx">Microphone, <a href="#Page_159">159</a>.</li>
-
-<li class="indx">Mines, submarine, <a href="#Page_283">283</a>.</li>
-
-<li class="indx">Mines, telephones in, <a href="#Page_169">169</a>.</li>
-
-<li class="indx">Mono-railway, <a href="#Page_89">89</a>.</li>
-
-<li class="indx">Morse, telegraph, <a href="#Page_130">130</a>;</li>
-<li class="isub1">experiments in wireless telegraphy, <a href="#Page_180">180</a>.</li>
-
-<li class="indx">Motor, electric, <a href="#Page_66">66</a>.</li>
-
-<li class="indx">Motor-car, electric, <a href="#Page_91">91</a>.</li>
-
-<li class="ifrst">Navy, use of wireless, <a href="#Page_274">274</a>;</li>
-<li class="isub1">of electricity, <a href="#Page_282">282</a>.</li>
-
-<li class="indx">Negative electricity, <a href="#Page_5">5</a>.</li>
-
-<li class="indx">Neon lamps, <a href="#Page_102">102</a>.</li>
-
-<li class="indx">Non-conductors, <a href="#Page_6">6</a>.</li>
-
-<li class="ifrst">Ohm, <a href="#Page_33">33</a>.</li>
-
-<li class="indx">Oil radiator, <a href="#Page_110">110</a>.</li>
-
-<li class="indx">Ozone, <a href="#Page_23">23</a>, <a href="#Page_247">247</a>.</li>
-
-<li class="indx">Ozone ventilation, <a href="#Page_249">249</a>.</li>
-
-<li class="ifrst">Petrol, motor, ignition in, <a href="#Page_253">253</a>.</li>
-
-<li class="indx">Photographophone, the, <a href="#Page_173">173</a>.</li>
-
-<li class="indx">Pile, voltaic, <a href="#Page_28">28</a>.</li>
-
-<li class="indx">Pipe locator, <a href="#Page_266">266</a>.</li>
-
-<li class="indx">Plant culture, electric, <a href="#Page_258">258</a>.</li>
-
-<li class="indx">Polarization, <a href="#Page_30">31</a>.</li>
-
-<li class="indx">Pollak-Virag telegraph, <a href="#Page_137">137</a>.</li>
-
-<li class="indx">Positive electricity, <a href="#Page_5">5</a>.</li>
-
-<li class="indx">Poulsen, Waldemar, <a href="#Page_171">171</a>, <a href="#Page_197">197</a>.</li>
-
-<li class="indx">Poultry, electro-culture of, <a href="#Page_264">264</a>.</li>
-
-<li class="indx">Power stations, <a href="#Page_75">75</a>.</li>
-
-<li class="indx">Preece, wireless experiments, <a href="#Page_180">180</a>.</li>
-
-<li class="indx">Primary and secondary coils, <a href="#Page_62">62</a>.</li>
-
-<li class="ifrst">Radiator, <a href="#Page_109">109</a>.</li>
-
-<li class="indx">Railways, electric, <a href="#Page_86">87</a>;</li>
-<li class="isub1">use of wireless, <a href="#Page_211">211</a>.</li>
-
-<li class="indx">Resistance, <a href="#Page_33">33</a>.</li>
-
-<li class="indx">Röntgen rays, <a href="#Page_228">228</a>, <a href="#Page_242">242</a>.</li>
-
-<li class="ifrst">Searchlights, <a href="#Page_98">98</a>.</li>
-
-<li class="indx">Ships, use of wireless, <a href="#Page_206">206</a>.</li>
-
-<li class="indx">Siphon recorder, the, <a href="#Page_252">252</a>.</li>
-
-<li class="indx">Sparking plug, <a href="#Page_154">154</a>.</li>
-
-<li class="indx">Static electricity, <a href="#Page_7">7</a>.</li>
-
-<li class="indx">Stations, wireless, <a href="#Page_204">204</a>.</li>
-
-<li class="indx">Steinheil telegraph, <a href="#Page_130">130</a>.</li>
-
-<li class="indx">Submarine telegraphy, <a href="#Page_144">144</a>.</li>
-
-<li class="indx">Submarine telephony, <a href="#Page_169">169</a>.</li>
-
-<li class="indx">Surface contact system, <a href="#Page_83">83</a>.</li>
-
-<li class="ifrst">Telefunken system, <a href="#Page_196">196</a>.</li>
-
-<li class="indx">Telegraph, the, <a href="#Page_128">128</a>, <a href="#Page_144">144</a>, <a href="#Page_171">171</a>, <a href="#Page_179">179</a>, <a href="#Page_203">203</a>.</li>
-
-<li class="indx">Telegraphone, <a href="#Page_171">171</a>.</li>
-
-<li class="indx">Telephone, the, <a href="#Page_154">154</a>, <a href="#Page_171">171</a>, <a href="#Page_179">179</a>, <a href="#Page_201">201</a>.</li>
-
-<li class="indx">Telephone exchange, <a href="#Page_160">160</a>.</li>
-
-<li class="indx">Thermopile, <a href="#Page_36">36</a>.</li>
-
-<li class="indx">Thermostat, <a href="#Page_120">121</a>.</li>
-
-<li class="indx">Thunderstorms, <a href="#Page_22">22</a>, <a href="#Page_194">194</a>.</li>
-
-<li class="indx">Trains, electric, <a href="#Page_86">87</a>;</li>
-<li class="isub1">the Bachelet, <a href="#Page_271">271</a>.</li>
-
-<li class="indx">Tramways, electric, <a href="#Page_78">78</a>, <a href="#Page_83">83</a>.</li>
-
-<li class="indx">Trolley system, <a href="#Page_83">83</a>.</li>
-
-<li class="indx">Tubes for X-rays, <a href="#Page_233">233</a>.</li>
-
-<li class="indx">Tuning in wireless telegraphy, <a href="#Page_191">191</a>.</li>
-
-<li class="indx">Tungsten lamps, <a href="#Page_104">104</a>.</li>
-
-<li class="ifrst">Volt, <a href="#Page_33">33</a>.</li>
-
-<li class="indx">Voltaic electricity, <a href="#Page_28">28</a>, <a href="#Page_129">129</a>, <a href="#Page_290">290</a>.</li>
-
-<li class="ifrst">War, electricity in, <a href="#Page_274">274</a>;</li>
-<li class="isub1">telegraph in, <a href="#Page_277">277</a>.</li>
-
-<li class="indx">Water, electrolysis of, <a href="#Page_38">38</a>.</li>
-
-<li class="indx">Water-power, <a href="#Page_80">81</a>.</li>
-
-<li class="indx">Waves, electric, <a href="#Page_181">181</a>, <a href="#Page_191">191</a>, <a href="#Page_199">199</a>.</li>
-
-<li class="indx">Welding, electric, <a href="#Page_114">114</a>.</li>
-
-<li class="indx">Welsbach lamp, <a href="#Page_103">103</a>.</li>
-
-<li class="indx">Wheatstone and Cooke telegraphs, <a href="#Page_130">130</a>.</li>
-
-<li class="indx">Wimshurst machine, <a href="#Page_12">12</a>.</li>
-
-<li class="indx">Wireless telegraphy and telephony, <a href="#Page_179">179</a>, <a href="#Page_203">203</a>, <a href="#Page_270">270</a>, <a href="#Page_280">280</a>.</li>
-
-<li class="indx">Wires, telegraph, <a href="#Page_141">141</a>.</li>
-
-<li class="ifrst">X-rays, <a href="#Page_231">231</a>, <a href="#Page_242">242</a>.</li>
-</ul>
-</div></div>
-
-<p class="p4 center wspace smaller">
-<span class="smcap">Morrison & Gibb Limited, Edinburgh</span><br>
-5/15 <span class="inend">2½</span>
-</p>
-
-<div class="chapter transnote">
-<h2 class="nobreak" id="Transcribers_Notes">Transcriber’s Notes</h2>
-
-<p>Punctuation, hyphenation, and spelling were made
-consistent when a predominant preference was found
-in the original book; otherwise they were not changed.</p>
-
-<p>Simple typographical errors were corrected; unbalanced
-quotation marks were remedied when the change was
-obvious, and otherwise left unbalanced.</p>
-
-<p>Illustrations in this eBook have been positioned
-between paragraphs and outside quotations. In versions
-of this eBook that support hyperlinks, the page
-references in the List of Plates lead to the
-corresponding illustrations. (There is no list
-of the other illustrations.)</p>
-
-<p id="plate_VIII"><span class="bold">Plate VIII.</span>, “Typical Electric Locomotives,” listed as being on
-<a href="#Page_90">page 90</a>, was not in the original book and therefore not in this ebook.</p>
-
-<p>The index was not checked for proper alphabetization
-or correct page references.</p>
-</div>
-<div style='text-align:center'>*** END OF THE PROJECT GUTENBERG EBOOK ELECTRICITY ***</div>
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+ padding: 1em; +} +.x-ebookmaker .transnote { + page-break-before: always; + page-break-after: always; + margin-left: 2%; + margin-right: 2%; + margin-top: 1em; + margin-bottom: 1em; + padding: .5em; +} + +.gesperrt { + letter-spacing: 0.2em; + margin-right: -0.2em; +} +.wspace {word-spacing: .3em;} + +span.locked {white-space:nowrap;} +.pagenum br {display: none; visibility: hidden;} +.sans {font-family: sans-serif, serif;} +.fright {float: right; padding-left: 4em;} +.x-ebookmaker-2 {float: none;} +.bb {border-bottom: thin solid black; padding-bottom: .1em;} + + /* ]]> */ </style> +</head> + +<body> +<div style='text-align:center'>*** START OF THE PROJECT GUTENBERG EBOOK ELECTRICITY ***</div> + +<div class="transnote section"> +<p class="center larger">Transcriber’s Note</p> + +<p>Larger versions of most illustrations may be seen by right-clicking them +and selecting an option to view them separately, or by double-tapping and/or +stretching them.</p> + +<p><a href="#Transcribers_Notes">Additional notes</a> will be found near the end of this ebook.</p> +<div> </div> +</div> + +<div class="section"> +<figure id="coversmall" class="figcenter" style="max-width: 30em;"> + <img src="images/coversmall.jpg" width="746" height="1024" alt="(cover)"></figure> +<div> </div> +</div> + +<div class="section p4"> +<p class="right l4 b2">“ROMANCE OF REALITY” SERIES<br> +<span style="padding-right: 2.5em;">Edited by <span class="smcap">Ellison Hawks</span></span></p> + +<h1>ELECTRICITY</h1> +<div> </div> +</div> + +<hr class="chap x-ebookmaker-drop"> + +<div class="chapter section"> +<p class="p1 in0 in4 b1 gesperrt"><span class="in2 wspace"><i>VOLUMES ALREADY ISSUED</i></span></p> +</div> + +<div class="hang"> +<p>1. THE AEROPLANE. By <span class="smcap">Grahame White</span> and <span class="smcap">Harry Harper</span>.</p> + +<p>2. THE MAN-OF-WAR. By Commander <span class="smcap">E. H. Currey</span>, R.N.<br></p> + +<p>3. MODERN INVENTIONS. By <span class="smcap">V. E. Johnson</span>, M.A.<br></p> + +<p>4. ELECTRICITY. By <span class="smcap">W. H. McCormick</span>.<br></p> + +<p>5. ENGINEERING. By <span class="smcap">Gordon D. Knox</span>.</p> +<div> </div> +</div> + +<div class="section p4"> +<figure id="plate_0" class="figcenter" style="max-width: 25em;"> + <img src="images/i_004.jpg" width="631" height="994" alt=" "> + <figcaption class="caption"><p class="floatr">(<a href="images/i_004large.jpg"><i>Larger</i></a>)</p><p class="clear">THE MARCONI TRANSATLANTIC WIRELESS STATION +AT GLACE BAY, NOVA SCOTIA</p> + +<p>Drawing by Irene Sutcliffe</p> +</figcaption></figure> +<div> </div> +</div> + +<hr class="chap x-ebookmaker-drop"> + +<div class="chapter center vspace"> +<p> +<i class="bb">“ROMANCE OF REALITY” SERIES</i></p> + +<p class="p1 xxlarge gesperrt bold">ELECTRICITY</p> + +<p class="p2">BY<br> +<span class="large">W. H. McCORMICK</span></p> + +<figure id="i_5" class="figcenter" style="max-width: 20em;"> + <img src="images/i_005.png" width="1531" height="1072" alt="X-ray tube"></figure> + +<p class="p2">NEW YORK<br> +<span class="larger">FREDERICK A. STOKES COMPANY</span><br> +PUBLISHERS +</p> +</div> + +<hr class="chap x-ebookmaker-drop"> + +<div class="chapter section p4"> +<p class="xsmall wspace center"><i>Printed in Great Britain</i></p> +<div> </div> +</div> + +<hr class="chap x-ebookmaker-drop"> + +<div class="chapter"> +<p><span class="pagenum" id="Page_v">v</span></p> +<h2 class="nobreak" id="PREFACE">PREFACE</h2> +</div> + +<p class="in0"><span class="firstword">I gladly</span> take this opportunity of acknowledging the generous +assistance I have received in the preparation of this book.</p> + +<p>I am indebted to the following firms for much useful information +regarding their various <span class="locked">specialities:—</span></p> + +<p>Chloride Electrical Storage Co. Ltd.; General Electric Co. Ltd.; +Union Electric Co. Ltd.; Automatic Electric Co., Chicago; Westinghouse +Cooper-Hewitt Co. Ltd.; Creed, Bille & Co. Ltd.; India +Rubber, Gutta Percha, and Telegraph Works Co. Ltd.; W. Canning +& Co.; C. H. F. Muller; Ozonair Ltd.; Universal Electric Supply +Co., Manchester; and the Agricultural Electric Discharge Co. Ltd.</p> + +<p>For illustrations my thanks are due <span class="locked">to:—</span></p> + +<p>Marconi’s Wireless Telegraph Co. Ltd.; Chloride Electrical +Storage Co. Ltd.; Harry W. Cox & Co. Ltd.; C. H. F. Muller; +W. Canning & Co.; Union Electric Co. Ltd.; Creed, Bille & Co. +Ltd.; Ozonair Ltd.; Kodak Ltd.; C. A. Parsons & Co.; Lancashire +Dynamo and Motor Co. Ltd.; Dick, Kerr & Co. Ltd.; +Siemens Brothers Dynamo Works Ltd.; Vickers Ltd.; and +Craven Brothers Ltd.</p> + +<p>Mr. Edward Maude and Mr. J. A. Robson have most kindly +prepared for me a number of the diagrams, and I am indebted +to Dr. Myer Coplans for particulars and a diagram of the heat-compensated +salinometer.</p> + +<p>I acknowledge also many important suggestions from Miss +E. C. Dudgeon on Electro-Culture, and from Mr. R. Baxter and +Mr. G. Clark on Telegraphy and Telephony.</p> + +<p>Amongst the many books I have consulted I am indebted<span class="pagenum" id="Page_vi">vi</span> +specially to <cite>Electricity in Modern Medicine</cite>, by Alfred C. Norman, +M.D.; <cite>Growing Crops and Plants by Electricity</cite>, by Miss E. C. +Dudgeon; and <cite>Wireless Telegraphy</cite> (Cambridge Manuals), by +Prof. C. L. Fortescue. I have derived great assistance also from +the <cite>Wireless World</cite>.</p> + +<p>Finally, I have to thank Mr. Albert Innes, A.I.E.E., of Leeds, +for a number of most valuable suggestions, and for his kindness in +reading through the proofs.</p> + +<p class="right"> +<span style="margin-right: 2em;">W. H. McC.</span> +</p> + +<p><span class="smcap">Leeds, 1915<span class="pagenum" id="Page_vii">vii</span></span></p> + +<hr class="chap x-ebookmaker-drop"> + +<div class="chapter"> +<h2 class="nobreak" id="CONTENTS">CONTENTS</h2> +</div> + +<table id="toc"> +<tr class="small"> + <td class="tdr">CHAPTER</td> + <td></td> + <td class="tdr">PAGE</td> +</tr> +<tr> + <td class="tdr top">I.</td> + <td class="tdl"><span class="smcap">The Birth of the Science of Electricity</span></td> + <td class="tdr"><a href="#toclink_1">1</a></td> +</tr> +<tr> + <td class="tdr top">II.</td> + <td class="tdl"><span class="smcap">Electrical Machines and the Leyden Jar</span></td> + <td class="tdr"><a href="#toclink_9">9</a></td> +</tr> +<tr> + <td class="tdr top">III.</td> + <td class="tdl"><span class="smcap">Electricity in the Atmosphere</span></td> + <td class="tdr"><a href="#toclink_18">18</a></td> +</tr> +<tr> + <td class="tdr top">IV.</td> + <td class="tdl"><span class="smcap">The Electric Current</span></td> + <td class="tdr"><a href="#toclink_27">27</a></td> +</tr> +<tr> + <td class="tdr top">V.</td> + <td class="tdl"><span class="smcap">The Accumulator</span></td> + <td class="tdr"><a href="#toclink_38">38</a></td> +</tr> +<tr> + <td class="tdr top">VI.</td> + <td class="tdl"><span class="smcap">Magnets and Magnetism</span></td> + <td class="tdr"><a href="#toclink_44">44</a></td> +</tr> +<tr> + <td class="tdr top">VII.</td> + <td class="tdl"><span class="smcap">The Production of Magnetism by Electricity</span></td> + <td class="tdr"><a href="#toclink_56">56</a></td> +</tr> +<tr> + <td class="tdr top">VIII.</td> + <td class="tdl"><span class="smcap">The Induction Coil</span></td> + <td class="tdr"><a href="#toclink_61">61</a></td> +</tr> +<tr> + <td class="tdr top">IX.</td> + <td class="tdl"><span class="smcap">The Dynamo and the Electric Motor</span></td> + <td class="tdr"><a href="#toclink_66">66</a></td> +</tr> +<tr> + <td class="tdr top">X.</td> + <td class="tdl"><span class="smcap">Electric Power Stations</span></td> + <td class="tdr"><a href="#toclink_75">75</a></td> +</tr> +<tr> + <td class="tdr top">XI.</td> + <td class="tdl"><span class="smcap">Electricity in Locomotion</span></td> + <td class="tdr"><a href="#toclink_83">83</a></td> +</tr> +<tr> + <td class="tdr top">XII.</td> + <td class="tdl"><span class="smcap">Electric Lighting</span></td> + <td class="tdr"><a href="#toclink_93">93</a></td> +</tr> +<tr> + <td class="tdr top">XIII.</td> + <td class="tdl"><span class="smcap">Electric Heating</span></td> + <td class="tdr"><a href="#toclink_109">109</a></td> +</tr> +<tr> + <td class="tdr top">XIV.</td> + <td class="tdl"><span class="smcap">Electric Bells and Alarms</span></td> + <td class="tdr"><a href="#toclink_116">116</a></td> +</tr> +<tr> + <td class="tdr top">XV.</td> + <td class="tdl"><span class="smcap">Electric Clocks</span></td> + <td class="tdr"><a href="#toclink_124">124</a></td> +</tr> +<tr> + <td class="tdr top">XVI.</td> + <td class="tdl"><span class="smcap">The Telegraph</span></td> + <td class="tdr"><a href="#toclink_128">128</a></td> +</tr> +<tr> + <td class="tdr top">XVII.</td> + <td class="tdl"><span class="smcap">Submarine Telegraphy</span></td> + <td class="tdr"><a href="#toclink_144">144</a></td> +</tr> +<tr> + <td class="tdr top">XVIII.</td> + <td class="tdl"><span class="smcap">The Telephone</span></td> + <td class="tdr"><a href="#toclink_154">154</a></td> +</tr> +<tr> + <td class="tdr top">XIX.</td> + <td class="tdl"><span class="smcap">Some Telegraphic and Telephonic Inventions</span></td> + <td class="tdr"><a href="#toclink_171">171</a></td> +</tr> +<tr> + <td class="tdr top">XX.</td> + <td class="tdl"><span class="smcap">Wireless Telegraphy and Telephony—Principles and Apparatus</span></td> + <td class="tdr"><a href="#toclink_179">179</a></td> +</tr> +<tr> + <td class="tdr top">XXI.</td> + <td class="tdl"><span class="smcap">Wireless Telegraphy—Practical Applications</span></td> + <td class="tdr"><a href="#toclink_203">203</a></td> +</tr> +<tr> + <td class="tdr top">XXII.</td> + <td class="tdl"><span class="smcap">Electroplating and Electrotyping</span></td> + <td class="tdr"><a href="#toclink_213">213</a></td> +</tr> +<tr> + <td class="tdr top">XXIII.</td> + <td class="tdl"><span class="smcap">Industrial Electrolysis</span></td> + <td class="tdr"><a href="#toclink_224">224</a></td> +</tr> +<tr> + <td class="tdr top">XXIV.</td> + <td class="tdl"><span class="smcap">The Röntgen Rays</span></td> + <td class="tdr"><a href="#toclink_228">228</a></td> +</tr> +<tr> + <td class="tdr top">XXV.</td> + <td class="tdl"><span class="smcap">Electricity in Medicine</span></td> + <td class="tdr"><a href="#toclink_241">241</a></td> +</tr> +<tr> + <td class="tdr top">XXVI.</td> + <td class="tdl"><span class="smcap">Ozone</span></td> + <td class="tdr"><a href="#toclink_247">247</a></td> +</tr> +<tr> + <td class="tdr top">XXVII.</td> + <td class="tdl"><span class="smcap">Electric Ignition</span></td> + <td class="tdr"><a href="#toclink_253">253</a></td> +</tr> +<tr> + <td class="tdr top">XXVIII.</td> + <td class="tdl"><span class="smcap">Electro-Culture</span></td> + <td class="tdr"><a href="#toclink_258">258</a></td> +</tr> +<tr> + <td class="tdr top">XXIX.</td> + <td class="tdl"><span class="smcap">Some Recent Applications of Electricity—An Electric Pipe Locator, etc.</span></td> + <td class="tdr"><a href="#toclink_266">266</a></td> +</tr> +<tr> + <td class="tdr top">XXX.</td> + <td class="tdl"><span class="smcap">Electricity in War</span></td> + <td class="tdr"><a href="#toclink_274">274</a></td> +</tr> +<tr> + <td class="tdr top">XXXI.</td> + <td class="tdl"><span class="smcap">What is Electricity?</span></td> + <td class="tdr"><a href="#toclink_287">287</a></td> +</tr> +<tr> + <td></td> + <td class="tdl"><span class="smcap">Index</span></td> + <td class="tdr"><a href="#toclink_295">295</a></td> +</tr> +</table> + +<hr class="chap x-ebookmaker-drop"> + +<div class="chapter"> +<p><span class="pagenum" id="Page_viii">viii</span></p> + +<h2 class="nobreak" id="LIST_OF_PLATES">LIST OF PLATES</h2> +</div> + +<table id="loi"> +<tr> + <td class="tdl norpad" colspan="2"><span class="smcap">Plate in Colour: +The Marconi Transatlantic Wireless Station at Glace Bay, Nova Scotia</span> + <span class="fright"><a href="#plate_0"><i>Frontispiece</i></a></span></td> +</tr> +<tr class="small"> + <td class="tdr" colspan="2">FACING PAGE</td> +</tr> +<tr> + <td class="tdl"><span class="smcap">Hydro-Electric Power Station</span></td> + <td class="tdr"><a href="#plate_I">30</a></td> +</tr> +<tr> + <td class="tdl">(<i>a</i>) <span class="smcap">Experiment to show Magnetic Induction</span></td> + <td class="tdr"><a href="#plate_IIa">48</a></td> +</tr> +<tr> + <td class="tdl">(<i>b</i>) <span class="smcap">Experiment to show the Production of Magnetism by an Electric Current</span></td> + <td class="tdr"><a href="#plate_IIb">48</a></td> +</tr> +<tr> + <td class="tdl">(<i>a</i>) <span class="smcap">Lines of Magnetic Force of Two Opposite Poles</span></td> + <td class="tdr"><a href="#plate_III">50</a></td> +</tr> +<tr> + <td class="tdl">(<i>b</i>) <span class="smcap">Lines of Magnetic Force of Two Similar Poles</span></td> + <td class="tdr"><a href="#plate_III">50</a></td> +</tr> +<tr> + <td class="tdl"><span class="smcap">A Typical Dynamo and its Parts</span></td> + <td class="tdr"><a href="#plate_IV">70</a></td> +</tr> +<tr> + <td class="tdl"><span class="smcap">Lots Road Electric Power Station, Chelsea</span></td> + <td class="tdr"><a href="#plate_V">76</a></td> +</tr> +<tr> + <td class="tdl"><span class="smcap">Power Station Battery of Accumulators</span></td> + <td class="tdr"><a href="#plate_VI">80</a></td> +</tr> +<tr> + <td class="tdl"><span class="smcap">Electric Colliery Railway</span></td> + <td class="tdr"><a href="#plate_VII">86</a></td> +</tr> +<tr> + <td class="tdl"><span class="smcap">Typical Electric Locomotives</span></td> + <td class="tdr"><a href="#plate_VIII">90</a></td> +</tr> +<tr> + <td class="tdl"><span class="smcap">Night Photographs, taken by the Light of the Arc Lamps</span></td> + <td class="tdr"><a href="#plate_IXa">96</a></td> +</tr> +<tr> + <td class="tdl"><span class="smcap">Where Electrical Machinery is made</span></td> + <td class="tdr"><a href="#plate_X">120</a></td> +</tr> +<tr> + <td class="tdl"><span class="smcap">Specimen of the Work of the Creed High-Speed Printing Telegraph</span></td> + <td class="tdr"><a href="#plate_XI">140</a></td> +</tr> +<tr> + <td class="tdl"><span class="smcap">Large Electric Travelling Crane at a Railway Works</span></td> + <td class="tdr"><a href="#plate_XII">164</a></td> +</tr> +<tr> + <td class="tdl">(<i>a</i>) <span class="smcap">Marconi Operator Receiving a Message</span></td> + <td class="tdr"><a href="#plate_XIIIa">188</a></td> +</tr> +<tr> + <td class="tdl">(<i>b</i>) <span class="smcap">Marconi Magnetic Detector</span></td> + <td class="tdr"><a href="#plate_XIIIb">188</a></td> +</tr> +<tr> + <td class="tdl"><span class="smcap">Röntgen Ray Photograph of British and Foreign Fountain Pens</span></td> + <td class="tdr"><a href="#plate_XIV">240</a></td> +</tr> +<tr> + <td class="tdl"><span class="smcap">Bachelet “Flying Train” and its Inventor</span></td> + <td class="tdr"><a href="#plate_XV">272</a></td> +</tr> +<tr> + <td class="tdl">(<i>a</i>) <span class="smcap">Cavalry Portable Wireless Cart Set</span></td> + <td class="tdr"><a href="#plate_XVIa">280</a></td> +</tr> +<tr> + <td class="tdl">(<i>b</i>) <span class="smcap">Aeroplane fitted with Wireless Telegraphy</span></td> + <td class="tdr"><a href="#plate_XVIb">280</a></td> +</tr> +</table> + +<hr class="chap x-ebookmaker-drop"> + +<div class="chapter"> +<p><span class="pagenum" id="Page_1">1</span></p> + +<h2 class="nobreak" id="toclink_1"><span class="larger">ELECTRICITY</span></h2> + +<h2 class="nobreak" id="chapter_I">CHAPTER I<br> + +<span class="subhead">THE BIRTH OF THE SCIENCE OF ELECTRICITY</span></h2> +</div> + +<p class="in0"><span class="firstword">Although</span> the science of electricity is of comparatively +recent date, electricity itself has existed from the beginning +of the world. There can be no doubt that man’s introduction +to electricity was brought about through the +medium of the thunderstorm, and from very early times +come down to us records of the terror inspired by thunder +and lightning, and of the ways in which the ancients tried +to account for the phenomena. Even to-day, although we +know what lightning is and how it is produced, a severe +thunderstorm fills us with a certain amount of awe, if not +fear; and we can understand what a terrifying experience +it must have been to the ancients, who had none of our +knowledge.</p> + +<p>These early people had simple minds, and from our +point of view they had little intelligence; but they possessed +a great deal of curiosity. They were just as anxious to +explain things as we are, and so they were not content +until they had invented an explanation of lightning and +thunder. Their favourite way of accounting for anything +they did not understand was to make up a sort of romance +about it. They believed that the heavens were inhabited +by various gods, who showed their pleasure or anger by<span class="pagenum" id="Page_2">2</span> +signs, and so they naturally concluded that thunder was +the voice of angry gods, and lightning the weapon with +which they struck down those who had displeased them. +Prayers and sacrifices were therefore offered to the gods, in +the hope of appeasing their wrath.</p> + +<p>Greek and Roman mythology contains many references +to thunder and lightning. For instance, we read about +the great god Zeus, who wielded thunder-bolts which had +been forged in underground furnaces by the giant Cyclops. +There was no doubt that the thunder-bolts were made in +this way, because one only had to visit a volcano in order +to see the smoke from the furnace, and hear the rumbling +echo of the far-off hammering. Then we are told the +tragic story of Phaeton, son of the Sun-god. This youth, +like many others since his time, was daring and venturesome, +and imagined that he could do things quite as well +as his father. On one occasion he tried to drive his father’s +chariot, and, as might have been expected, it got beyond +his control, and came dangerously near the Earth. The +land was scorched, the oceans were dried up, and the whole +Earth was threatened with utter destruction. In order to +prevent such a frightful catastrophe, Jupiter, the mighty +lord of the heavens, hurled a thunder-bolt at Phaeton, and +struck him from the chariot into the river Po. A whole +book could be written about these ancient legends concerning +the thunderstorm, but, interesting as they are, they +have no scientific value, and many centuries were to elapse +before the real nature of lightning was understood.</p> + +<p>In order to trace the first glimmerings of electrical +knowledge we must leave the thunderstorm and pass on to +more trivial matters. On certain sea-coasts the ancients +found a transparent yellow substance capable of taking a +high polish, and much to be desired as an ornament; and +about 600 years <span class="allsmcap">B.C.</span> it was discovered that this substance,<span class="pagenum" id="Page_3">3</span> +when rubbed, gained the power of drawing to it bits of +straw, feathers, and other light bodies. This discovery is +generally credited to a Greek philosopher named Thales, +941–563 <span class="allsmcap">B.C.</span>, and it must be regarded as the first step +towards the foundation of electrical science. The yellow +substance was amber. We now know it to be simply a +sort of fossilized resin, but the Greeks gave it a much more +romantic origin. When Phaeton’s rashness brought him +to an untimely end, his sorrowing sisters, the Heliades, +were changed into poplar trees, and their tears into amber. +Amongst the names given to the Sun-god was Alector, +which means the shining one, and so the tears of the +Heliades came to have the name Electron, or the shining +thing. Unlike most of the old legends, this story of the +fate of the Sun-maidens is of great importance to us, for +from the word “electron” we get the name Electricity.</p> + +<p>Thales and his contemporaries seem to have made no +serious attempts to explain the attraction of the rubbed +amber, and indeed so little importance was attached to the +discovery that it was completely forgotten. About 321 <span class="allsmcap">B.C.</span> +one Theophrastus found that a certain mineral called +“lyncurium” gained attractive powers when rubbed, but +again little attention was paid to the matter, and astonishing +as it may seem, no further progress worth mention was +made until towards the close of the sixteenth century, when +Doctor Gilbert of Colchester began to experiment seriously. +This man was born about 1543, and took his degree of +doctor of medicine at Cambridge in 1569. He was very +successful in his medical work, and became President of the +College of Physicians, and later on physician to Queen +Elizabeth. He had a true instinct for scientific research, +and was not content to accept statements on the authority +of others, but tested everything for himself. He found +that sulphur, resin, sealing-wax, and many other substances<span class="pagenum" id="Page_4">4</span> +behaved like amber when rubbed, but he failed to get any +results from certain other substances, such as the metals. +He therefore called the former substances “electrics,” and +the latter “anelectrics,” or non-electrics. His researches +were continued by other investigators, and from him dates +the science of electricity.</p> + +<figure id="fig_1" class="figleft" style="max-width: 8em;"> + <img src="images/i_014.png" width="609" height="1021" alt=" "> + <figcaption class="caption hang"><span class="smcap">Fig. 1.</span>—Suspended +pith ball for showing electric attraction. +</figcaption></figure> + +<p>Leaving historical matters for the present, we will +examine the curious power which is gained by substances +as the result of rubbing. Amber is not always obtainable, +and so we will use in its place a glass +rod and a stick of sealing-wax. If the +glass rod is rubbed briskly with a dry +silk handkerchief, and then held close +to a number of very small bits of paper, +the bits are immediately drawn to the +rod, and the same thing occurs if the +stick of sealing-wax is substituted for +the glass. This power of attraction is +due to the presence of a small charge +of electricity on the rubbed glass and +sealing-wax, or in other words, the two +substances are said to be electrified. +Bits of paper are unsatisfactory for careful +experimenting, and instead of them +we will use the simple piece of apparatus shown in <a href="#fig_1">Fig. 1</a>. +This consists of a ball of elder pith, suspended from a glass +support by means of a silk thread. If now we repeat our +experiments with the electrified glass or sealing-wax we +find that the little ball is attracted in the same way as the +bits of paper. But if we look carefully we shall notice that +attraction is not the only effect, for as soon as the ball +touches the electrified body it is driven away or repelled. +Now let us suspend, by means of a thread, a glass rod +which has been electrified by rubbing it with silk, and bring<span class="pagenum" id="Page_5">5</span> +near it in turn another silk-rubbed glass rod and a stick of +sealing-wax rubbed with flannel. The two glass rods are +found to repel one another, whereas the sealing-wax attracts +the glass. If the experiment is repeated with a suspended +stick of sealing-wax rubbed with flannel, the glass and the +sealing-wax attract each other, but the two sticks of wax +repel one another. Both glass and sealing-wax are +electrified, as may be seen by bringing them near the pith +ball, but there must be some difference between them as +we get attraction in one case and repulsion in the other.</p> + +<p>The explanation is that the electric charges on the silk-rubbed +glass and on the flannel-rubbed sealing-wax are of +different kinds, the former being called positive, and the +latter negative. Bodies with similar charges, such as the +two glass rods, repel one another; while bodies with unlike +charges, such as the glass and the sealing-wax, attract each +other. We can now see why the pith ball was first +attracted and then repelled. To start with, the ball was +not electrified, and was attracted when the rubbed glass or +sealing-wax was brought near it. When however the +ball touched the electrified body it received a share of the +latter’s electricity, and as similar charges repel one another, +the ball was driven away.</p> + +<p>The kind of electricity produced depends not only on +the substance rubbed, but also on the material used as the +rubber. For instance, we can give glass a negative charge +by rubbing it with flannel, and sealing-wax becomes +positively charged when rubbed with silk. The important +point to remember is that there are only two kinds of +electricity, and that every substance electrified by rubbing +is charged either positively, like the silk-rubbed glass, or +negatively, like the flannel-rubbed sealing-wax.</p> + +<p>If we try to electrify a metal rod by holding it in the +hand and rubbing it, we get no result, but if we fasten to<span class="pagenum" id="Page_6">6</span> +the metal a handle of glass, and hold it by this while +rubbing, we find that it becomes electrified in the same way +as the glass rod or the sealing-wax. Substances such as +glass do not allow electricity to pass along them, so that in +rubbing a glass rod the part rubbed becomes charged, and +the electricity stays there, being unable to spread to the +other parts of the rod. Substances such as metals allow +electricity to pass easily, so that when a metal rod is +rubbed electricity is produced, but it immediately spreads +over the whole rod, reaches the hand, and escapes. If we +wish the metal to retain its charge we must provide it with +a handle of glass or of some other material which does not +allow electricity to pass. Dr. Gilbert did not know this, +and so he came to the conclusion that metals were non-electrics, +or could not be electrified.</p> + +<p>Substances which allow electricity to pass freely are +called conductors, and those which do not are called non-conductors; +while between the two extremes are many +substances which are called partial conductors. It may be +said here that no substance is quite perfect in either respect, +for all conductors offer some resistance to the passage of +electricity, while all non-conductors possess some conducting +power. Amongst conductors are metals, acids, water, +and the human body; cotton, linen, and paper are partial +conductors; and air, resin, silk, glass, sealing-wax, and +gutta-percha are non-conductors. When a conductor is +guarded by a non-conductor so that its electricity cannot +escape, it is said to be insulated, from Latin, <i lang="la">insula</i>, an +island; and non-conductors are also called “insulators.”</p> + +<p>So far we have mentioned only the electric charge +produced on the substance rubbed, but the material used as +rubber also becomes electrified. The two charges, however, +are not alike, but one is always positive and the other +negative. For instance, if glass is rubbed with silk, the<span class="pagenum" id="Page_7">7</span> +glass receives a positive, and the silk a negative charge. +It also can be shown that the two opposite charges are +always equal in quantity.</p> + +<p>The two kinds of electricity are generally represented +by the signs + and -, the former standing for positive +and the latter for negative electricity.</p> + +<p>The electricity produced by rubbing, or friction, is +known as Static Electricity; that is, electricity in a state of +rest, as distinguished from electricity in motion, or current +electricity. The word static is derived from a Greek word +meaning to stand. At the same time it must be understood +that this electricity of friction is at rest only in the +sense that it is a prisoner, unable to move. When we +produce a charge of static electricity on a glass rod, by +rubbing it, the electricity would escape fast enough if it +could. It has only two possible ways of escape, along the +rod and through the air, and as both glass and air are non-conductors +it is obliged to remain at rest where it was +produced. On the other hand, as we have seen, the +electricity produced by rubbing a metal rod which is not +protected by an insulating handle escapes instantly, because +the metal is a good conductor.</p> + +<p>When static electricity collects in sufficient quantities +it discharges itself in the form of a bright spark, and we +shall speak of these sparks in <a href="#chapter_III">Chapter III</a>. Static electricity +is of no use for doing useful work, such as ringing bells or +driving motors, and in fact, except for scientific purposes, +it is more of a nuisance than a help. It collects almost +everywhere, and its power of attraction makes it very +troublesome at times. In the processes of textile manufacture +static electricity is produced in considerable +quantities, and it makes its presence known by causing the +threads to stick together in the most annoying fashion. In +printing rooms too it plays pranks, making the sheets of<span class="pagenum" id="Page_8">8</span> +paper stick together so that the printing presses have to be +stopped.</p> + +<p>Curiously enough, static electricity has been detected in +the act of interfering with the work of its twin brother, +current electricity. A little while ago it was noticed that +the electric incandescent lamps in a certain building were +lasting only a very short time, the filaments being found +broken after comparatively little use. Investigations +showed that the boy was in the habit of dusting the lamp +globes with a feather duster. The friction set up in this +way produced charges of electricity on the glass, and this +had the effect of breaking the filaments. When this +method of dusting was discontinued the trouble ceased, and +the lamps lasted their proper number of hours.</p> +<hr class="chap x-ebookmaker-drop"> + +<div class="chapter"> +<p><span class="pagenum" id="Page_9">9</span></p> + +<h2 class="nobreak" id="toclink_9"><a id="chapter_II"></a>CHAPTER II<br> + +<span class="subhead">ELECTRICAL MACHINES AND THE LEYDEN JAR</span></h2> +</div> + +<p class="in0"><span class="firstword">The</span> amount of electricity produced by the rubbing of glass +or sealing-wax rods is very small, and experimenters soon +felt the need of apparatus to produce larger quantities. In +1675 the first electrical machine was made by Otto von +Guericke, the inventor of the air-pump. His machine consisted +of a globe of sulphur fixed on a spindle, and rotated +while the hands were pressed against it to provide the +necessary friction. Globes and cylinders of glass soon +replaced the sulphur globe, and the friction was produced +by cushions instead of by the hands. Still later, revolving +plates of glass were employed. These machines worked +well enough in a dry atmosphere, but were very troublesome +in wet weather, and they are now almost entirely +superseded by what are known as <em>influence</em> machines.</p> + +<p>In order to understand the working of influence +machines, it is necessary to have a clear idea of what is +meant by the word influence as used in an electrical sense. +In the previous chapter we saw that a pith ball was +attracted by an electrified body, and that when the ball +touched that body it received a charge of electricity. +We now have to learn that one body can receive a charge +from another body without actual contact, by what is called +“influence,” or electro-static induction. In <a href="#fig_2">Fig. 2</a> is seen a +simple arrangement for showing this influence or induction. +A is a glass ball, and BC a piece of metal, either solid or<span class="pagenum" id="Page_10">10</span> +hollow, made somewhat in the shape of a sausage, and +insulated by means of its glass support. Three pairs of +pith balls are suspended from BC as shown. If A is +electrified positively, and brought near BC, the pith balls +at B and C repel one another, showing that the ends of +BC are electrified. No repulsion takes place between the +two pith balls at the middle, indicating that this part of +BC is not electrified. If the charges at B and C are tested +they are found to be of opposite kinds, that at B being +negative, and that at C positive. Thus it appears that +the positive charge on A has attracted a negative charge +to B, and repelled a positive one to C. If A is taken +away, the two opposite charges on BC unite and neutralise +one another, and BC is left in its original uncharged condition, +while A is found to have lost none of its own charge. +If BC is made in two parts, and if these are separated while +under the influence of A, the two charges cannot unite +when A is removed, but remain each on its own half of +BC. In this experiment A is said to have induced electrification +on BC. Induction will take place across a considerable +distance, and it is not stopped by the interposition +of obstacles such as a sheet of glass.</p> + +<figure id="fig_2" class="figcenter" style="max-width: 26em;"> + <img src="images/i_020.png" width="2014" height="892" alt=" "> + <figcaption class="caption"><span class="smcap">Fig. 2.</span>—Diagram to illustrate Electro-static Induction. +</figcaption></figure> + +<p><span class="pagenum" id="Page_11">11</span></p> + +<p>We can now understand why an electrified body +attracts an unelectrified body, as in our pith ball experiments. +If we bring a positively charged glass rod near +a pith ball, the latter becomes electrified by induction, the +side nearer the rod receiving a negative, and the farther +side a positive charge. One half of the ball is therefore +attracted and the other half repelled, but as the attracted +half is the nearer, the attraction is stronger than the repulsion, +and so the ball moves towards the rod.</p> + +<figure id="fig_3" class="figright" style="max-width: 13em;"> + <img src="images/i_021.png" width="1007" height="754" alt=" "> + <figcaption class="caption"><span class="smcap">Fig. 3.</span>—The Electrophorus. +</figcaption></figure> + +<p><a href="#fig_3">Fig. 3</a> shows an appliance for obtaining strong charges +of electricity by influence or induction. It is called the +<em>electrophorus</em>, the name coming from two Greek words, +<em>electron</em>, amber, and <em>phero</em>, +I yield or bear; and it was +devised in 1775 by Volta, an +Italian professor of physics. +The apparatus consists of a +round cake, A, of some +resinous material contained +in a metal dish, and a round +disc of metal, B, of slightly +smaller diameter, fitted with +an insulating handle. A simple electrophorus may be +made by filling with melted sealing-wax the lid of a +round tin, the disc being made of a circular piece of +copper or brass, a little smaller than the lid, fastened to +the end of a stick of sealing-wax. To use the electrophorus, +the sealing-wax is electrified negatively by rubbing +it with flannel. The metal disc is then placed on the +sealing-wax, touched for an instant with the finger, and +lifted away. The disc is now found to be electrified +positively, and it may be discharged and the process repeated +many times without recharging the sealing-wax. +The charge on the latter is not used up in the process,<span class="pagenum" id="Page_12">12</span> +but it gradually leaks away, and after a time it has to +be renewed.</p> + +<p>The theory of the electrophorus is easy to understand +from what we have already learnt about influence. When +the disc B is placed on the charged cake A, the two surfaces +are really in contact at only three or four points, +because neither of them is a true plane; so that on the +whole the disc and the cake are like A and BC in <a href="#fig_2">Fig. 2</a>, +only much closer together. The negative charge on +A acts by induction +on the disc B, attracting +a positive charge +to the under side, and +repelling a negative +charge to the upper +side. When the disc +is touched, the negative +charge on the +upper side escapes, but +the positive charge +remains, being as it +were held fast by the +attraction of the negative +charge on A. If +the disc is now raised, the positive charge is no longer +bound on the under side, and it therefore spreads over +both surfaces, remaining there because its escape is cut +off by the insulating handle.</p> + +<figure id="fig_4" class="figleft" style="max-width: 16em;"> + <img src="images/i_022.jpg" width="1229" height="1207" alt=" "> + <figcaption class="caption"><span class="smcap">Fig. 4.</span>—Wimshurst Machine. +</figcaption></figure> + +<p>We may now try to understand the working of influence +machines, which are really mechanically worked electrophori. +There are various types of such machines, but the +one in most general use in this country is that known as +the Wimshurst machine, <a href="#fig_4">Fig. 4</a>, and we will therefore +confine ourselves to this. It consists of two circular plates<span class="pagenum" id="Page_13">13</span> +of varnished glass or of ebonite, placed close together and +so geared that they rotate in opposite directions. On the +outer surfaces of the plates are cemented sectors of metal +foil, at equal distances apart. Each plate has the same +number of sectors, so that at any given moment the sectors +on one plate are exactly opposite those on the other. +Across the outer surface of each plate is fixed a rod of +metal carrying at its ends light tinsel brushes, which are +adjusted to touch the sectors as they pass when the plates +are rotated. These rods are placed at an angle to each +other of from sixty to ninety degrees, and the brushes are +called neutralizing brushes. The machine is now complete +for generating purposes, but in order to collect the electricity +two pairs of insulated metal combs are provided, one pair +at each end of the horizontal diameter, with the teeth +pointing inward towards the plates, but not touching them. +The collecting combs are fitted with adjustable discharging +rods terminating in round knobs.</p> + +<p>The principle upon which the machine works will be +best understood by reference to <a href="#fig_5">Fig. 5</a>. In this diagram +the inner circle represents the front plate, with neutralizing +brushes A and B, and the outer one represents the back +plate, with brushes C and D. The sectors are shown +heavily shaded. E and F are the pairs of combs, and the +plates rotate in the direction of the arrows. Let us suppose +one of the sectors at the top of the back plate to have a +slight positive charge. As the plates rotate this sector will +come opposite to a front plate sector touched by brush A, +and it will induce a slight negative charge on the latter +sector, at the same time repelling a positive charge along +the rod to the sector touched by brush B. The two sectors +carrying the induced charges now move on until opposite +back plate sectors touched by brushes C and D, and these +back sectors will receive by induction positive and negative<span class="pagenum" id="Page_14">14</span> +charges respectively. This process continues as the plates +rotate, and finally all the sectors moving towards comb E +will be positively charged, while those approaching comb +F will be negatively charged. The combs collect these +charges, and the discharging rods K and L become highly +electrified, K positively and L negatively, and if they are +near enough together sparks will pass between them.</p> + +<figure id="fig_5" class="figcenter" style="max-width: 27em;"> + <img src="images/i_024.jpg" width="2086" height="1762" alt=" "> + <figcaption class="caption"><span class="smcap">Fig. 5.</span>—Diagram to illustrate working of a Wimshurst Machine. +</figcaption></figure> + +<p>At the commencement we supposed one of the sectors +to have a positive charge, but it is not necessary to charge +a sector specially, for the machine is self-starting. Why +this is the case is not yet thoroughly understood, but probably +the explanation is that at any particular moment no +two places in the atmosphere are in exactly the same<span class="pagenum" id="Page_15">15</span> +electro-static condition, so that an uneven state of charge +exists permanently amongst the sectors.</p> + +<p>The Wimshurst machine provides us with a plentiful +supply of electricity, and the question naturally arises, +“Can this electricity be stored up in any way?” In 1745, +long before the days of influence machines, a certain Bishop +of Pomerania, Von Kleist by name, got the idea that if he +could persuade a charge of electricity to go into a glass +bottle he would be able to capture it, because glass was a +non-conductor. So he partly filled a bottle with water, led +a wire down into the water, and while holding the bottle in +one hand connected the wire to a primitive form of electric +machine. When he thought he had got enough electricity +he tried to remove his bottle in order to examine the contents, +and in so doing he received a shock which scared +him considerably. He had succeeded in storing electricity +in his bottle. Shortly afterwards the bishop’s experiment +was repeated by Professor Muschenbrock of Leyden, and +by his pupil Cuneus, the former being so startled by the +shock that he wrote, “I would not take a second shock for +the kingdom of France.” But in spite of shocks the end +was achieved; it was proved that electricity could be collected +and stored up, and the bottle became known as the +Leyden jar. The original idea was soon improved upon, +water being replaced by a coating of tinfoil, and it was +found that better results were obtained by coating the +outside of the bottle as well as the inside.</p> + +<p>As now used the Leyden jar consists of a glass jar +covered inside and outside with tinfoil up to about two-thirds +of its height. A wooden lid is fitted, through which passes +a brass rod terminating above in a brass knob, and below +in a piece of brass chain long enough to touch the foil +lining. A Leyden jar is charged by holding it in one +hand with its knob presented to the discharging ball of a<span class="pagenum" id="Page_16">16</span> +Wimshurst machine, and even if the machine is small and +feeble the jar will accumulate electricity until it is very +highly charged. It may now be put down on the table, +and if it is clean and quite dry it will hold its charge for +some time. If the outer and inner coatings of the jar are +connected by means of a piece of metal, the electricity will +be discharged in the form of a bright spark. A Leyden +jar is usually discharged by means of discharging tongs, +consisting of a jointed brass rod with brass terminal +knobs and glass handles. One knob is placed in contact +with the outer coating of foil, and the other brought near +to the knob of the jar, which of course is connected with +the inner coating.</p> + +<p>The electrical capacity of even a small Leyden jar is +surprisingly great, and this is due to the mutual attraction +between opposite kinds of electricity. If we stick a piece +of tinfoil on the centre of each face of a pane of glass, and +charge one positively and the other negatively, the two +charges attract each other through the glass; and in fact +they hold on to each other so strongly that we can get very +little electricity by touching either piece of foil. This +mutual attraction enables us to charge the two pieces of +foil much more strongly than if they were each on a +separate pane, and this is the secret of the working of the +Leyden jar. If the knob of the jar is held to the positive +ball of a Wimshurst, the inside coating receives a positive +charge, which acts inductively on the outside coating, +attracting a negative charge to the inner face of the latter, +and repelling a positive charge to its outer face, and thence +away through the hand. The electricity is entirely confined +to the sides of the jar, the interior having no charge +whatever.</p> + +<p>Leyden jars are very often fitted to a Wimshurst +machine as shown at A, A, <a href="#fig_4">Fig. 4</a>, and arranged so that they<span class="pagenum" id="Page_17">17</span> +can be connected or disconnected to the collecting combs +as desired. When the jars are disconnected the machine +gives a rapid succession of thin sparks, but when the jars +are connected to the combs they accumulate a number of +charges before the discharge takes place, with the result +that the sparks are thicker, but occur at less frequent +intervals.</p> + +<p>It will have been noticed that the rod of a Leyden jar +and the discharging rods of a Wimshurst machine are +made to terminate not in points, but in rounded knobs or +balls. The reason of this is that electricity rapidly leaks +away from points. If we electrify a conductor shaped like +a cone with a sharp point, the density of the electricity is +greatest at that point, and when it becomes sufficiently +great the particles of air near the point become electrified +and repelled. Other particles take their place, and are +electrified and repelled in the same way, and so a constant +loss of electricity takes place. This may be shown in an +interesting way by fastening with wax a needle to the knob +of a Wimshurst. If a lighted taper is held to the point of +the needle while the machine is in action, the flame is +blown aside by the streams of repelled air, which form a +sort of electric wind.</p> +<hr class="chap x-ebookmaker-drop"> + +<div class="chapter"> +<p><span class="pagenum" id="Page_18">18</span></p> + +<h2 class="nobreak" id="toclink_18"><a id="chapter_III"></a>CHAPTER III<br> + +<span class="subhead">ELECTRICITY IN THE ATMOSPHERE</span></h2> +</div> + +<p class="in0"><span class="firstword">If</span> the Leyden jars of a Wimshurst machine are connected +up and the discharging balls placed at a suitable distance +apart, the electricity produced by rotating the plates is +discharged in the form of a brilliant zigzag spark between +the balls, accompanied by a sharp crack. The resemblance +between this spark and forked lightning is at once evident, +and in fact it is lightning in miniature. The discharging +balls are charged, as we have seen, with opposite kinds of +electricity, and these charges are constantly trying to reach +one another across the intervening air, which, being an +insulator, vigorously opposes their passage. There is thus +a kind of struggle going on between the air and the two +charges of electricity, and this keeps the air in a state of +constant strain. But the resisting power of the air is +limited, and when the charges reach a certain strength the +electricity violently forces its way across, literally rupturing +or splitting the air. The particles of air along the path of +the discharge are rendered incandescent by the heat produced +by the passage of the electricity, and so the brilliant +flash is produced. Just as a river winds about seeking the +easiest course, so the electricity takes the path of least +resistance, which probably is determined by the particles +of dust in the air, and also by the density of the air, which +becomes compressed in front, leaving less dense air and +therefore an easier path on each side.</p> + +<p><span class="pagenum" id="Page_19">19</span></p> + +<p>The connexion between lightning and the sparks from +electrified bodies and electrical machines was suspected by +many early observers, but it remained for Benjamin +Franklin to prove that lightning was simply a tremendous +electric discharge, by actually obtaining electricity from a +thunder-cloud. Franklin was an American, born at Boston +in 1706. He was a remarkable man in every way, and +quite apart from his investigations in electricity, will always +be remembered for the great public services he rendered to +his country in general and to Philadelphia in particular. +He founded the Philadelphia Library, the American Philosophical +Society, and the University of Pennsylvania.</p> + +<p>Franklin noticed many similarities between electricity +and lightning. For instance, both produced zigzag sparks, +both were conducted by metals, both set fire to inflammable +materials, and both were capable of killing animals. These +resemblances appeared to him so striking that he was +convinced that the two were the same, and he resolved to +put the matter to the test. For this purpose he hit upon +the idea of using a kite, to the top of which was fixed a +pointed wire. At the lower end of the flying string was +tied a key, insulated by a piece of silk ribbon. In June +1752, Franklin flew his kite, and after waiting a while he +was rewarded by finding that when he brought his knuckle +near to the key a little spark made its appearance. This +spark was exactly like the sparks obtained from electrified +bodies, but to make things quite certain a Leyden jar was +charged from the key. Various experiments were then +performed with the jar, and it was proved beyond all doubt +that lightning and electricity were one and the same.</p> + +<p>Lightning is then an enormous electric spark between a +cloud and the Earth, or between two clouds, produced when +opposite charges become so strong that they are able to +break down the intervening non-conducting layer of air.<span class="pagenum" id="Page_20">20</span> +The surface of the Earth is negatively electrified, the +electrification varying at different times and places; while +the electricity of the air is usually positive, but frequently +changes to negative in rainy weather and on other occasions. +As the clouds float about they collect the electricity from +the air, and thus they may be either positively or negatively +electrified, so that a discharge may take place between one +cloud and another, as well as between a cloud and the Earth.</p> + +<p>Lightning flashes take different forms, the commonest +being forked or zigzag lightning, and sheet lightning. +The zigzag form is due to the discharge taking the easiest +path, as in the case of the spark from a Wimshurst machine. +Sheet lightning is probably the reflection of a flash taking +place at a distance. It may be unaccompanied by thunder, +as in the so-called “summer lightning,” seen on the horizon +at night, which is the reflection of a storm too far off for the +thunder to be heard. A much rarer form is globular or +ball lightning, in which the discharge takes the shape of a +ball of light, which moves slowly along and finally disappears +with a sudden explosion. The cause of this form +of lightning is not yet understood, but it is possible that the +ball of light consists of intensely heated and extremely +minute fragments of ordinary matter, torn off by the +violence of the lightning discharge. Another uncommon +form is multiple lightning, which consists of a number of +separate parallel discharges having the appearance of a +ribbon.</p> + +<p>A lightning flash probably lasts from about 1/100,000 to +1/1,000,000 of a second, and in the majority of cases the +discharge is oscillatory; that is to say, it passes several times +backwards and forwards between two clouds or between a +cloud and the Earth. At times it appears as though we +could see the lightning start downwards from the cloud or +upwards from the Earth, but this is an optical illusion, and<span class="pagenum" id="Page_21">21</span> +it is really quite impossible to tell at which end the flash +starts.</p> + +<p>Death by lightning is instantaneous, and therefore +quite painless. We are apt to think that pain is felt at the +moment when a wound is inflicted. This is not the case +however, for no pain is felt until the impression reaches the +brain by way of the nerves, and this takes an appreciable +time. The nerves transmit sensations at a speed of only +about one hundred feet per second, so that in the case of a +man killed by a bullet through the brain, no pain would be +felt, because the brain would be deprived of sensibility +before the sensation could reach it. Lightning is infinitely +swifter than any bullet, so life would be destroyed by it +before any pain could be felt.</p> + +<p>On one occasion Professor Tyndall, the famous +physicist, received accidentally a very severe shock from +a large battery of Leyden jars while giving a public lecture. +His account of his sensations is very interesting. “Life +was absolutely blotted out for a very sensible interval, +without a trace of pain. In a second or so consciousness +returned; I saw myself in the presence of the audience and +apparatus, and, by the help of these external appearances, +immediately concluded that I had received the battery discharge. +The intellectual consciousness of my position was +restored with exceeding rapidity, but not so the optical +consciousness. To prevent the audience from being +alarmed, I observed that it had often been my desire to +receive accidentally such a shock, and that my wish had at +length been fulfilled. But, while making this remark, the +appearance which my body presented to myself was that of +a number of separate pieces. The arms, for example, were +detached from the trunk, and seemed suspended in the air. +In fact, memory and the power of reasoning appeared to +be complete long before the optic nerve was restored to<span class="pagenum" id="Page_22">22</span> +healthy action. But what I wish chiefly to dwell upon +here is, the absolute painlessness of the shock; and there +cannot be a doubt that, to a person struck dead by lightning, +the passage from life to death occurs without consciousness +being in the least degree implicated. It is an abrupt +stoppage of sensation, unaccompanied by a pang.”</p> + +<p>Occasionally branched markings are found on the +bodies of those struck by lightning, and these are often +taken to be photographic impressions of trees under which +the persons may have been standing at the time of the +flash. The markings however are nothing of the kind, +but are merely physiological effects due to the passage of +the discharge.</p> + +<p>During a thunderstorm it is safer to be in the house +than out in the open. It is probable that draughts are a +source of some danger, and the windows and doors of the +room ought to be shut. Animals are more liable to be +struck by lightning than men, and a shed containing +horses or cows is a dangerous place in which to take +shelter; in fact it is better to remain in the open. If one +is caught in a storm while out of reach of a house or other +building free from draughts and containing no animals, +the safest plan is to lie down, not minding the rain. +Umbrellas are distinctly dangerous, and never should be +used during a storm. Wire fences, hedges, and still or +running water should be given a wide berth, and it is +safer to be alone than in company with a crowd of people. +It is extremely foolish to take shelter under an isolated +tree, for such trees are very liable to be struck. Isolated +beech trees appear to have considerable immunity from +lightning, but any tree standing alone should be avoided, +the oak being particularly dangerous. On the other hand, +a fairly thick wood is comparatively safe, and failing a +house, should be chosen before all other places of refuge.<span class="pagenum" id="Page_23">23</span> +Horses are liable to be struck, and if a storm comes on +while one is out driving it is safer to keep quite clear of the +animals.</p> + +<p>When a Wimshurst machine has been in action for a +little time a peculiar odour is noticed. This is due to the +formation of a modified and chemically more active form of +oxygen, called <em>ozone</em>, the name being derived from the +Greek <em>ozein</em>, “to smell.” Ozone has very invigorating effects +when breathed, and it is also a powerful germicide, capable +of killing the germs which give rise to contagious diseases. +During a thunderstorm ozone is produced in large +quantities by the electric discharges, and thus the air +receives as it were a new lease of life, and we feel the +refreshing effects when the storm is over. We shall speak +again of ozone in <a href="#chapter_XXV">Chapter XXV</a>.</p> + +<p>Thunder probably is caused by the heating and sudden +expansion of the air in the path of the discharge, which +creates a partial vacuum into which the surrounding air +rushes violently. Light travels at the rate of 186,000 +miles per second, and therefore the flash reaches us +practically instantaneously; but sound travels at the rate of +only about 1115 feet per second, so that the thunder takes +an appreciable time to reach us, and the farther away the +discharge the greater the interval between the flash and +the thunder. Thus by multiplying the number of seconds +which elapse between the flash and the thunder by 1115, +we may calculate roughly the distance in feet of the +discharge. A lightning flash may be several miles in +length, the greatest recorded length being about ten miles. +The sounds produced at different points along its path +reach us at different times, producing the familiar sharp +rattle, and the following rolling and rumbling is produced +by the echoes from other clouds. The noise of a thunder-clap +is so tremendous that it seems as though the sound<span class="pagenum" id="Page_24">24</span> +would be heard far and wide, but the greatest distance at +which thunder has been heard is about fifteen miles. In +this respect it is interesting to compare the loudest +thunder-clap we ever heard with the noise of the famous +eruption of Krakatoa, in 1883, which was heard at the +enormous distance of nearly three thousand miles.</p> + +<p>When Franklin had demonstrated the nature of +lightning, he began to consider the possibility of protecting +buildings from the disastrous effects of the lightning stroke. +At that time the amount of damage caused by lightning +was very great. Cathedrals, churches, public buildings, +and in fact all tall edifices were in danger every time a +severe thunderstorm took place in their neighbourhood, for +there was absolutely nothing to prevent their destruction if +the lightning chanced to strike them. Ships at sea, too, +were damaged very frequently by lightning, and often +some of the crew were killed or disabled. To-day, thanks +to the lightning conductor, it is an unusual occurrence for +ships or large buildings to be damaged by lightning. The +lightning strikes them as before, but in the great majority +of cases it is led away harmlessly to earth.</p> + +<p>Franklin was the first to suggest the possibility of +protecting buildings by means of a rod of some conducting +material terminating in a point at the highest part of the +building, and leading down, outside the building, into the +earth. Lightning conductors at the present day are +similar to Franklin’s rod, but many improvements have +been made from time to time as our knowledge of the +nature and action of the lightning discharge has increased. +A modern lightning conductor generally consists of one or +more pointed rods fixed to the highest parts of the building, +and connected to a cable running directly to earth. This +cable is kept as straight as possible, because turns and +bends offer a very high resistance to the rapidly oscillating<span class="pagenum" id="Page_25">25</span> +discharge; and it is connected to large copper plates +buried in permanently moist ground or in water, or to +water or gas mains. Copper is generally used for the +cable, but iron also may be employed. In any case, the +cable must be of sufficient thickness to prevent the +possibility of its being deflagrated by the discharge. In +ships the arrangements are similar, except that the cable is +connected to the copper sheathing of the bottom.</p> + +<p>The fixing of lightning conductors must be carried out +with great care, for an improperly fixed conductor is not +only useless, but may be a source of actual danger. +Lightning flashes vary greatly in character, and while a +carefully erected lightning conductor is capable of dealing +with most of them, there are unfortunately certain kinds of +discharge with which it now and then is unable to deal. +The only absolutely certain way of protecting a building is +to surround it completely by a sort of cage of metal, but +except for buildings in which explosives are stored this +plan is usually impracticable.</p> + +<p>The electricity of the atmosphere manifests itself in +other forms beside the lightning. The most remarkable +of these manifestations is the beautiful phenomenon known +in the Northern Hemisphere as the Aurora Borealis, and +in the Southern Hemisphere as the Aurora Australis. +Aurora means the morning hour or dawn, and the phenomenon +is so called from its resemblance to the dawn of +day. The aurora is seen in its full glory only in high +latitudes, and it is quite unknown at the equator. It +assumes various forms, sometimes appearing as an arch of +light with rapidly moving streamers of different colours, +and sometimes taking the form of a luminous curtain +extending across the sky. The light of the aurora is never +very strong, and as a rule stars can be seen through it. +Auroras are sometimes accompanied by rustling or crackling<span class="pagenum" id="Page_26">26</span> +sounds, but the sounds are always extremely faint. +Some authorities assert that these sounds do not exist, and +that they are the result of imagination, but other equally +reliable observers have heard the sounds quite plainly on +several occasions. Probably the explanation of this confliction +of evidence is that the great majority of auroras are +silent, so that an observer might witness many of them +without hearing any sounds. The height at which auroras +occur is a disputed point, and one which it is difficult to +determine accurately; but most observers agree that it +is generally from 60 to 125 miles above the Earth’s +surface.</p> + +<p>There is little doubt that the aurora is caused by the +passage of electric discharges through the higher regions +of the atmosphere, where the air is so rarefied as to act as +a partial conductor; and its effects can be imitated in some +degree by passing powerful discharges through tubes from +which the air has been exhausted to a partial vacuum. +Auroral displays are usually accompanied by magnetic +disturbances, which sometimes completely upset telegraphic +communication. Auroras and magnetic storms appear to +be connected in some way with solar disturbances, for they +are frequently simultaneous with an unusual number of +sunspots, and all three run in cycles of about eleven and a +half years.</p> +<hr class="chap x-ebookmaker-drop"> + +<div class="chapter"> +<p><span class="pagenum" id="Page_27">27</span></p> + +<h2 class="nobreak" id="toclink_27"><a id="chapter_IV"></a>CHAPTER IV<br> + +<span class="subhead">THE ELECTRIC CURRENT</span></h2> +</div> + +<p class="in0"><span class="firstword">In</span> the previous chapters we have dealt with electricity in +charged bodies, or static electricity, and now we must turn +to electricity in motion, or current electricity. In <a href="#chapter_I">Chapter I</a>. +we saw that if a metal rod is held in the hand and rubbed, +electricity is produced, but it immediately escapes along the +rod to the hand, and so to the earth. In other words, the +electricity flows away along the conducting path provided +by the rod and the hand. When we see the word “flow” we +at once think of a fluid of some kind, and we often hear +people speak of the “electric fluid.” Now, whatever +electricity may be it certainly is not a fluid, and we use +the word “flow” in connexion with electricity simply because +it is the most convenient word we can find for the purpose. +Just in the same way we might say that when we hold a +poker with its point in the fire, heat flows along it towards +our hand, although we know quite well that heat is not a +fluid. In the experiment with the metal rod referred to +above, the electricity flows away instantly, leaving the rod +unelectrified; but if we arrange matters so that the +electricity is renewed as fast as it flows away, then we get +a continuous flow, or current.</p> + +<p>Somewhere about the year 1780 an Italian anatomist, +Luigi Galvani, was studying the effects of electricity upon +animal organisms, using for the purpose the legs of freshly +killed frogs. In the course of his experiments he happened +to hang against an iron window rail a bundle of frogs’ legs<span class="pagenum" id="Page_28">28</span> +fastened together with a piece of copper wire, and he +noticed that the legs began to twitch in a peculiar manner. +He knew that a frog’s leg would twitch when electricity +was applied to it, and he concluded that the twitchings in +this case were caused in the same way. So far he was +quite right, but then came the problem of how any +electricity could be produced in these circumstances, and +here he went astray. It +never occurred to him that +the source of the electricity +might be found in something +quite apart from the legs, +and so he came to the conclusion +that the phenomenon +was due to electricity produced +in some mysterious +way in the tissues of the +animal itself. He therefore +announced that he had discovered +the existence of a +kind of animal electricity, +and it was left for his fellow-countryman, +Alessandro +Volta, to prove that the +twitchings were due to electricity +produced by the contact +of the two metals, the iron of the window rail and the +copper wire.</p> + +<figure id="fig_6" class="figleft" style="max-width: 14em;"> + <img src="images/i_038.jpg" width="1092" height="1523" alt=" "> + <figcaption class="caption"><span class="smcap">Fig. 6.</span>—Voltaic Pile. +</figcaption></figure> + +<p>Volta found that when two different metals were placed in +contact in air, one became positively charged, and the other +negatively. These charges however were extremely feeble, +and in his endeavours to obtain stronger results he hit upon +the idea of using a number of pairs of metals, and he constructed +the apparatus known as the Voltaic pile, <a href="#fig_6">Fig. 6</a>.<span class="pagenum" id="Page_29">29</span> +This consists of a number of pairs of zinc and copper +discs, each pair being separated from the next pair by a +disc of cloth moistened with salt water. These are piled +up and placed in a frame, as shown in the figure. One +end of the pile thus terminates in a zinc disc, and the other +in a copper disc, and as soon as the two are connected by +a wire or other conductor a continuous current of electricity +is produced. The cause of the electricity produced by the +voltaic pile was the subject of +a long and heated controversy. +There were two main theories; +that of Volta himself, which +attributed the electricity to the +mere contact of unlike metals, +and the chemical theory, which +ascribed it to chemical action. +The chemical theory is now +generally accepted, but certain +points, into which we need not +enter, are still in dispute.</p> + +<p>There is a curious experiment +which some of my readers +may like to try. Place a copper +coin on a sheet of zinc, and set an +ordinary garden snail to crawl +across the zinc towards the coin. As soon as the snail +comes in contact with the copper it shrinks back, and shows +every sign of having received a shock. One can well +imagine that an enthusiastic gardener pestered with snails +would watch this experiment with great glee.</p> + +<figure id="fig_7" class="figright" style="max-width: 11em;"> + <img src="images/i_039.jpg" width="830" height="1338" alt=" "> + <figcaption class="caption"><span class="smcap">Fig. 7.</span>—Simple Voltaic Cell. +</figcaption></figure> + +<p>Volta soon found that it was not necessary to have his +pairs of metals in actual metallic contact, and that better +results were got by placing them in a vessel filled with +dilute acid. <a href="#fig_7">Fig. 7</a> is a diagram of a simple voltaic cell of<span class="pagenum" id="Page_30">30</span> +this kind, and it shows the direction of the current when +the zinc and the copper are connected by the wire. In +order to get some idea of the reason why a current flows +we must understand the meaning of electric potential. If +water is poured into a vessel, a certain water pressure is +produced. The amount of this pressure depends upon the +level of the water, and this in turn depends upon the +quantity of water and the capacity of the vessel, for a +given quantity of water will reach a higher level in a small +vessel than in a larger one. In the same way, if electricity +is imparted to a conductor an electric pressure is +produced, its amount depending upon the quantity of +electricity and the electric capacity of the conductor, for +conductors vary in capacity just as water vessels do.</p> + +<p>This electric pressure is called “potential,” and electricity +tends to flow from a conductor of higher to one of lower +potential. When we say that a place is so many feet +above or below sea-level we are using the level of the sea +as a zero level, and in estimating electric potential we take +the potential of the earth’s surface as zero; and we regard +a positively electrified body as one at a positive or relatively +high potential, and a negatively electrified body as +one at a negative or relatively low potential. This may be +clearer if we think of temperature and the thermometer. +Temperatures above zero are positive and represented by +the sign +, and those below zero are negative and represented +by the sign -. Thus we assume that an electric +current flows from a positive to a negative conductor.</p> + +<figure id="plate_I" class="figcenter" style="max-width: 40em;"> + <p class="caption">PLATE I.</p> + <img src="images/i_041.jpg" width="3173" height="2031" alt=" "> + <figcaption class="caption"> + +<p class="floatl"><i>By permission of</i></p> +<p class="floatr"><i>Dick, Kerr & Co. Ltd.</i></p> + +<p class="floatc">HYDRO-ELECTRIC POWER STATION.</p> +</figcaption></figure> + +<p>In a voltaic cell the plates are at different potentials, so +that when they are connected by a wire a current flows, +and we say that the current leaves the cell at the positive +terminal, and enters it again at the negative terminal. As +shown in <a href="#fig_7">Fig. 7</a>, the current moves in opposite directions +inside and outside the cell, making a complete round called +a <em>circuit</em>, and if the circuit is broken anywhere the current +ceases to flow. If the circuit is complete the current keeps +on flowing, trying to equalize the electric pressure or +potential, but it is unable to do this because the chemical +action between the acid and the zinc maintains the difference +of potential between the plates. This chemical action +results in wasting of the zinc and weakening of the acid, +and as long as it continues the current keeps on flowing. +When we wish to stop the current we break the circuit by +disconnecting the wire joining the terminals, and the cell +then should be at rest; but owing to the impurities in +ordinary commercial zinc chemical action still continues. +In order to prevent wasting when the current is not required +the surface of the zinc is coated with a thin film of +mercury. The zinc is then said to be amalgamated, and +it is not acted upon by the acid so long as the circuit +remains broken.</p> + +<p>The current from a simple voltaic cell does not remain +at a constant strength, but after a short time it begins to +weaken rapidly. The cell is then said to be polarized, and +this polarization is caused by bubbles of hydrogen gas +which accumulate on the surface of the copper plate during +the chemical action. These bubbles of gas weaken the +current partly by resisting its flow, for they are bad conductors, +and still more by trying to set up another current +in the opposite direction. For this reason the simple +voltaic cell is unsuitable for long spells of work, and many +cells have been devised to avoid the polarization trouble. +One of the most successful of these is the Daniell cell. It +consists of an outer vessel of copper, which serves as the +copper plate, and an inner porous pot containing a zinc +rod. Dilute sulphuric acid is put into the porous pot and +a strong solution of copper sulphate into the outer jar. +When the circuit is closed, the hydrogen liberated by the<span class="pagenum" id="Page_32">32</span> +action of the zinc on the acid passes through the porous +pot, and splits up the copper sulphate into copper and +sulphuric acid. In this way pure copper, instead of +hydrogen, is deposited on the copper plate, no polarization +takes place, and the current is constant.</p> + +<p>Other cells have different combinations of metals, such +as silver-zinc, or platinum-zinc, and carbon is also largely +used in place of one metal, as in the familiar carbon-zinc +Leclanché cell, used for ringing electric bells. This cell +consists of an inner porous pot containing a carbon plate +packed round with a mixture of crushed carbon and manganese +dioxide, and an outer glass jar containing a zinc +rod and a solution of sal-ammoniac. Polarization is +checked by the oxygen in the manganese dioxide, which +seizes the hydrogen on its way to the carbon plate, and +combines with it. If the cell is used continuously however +this action cannot keep pace with the rate at which the +hydrogen is produced, and so the cell becomes polarized; +but it soon recovers after a short rest.</p> + +<p>The so-called “dry” cells so much used at the present +time are not really dry at all; if they were they would give +no current. They are in fact Leclanché cells, in which +the containing vessel is made of zinc to take the place of a +zinc rod; and they are dry only in the sense that the liquid +is taken up by an absorbent material, so as to form a moist +paste. Dry cells are placed inside closely fitting cardboard +tubes, and are sealed up at the top. Their chief advantage +lies in their portability, for as there is no free liquid to +spill they can be carried about and placed in any position.</p> + +<p>We have seen that the continuance of the current from +a voltaic cell depends upon the keeping up of a difference +of potential between the plates. The force which serves +to maintain this difference is called the electro-motive force, +and it is measured in volts. The actual flow of electricity<span class="pagenum" id="Page_33">33</span> +is measured in amperes. Probably all my readers are +familiar with the terms volt and ampere, but perhaps some +may not be quite clear about the distinction between the +two. When water flows along a pipe we know that it is +being forced to do so by pressure resulting from a difference +of level. That is to say, a difference of level produces +a water-moving or water-motive force; and in a +similar way a difference of potential produces an electricity-moving +or electro-motive force, which is measured +in volts. If we wish to describe the rate of flow of water +we state it in gallons per second, and the rate of flow of +electricity is stated in amperes. Volts thus represent the +pressure at which a current is supplied, while the current +itself is measured in amperes.</p> + +<p>We may take this opportunity of speaking of electric +resistance. A current of water flowing through a pipe is +resisted by friction against the inner surface of the pipe; +and a current of electricity flowing through a circuit also +meets with a resistance, though this is not due to friction. +In a good conductor this resistance is small, but in a bad +conductor or non-conductor it is very great. The resistance +also depends upon length and area of cross-section; so that +a long wire offers more resistance than a short one, and a +thin wire more than a thick one. Before any current can +flow in a circuit the electro-motive force must overcome +the resistance, and we might say that the volts drive the +amperes through the resistance. The unit of resistance is +the ohm, and the definition of a volt is that electro-motive +force which will cause a current of one ampere to flow +through a conductor having a resistance of one ohm. These +units of measurement are named after three famous scientists, +Volta, Ampère, and Ohm.</p> + +<figure id="fig_8" class="figcenter" style="max-width: 26em;"> + <img src="images/i_046.jpg" width="2023" height="685" alt=" "> + <figcaption class="caption"><span class="smcap">Fig. 8.</span>—Cells connected in Parallel. +</figcaption></figure> + +<p>A number of cells coupled together form a battery, and +different methods of coupling are used to get different<span class="pagenum" id="Page_34">34</span> +results. In addition to the resistance of the circuit outside +the cell, the cell itself offers an internal resistance, and part +of the electro-motive force is used up in overcoming this +resistance. If we can decrease this internal resistance we +shall have a larger current at our disposal, and one way of +doing this is to increase the size of the plates. This of +course means making the cell larger, and very large cells +take up a lot of room and are troublesome to move about. +We can get the same effect however by coupling. If we +connect together all the positive terminals and all the +negative terminals of several cells, that is, copper to copper +and zinc to zinc in Daniell cells, we get the same result as +if we had one very large cell. The current is much larger, +but the electro-motive force remains the same as if only +one cell were used, or in other words we have more amperes +but no more volts. This is called connecting in “parallel,” +and the method is shown in <a href="#fig_8">Fig. 8</a>. On the other hand, +if, as is usually the case, we want a larger electro-motive +force, we connect the positive terminal of one cell to the +negative terminal of the next, or copper to zinc all through. +In this way we add together the electro-motive forces of all +the cells, but the amount of current remains that of a single +cell; that is, we get more volts but no more amperes. This is +called connecting in “series,” and the arrangement is shown<span class="pagenum" id="Page_35">35</span> +in <a href="#fig_9">Fig. 9</a>. We can also increase both volts and amperes by +combining the two methods.</p> + +<figure id="fig_9" class="figcenter" style="max-width: 24em;"> + <img src="images/i_047.png" width="1879" height="835" alt=" "> + <figcaption class="caption"><span class="smcap">Fig. 9.</span>—Cells connected in Series. +</figcaption></figure> + +<p>A voltaic cell gives us a considerable quantity of +electricity at low pressure, the electro-motive force of a +Leclanché cell being about 1½ volts, and that of a Daniell +cell about 1 volt. We may perhaps get some idea of the +electrical conditions existing during a thunderstorm from +the fact that to produce a spark one mile long through air +at ordinary pressure we should require a battery of more +than a thousand million Daniell cells. Cells such as we +have described in this chapter are called primary cells, as +distinguished from accumulators, which are called secondary +cells. Some of the practical applications of primary cells +will be described in later chapters.</p> + +<p>Besides the voltaic cell, in which the current is produced +by chemical action, there is the thermo-electric battery, or +thermopile, which produces current directly from heat +energy. About 1822 Seebeck was experimenting with +voltaic pairs of metals, and he found that a current could +be produced in a complete metallic circuit consisting of +different metals joined together, by keeping these joinings +at different temperatures. <a href="#fig_10">Fig. 10</a> shows a simple arrangement +for demonstrating this effect, which is known as the<span class="pagenum" id="Page_36">36</span> +“Seebeck effect.” A slab of bismuth, BB, has placed upon it +a bent strip of copper, C. If one of the junctions of the +two metals is heated as shown, a current flows; and the +same effect is produced +by cooling one of the +junctions. This current +continues to flow +as long as the two junctions +are kept at different +temperatures. In +1834 another scientist, +Peltier, discovered that +if a current was passed +across a junction of two different metals, this junction was +either heated or cooled, according to the direction in which +the current flowed. In <a href="#fig_10">Fig. 10</a> the current across the +heated junction tends to cool the junction, while the Bunsen +burner opposes this cooling, and keeps up the temperature. +A certain amount of the heat energy is thus transformed +into electrical +energy. At the +other junction +the current +produces a +heating effect, +so that some of +the electrical +energy is retransformed +into heat.</p> + +<figure id="fig_10" class="figleft" style="max-width: 16em;"> + <img src="images/i_048.png" width="1266" height="711" alt=" "> + <figcaption class="caption"><span class="smcap">Fig. 10.</span>—Diagram to illustrate the +Seebeck effect. +</figcaption></figure> + +<figure id="fig_11" class="figright" style="max-width: 20em;"> + <img src="images/i_048b.png" width="1534" height="684" alt=" "> + <figcaption class="caption"><span class="smcap">Fig. 11.</span>—Diagram to show arrangement of two +different metals in Thermopile. +</figcaption></figure> + +<p>A thermopile consists of a number of alternate bars or +strips of two unlike metals, joined together as shown +diagrammatically in <a href="#fig_11">Fig. 11</a>. The arrangement is such +that the odd junctions are at one side, and the even ones<span class="pagenum" id="Page_37">37</span> +at the other. The odd junctions are heated, and the even +ones cooled, and a current flows when the circuit is completed. +By using a larger number of junctions, and by +increasing the difference of temperature between them, the +voltage of the current may be increased. Thermopiles are +nothing like so efficient as voltaic cells, and they are more +costly. They are used to a limited extent for purposes +requiring a very small and constant current, but for +generating considerable quantities of current at high +pressure they are quite useless. The only really important +practical use of the thermopile is in the detection and +measurement of very minute differences of temperature, +which are beyond the capabilities of the ordinary thermometer. +Within certain limits, the electro-motive force of a +thermopile is exactly proportionate to the difference of +temperature. The very slightest difference of temperature +produces a current, and by connecting the wires from a +specially constructed thermopile to a delicate instrument +for measuring the strength of the current, temperature +differences of less than one-millionth of a degree can be +detected.</p> +<hr class="chap x-ebookmaker-drop"> + +<div class="chapter"> +<p><span class="pagenum" id="Page_38">38</span></p> + +<h2 class="nobreak" id="toclink_38"><a id="chapter_V"></a>CHAPTER V<br> + +<span class="subhead">THE ACCUMULATOR</span></h2> +</div> + +<p class="in0"><span class="firstword">If</span> we had two large water tanks, one of which could be +emptied only by allowing the bottom to fall completely out, +and the other by means of a narrow pipe, it is easy to see +which would be the more useful to us as a source of water +supply. If both tanks were filled, then from the first we +could get only a sudden uncontrollable rush of water, but +from the other we could get a steady stream extending over +a long period, and easily controlled. The Leyden jar stores +electricity, but in yielding up its store it acts like the first +tank, giving a sudden discharge in the form of a bright +spark. We cannot control the discharge, and therefore we +cannot make it do useful work for us. For practical +purposes we require a storing arrangement that will act like +the second tank, giving us a steady current of electricity +for a long period, and this we have in the accumulator or +storage cell.</p> + +<p>A current of electricity has the power of decomposing +certain liquids. If we pass a current through water, the +water is split up into its two constituent gases, hydrogen +and oxygen, and this may be shown by the apparatus seen +in <a href="#fig_12">Fig. 12</a>. It consists of a glass vessel with two strips of +platinum to which the current is led. The vessel contains +water to which has been added a little sulphuric acid to +increase its conducting power, and over the strips are inverted +two test-tubes filled with the acidulated water. The<span class="pagenum" id="Page_39">39</span> +platinum strips, which are called <em>electrodes</em>, are connected +to a battery of Daniell cells. When the current passes, +the water is decomposed, and oxygen collects at the electrode +connected to the positive terminal of the battery, and +hydrogen at the other electrode. The two gases rise up +into the test-tubes and displace the water in them, and the +whole process is called the electrolysis of water. If now +we disconnect the battery and join the two electrodes by +a wire, we find that a current flows from the apparatus +as from a voltaic cell, but +in the opposite direction +from the original battery +current.</p> + +<figure id="fig_12" class="figright" style="max-width: 14em;"> + <img src="images/i_051.png" width="1070" height="1191" alt=" "> + <figcaption class="caption"><span class="smcap">Fig. 12.</span>—Diagram showing Electrolysis +of Water. +</figcaption></figure> + +<p>It will be remembered +that one of the troubles +with a simple voltaic cell +was polarization, caused +by the accumulation of +hydrogen; and that this +weakened the current by +setting up an opposing +electro-motive force tending +to produce another +current in the opposite +direction. In the present +case a similar opposing or back electro-motive force is +produced, and as soon as the battery current is stopped +and the electrodes are connected, we get a current in the +reverse direction, and this current continues to flow until +the two gases have recombined, and the electrodes have +regained their original condition. Consequently we can +see that in order to electrolyze water, our battery must +have an electro-motive force greater than that set up in +opposition to it, and at least two Daniell cells are required.</p> + +<p><span class="pagenum" id="Page_40">40</span></p> + +<p>This apparatus thus may be made to serve to some +extent as an accumulator or storage cell, and it also serves +to show that an accumulator does not store up or accumulate +electricity. In a voltaic cell we have chemical energy +converted into electrical energy, and here we have first +electrical energy converted into chemical energy, and then +the chemical energy converted back again into electrical +energy. This is a rough-and-ready way of putting the +matter, but it is good enough for practical purposes, and at +any rate it makes it quite clear that what an accumulator +really stores up is not electricity, but energy, which is given +out in the form of electricity.</p> + +<p>The apparatus just described is of little use as a source +of current, and the first really practical accumulator was +made in 1878 by Gaston Planté. The electrodes were two +strips of sheet lead placed one upon the other, but separated +by some insulating material, and made into a roll. This +roll was placed in dilute sulphuric acid, and one strip or +plate connected to the positive, and the other to the +negative terminal of the source of current. The current +was passed for a certain length of time, and then the accumulator +partly discharged; after which current was passed +again, but in the reverse direction, followed by another period +of discharge. This process, which is called <em>forming</em>, +was continued for several days, and its effect was to change +one plate into a spongy condition, and to form a coating +of peroxide of lead on the other. When the plates were +properly formed the accumulator was ready to be fully +charged and put into use. The effect of charging was to +rob one plate of its oxygen, and to transfer this oxygen to +the other plate, which thus received an overcharge of the +gas. During the discharge of the accumulator the excess +of oxygen went back to the place from which it had been +taken, and the current continued until the surfaces of both<span class="pagenum" id="Page_41">41</span> +plates were reduced to a chemically inactive state. The +accumulator could be charged and discharged over and +over again as long as the plates remained in good order.</p> + +<p>In 1881, Faure hit upon the idea of coating the plates +with a paste of red-lead, and this greatly shortened the +time of forming. At first it was found difficult to make the +paste stick to the plates, but this trouble was got rid of by +making the plates in the form of grids, and pressing the +paste into the perforations. Many further improvements +have been made from time to time, but instead of tracing +these we will go on at once to the description of a present-day +accumulator. There are now many excellent accumulators +made, but we have not space to consider more than +one, and we will select that known as the “Chloride” +accumulator.</p> + +<p>The positive plate of this accumulator is of the Planté +type, but it is not simply a casting of pure lead, but is made +by a building-up process which allows of the use of a +lead-antimony mixture for the grids. This gives greater +strength, and the grids themselves are unaffected by the +chemical changes which take place during the charging and +discharging of the cell. The active material, that is the +material which undergoes chemical change, is pure lead +tape coiled up into rosettes, which are so designed that the +acid can circulate through the plates. These rosettes are +driven into the perforations of the grid by a hydraulic press, +and during the process of forming they expand and thus +become very firmly fixed. The negative plate has a frame +made in two parts, which are riveted together after the +insertion of the active material, which is thus contained in +a number of small cages. The plate is covered outside +with a finely perforated sheet of lead, which prevents the +active material from falling out. It is of the utmost +importance that the positive and negative plates should be<span class="pagenum" id="Page_42">42</span> +kept apart when in the cell, and in the Chloride accumulator +this is ensured by the use of a patent separator made of +a thin sheet of wood the size of the plates. Before being +used the wood undergoes a special treatment to remove all +substances which might be harmful, and it then remains +unchanged either in appearance or composition. Other +insulating substances, such as glass rods or ebonite forks, +can be used as separators, but it is claimed that the wood +separator is not only more satisfactory, but that in some +unexplained way it actually helps to keep up the capacity +of the cell. The plates are placed in glass, or lead-lined +wood or metal boxes, and are suspended from above the +dilute sulphuric acid with which the cells are filled. A +space is left below the plates for the sediment which +accumulates during the working of the cell.</p> + +<p>In all but the smallest cells several pairs of plates are +used, all the positive plates being connected together and +all the negative plates. This gives the same effect as two +very large plates, on the principle of connecting in parallel, +spoken of in <a href="#chapter_IV">Chapter IV</a>. A single cell, of whatever size, +gives current at about two volts, and to get higher voltages +many cells are connected in series, as with primary cells. +The capacity is generally measured in ampere-hours. For +instance, an accumulator that will give a current of eight +amperes for one hour, or of four amperes for two hours, or +one ampere for eight hours, is said to have a capacity of +eight ampere-hours.</p> + +<p>Accumulators are usually charged from a dynamo or +from the public mains, and the electro-motive force of +the charging current must be not less than 2½ volts for +each cell, in order to overcome the back electro-motive +force of the cells themselves. It is possible to charge +accumulators from primary cells, but except on a very +small scale the process is comparatively expensive. Non-polarizing<span class="pagenum" id="Page_43">43</span> +cells, such as the Daniell, must be used for this +purpose.</p> + +<p>The practical applications of accumulators are almost +innumerable, and year by year they increase. As the most +important of these are connected with the use of electricity +for power and light, it will be more convenient to speak of +them in the chapters dealing with this subject. Minor +uses of accumulators will be referred to briefly from time +to time in other chapters.</p> +<hr class="chap x-ebookmaker-drop"> + +<div class="chapter"> +<p><span class="pagenum" id="Page_44">44</span></p> + +<h2 class="nobreak" id="toclink_44"><a id="chapter_VI"></a>CHAPTER VI<br> + +<span class="subhead">MAGNETS AND MAGNETISM</span></h2> +</div> + +<p class="in0"><span class="firstword">In</span> many parts of the world there is to be found a kind of +iron ore, some specimens of which have the peculiar power +of attracting iron, and of turning to the north if suspended +freely. This is called the <em>lodestone</em>, and it has been +known from very remote times. The name Magnetism has +been given to this strange property of the lodestone, but the +origin of the name is not definitely known. There is an +old story about a shepherd named Magnes, who lived in +Phrygia in Asia Minor. One day, while tending his sheep +on Mount Ida, he happened to touch a dark coloured rock +with the iron end of his crook, and he was astonished and +alarmed to find that the rock was apparently alive, for it +gripped his crook so firmly that he could not pull it away. +This rock is said to have been a mass of lodestone, and +some people believe that the name magnet comes from the +shepherd Magnes. Others think that the name is derived +from Magnesia, in Asia Minor, where the lodestone was +found in large quantities; while a third theory finds the +origin in the Latin word <i lang="la">magnus</i>, heavy, on account of the +heavy nature of the lodestone. The word lodestone itself +comes from the Saxon <i lang="osx">laeden</i>, meaning to lead.</p> + +<p>It is fairly certain that the Chinese knew of the lodestone +long before Greek and Roman times, and according +to ancient Chinese records this knowledge extends as far +back as 2600 <span class="allsmcap">B.C.</span> Humboldt, in his <cite>Cosmos</cite>, states that a<span class="pagenum" id="Page_45">45</span> +miniature figure of a man which always turned to the south +was used by the Chinese to guide their caravans across the +plains of Tartary as early as 1000 <span class="allsmcap">B.C.</span> The ancient Greek +and Roman writers frequently refer to the lodestone. +Thales, of whom we spoke in <a href="#chapter_I">Chapter I</a>., believed that its +mysterious power was due to the possession of a soul, and +the Roman poet Claudian imagined that iron was a food +for which the lodestone was hungry. Our limited space +will not allow of an account of the many curious speculations +to which the lodestone has given rise, but the following +suggestion of one Famianus Strada, quoted from +Houston’s <cite>Electricity in Every-Day Life</cite>, is really too +good to be omitted.</p> + +<p>“Let there be two needles provided of an equal Length +and Bigness, being both of them touched by the same +lodestone; let the Letters of the Alphabet be placed on the +Circles on which they are moved, as the Points of the +Compass under the needle of the Mariner’s Chart. Let +the Friend that is to travel take one of these with him, first +agreeing upon the Days and Hours wherein they should +confer together; at which times, if one of them move the +Needle, the other Needle, by Sympathy, will move unto +the same letter in the other instantly, though they are +never so far distant; and thus, by several Motions of the +Needle to the Letters, they may easily make up any Words +or Sense which they have a mind to express.” This is +wireless telegraphy in good earnest!</p> + +<p>The lodestone is a natural magnet. If we rub a piece +of steel with a lodestone we find that it acquires the same +properties as the latter, and in this way we are able to +make any number of magnets, for the lodestone does not +lose any of its own magnetism in the process. Such +magnets are called artificial magnets. Iron is easier to +magnetize than steel, but it soon loses its magnetism,<span class="pagenum" id="Page_46">46</span> +whereas steel retains it; and the harder the steel the better +it keeps its magnetism. Artificial magnets, therefore, are +made of specially hardened steel. In this chapter we shall +refer only to steel magnets, as they are much more convenient +to use than the lodestone, but it should be +remembered that both act in exactly the same way. We +will suppose that we have a pair of bar magnets, and a +horse-shoe magnet, as shown in <a href="#fig_13">Fig. 13</a>.</p> + +<figure id="fig_13" class="figleft" style="max-width: 16em;"> + <img src="images/i_058.png" width="1272" height="1120" alt=" "> + <figcaption class="caption"><span class="smcap">Fig. 13.</span>—Horse-shoe and Bar Magnets, +with Keepers. +</figcaption></figure> + +<p>If we roll a bar magnet amongst iron filings we find +that the filings remain +clinging to it in two +tufts, one at each +end, and that few or +none adhere to the +middle. These two +points towards which +the filings are attracted +are called the +poles of the magnet. +Each pole attracts +filings or ordinary +needles, and one or +two experiments will +show that the attraction +becomes evident while the magnet is still some little +distance away. If, however, we test our magnet with other +substances, such as wood, glass, paper, brass, etc., we see +that there is no attraction whatever.</p> + +<p>If one of our bar magnets is suspended in a sort of +stirrup of copper wire attached to a thread, it comes to rest +in a north and south direction, and it will be noticed that +the end which points to the north is marked, either with a +letter N or in some other way. This is the north pole of +the magnet, and of course the other is the south pole. If<span class="pagenum" id="Page_47">47</span> +now we take our other magnet and bring its north pole +near each pole of the suspended magnet in turn, we find +that it repels the other north pole, but attracts the south +pole. Similarly, if we present the south pole, it repels the +other south pole, but attracts the north pole. From these +experiments we learn that both poles of a magnet attract +filings or needles, and that in the case of two magnets +unlike poles attract, but similar poles repel one another. +It will be noticed that this corresponds closely with the +results of our experiments in <a href="#chapter_I">Chapter I</a>., which showed +that an electrified body attracts unelectrified bodies, such +as bits of paper or pith balls, and that unlike charges +attract, and similar charges repel each other. So far as we +have seen, however, a magnet attracts only iron or steel, +whereas an electrified body attracts any light substance. +As a matter of fact, certain other substances, such as nickel +and cobalt, are attracted by a magnet, but not so readily as +iron and steel; while bismuth, antimony, phosphorus, and +a few other substances are feebly repelled.</p> + +<p>The simplest method of magnetizing a piece of steel by +means of one of our bar magnets is the following: Lay the +steel on the table, and draw one pole of the magnet along +it from end to end; lift the magnet clear of the steel, and +repeat the process several times, always starting at the +same end and treating each surface of the steel in turn. A +thin, flat bar of steel is the best for the purpose, but steel +knitting needles may be made in this way into useful +experimental magnets.</p> + +<p>We have seen that a magnet has two poles or points +where the magnetism is strongest. It might be thought +that by breaking a bar magnet in the middle we should get +two small bars each with a single pole, but this is not the +case, for the two poles are inseparable. However many +pieces we break a magnet into, each piece is a perfect<span class="pagenum" id="Page_48">48</span> +magnet having a north and south pole. Thus while we +can isolate a positive or a negative charge of electricity, we +cannot isolate north or south magnetism.</p> + +<p>If we place the north pole of a bar magnet near to, but +not touching, a bar of soft iron, as in <a href="#plate_IIa">Plate II.<i>a</i></a>, we find that +the latter becomes a magnet, as shown by its ability to +support filings; and that as soon as the magnet is removed +the filings drop off, showing that the iron has lost its +magnetism. If the iron is tested while the magnet is in +position it is found to have a south pole at the end nearer +the magnet, and a north pole at the end farther away; and +if the magnet is reversed, so as to bring its south pole +nearer the iron, the poles of the latter are found to reverse +also. The iron has gained its new properties by magnetic +induction, and we cannot fail to notice the similarity between +this experiment and that in <a href="#fig_2">Fig. 2</a>, <a href="#chapter_II">Chapter II</a>., which +showed electro-static induction. A positively or a negatively +electrified body induces an opposite charge at the +nearer end, and a similar charge at the further end of a +conductor, and a north or a south pole of a magnet +induces opposite polarity at the nearer end, and a +similar polarity at the further end of a bar of iron. In +<a href="#chapter_II">Chapter II</a>. we showed that the attraction of a pith ball +by an electrified body was due to induction, and from what +we have just learnt about magnetic induction the reader +will have no difficulty in understanding why a magnet +attracts filings or needles.</p> + +<figure id="plate_IIa" class="figcenter" style="max-width: 26em;"> + <p class="caption">PLATE II.</p> + <img src="images/i_061.jpg" width="2060" height="1021" alt=" "> + <figcaption class="caption"> + +<p>(<i>a</i>) EXPERIMENT TO SHOW MAGNETIC INDUCTION.</p> +</figcaption></figure> + +<figure id="plate_IIb" class="figcenter" style="max-width: 26em;"> + <img src="images/i_061b.jpg" width="2060" height="1348" alt=" "> + <figcaption class="caption">(<i>b</i>) EXPERIMENT TO SHOW THE PRODUCTION OF MAGNETISM BY AN +ELECTRIC CURRENT. +</figcaption></figure> + +<p>Any one who experiments with magnets must be struck +with the distance at which one magnet can influence filings +or another magnet. If a layer of iron filings is spread on a +sheet of paper, and a magnet brought gradually nearer +from above, the filings soon begin to move about restlessly, +and when the magnet comes close enough they fly up to it +as if pulled by invisible strings. A still more striking<span class="pagenum" id="Page_49">49</span> +experiment consists in spreading filings thinly over a sheet +of cardboard and moving a magnet to and fro underneath +the sheet. The result is most amusing. The filings seem +to stand up on their hind legs, and they march about like +regiments of soldiers. Here again invisible strings are +suggested, and we might wonder whether there really is +anything of the kind. Yes, there is. To put the matter +in the simplest way, the magnet acts by means of strings +or lines of force, which emerge from it in definite directions, +and in a most interesting way we can see some of these +lines of force actually at work.</p> + +<p>Place a magnet, or any arrangement of magnets, underneath +a sheet of glass, and sprinkle iron filings from a +muslin bag thinly and evenly all over the glass. Then tap +the glass gently with a pencil, and the filings at once +arrange themselves in a most remarkable manner. All the +filings become magnetized by induction, and when the tap +sets them free for an instant from the friction of the glass +they take up definite positions under the influence of the +force acting upon them. In this way we get a map of +the general direction of the magnetic lines of force, which +are our invisible strings.</p> + +<p>Many different maps may be made in this way, but we +have space for only two. <a href="#plate_III">Plate III.<i>a</i></a> shows the lines of two +opposite poles. Notice how they appear to stream across +from one pole to the other. It is believed that there is a +tension along the lines of force not unlike that in stretched +elastic bands, and if this is so it is easy to see from the +figure why opposite poles attract each other.</p> + +<p><a href="#plate_III">Plate III.<i>b</i></a> shows the lines of force of two similar poles. +In this case they do not stream from pole to pole, but turn +aside as if repelling one another, and from this figure we +see why there is repulsion between two similar poles. It +can be shown, although in a much less simple manner, that<span class="pagenum" id="Page_50">50</span> +lines of electric force proceed from electrified bodies, and +in electric attraction and repulsion between two charged +bodies the lines of force take paths which closely resemble +those in our two figures. A space filled with lines of +magnetic force is called a <em>magnetic field</em>, and one filled +with lines of electric force is called an <em>electric field</em>.</p> + +<p>A horse-shoe magnet, which is simply a bar of steel +bent into the shape of a horse-shoe before being magnetized, +gradually loses its magnetism if left with its poles +unprotected, but this loss is prevented if the poles are +connected by a piece of soft iron. The same loss occurs +with a bar magnet, but as the two poles cannot be connected +in this way it is customary to keep two bar magnets side +by side, separated by a strip of wood; with opposite poles +together and a piece of soft iron across the ends. Such +pieces of iron are called <em>keepers</em>, and <a href="#fig_13">Fig. 13</a> shows a +horse-shoe magnet and a pair of bar magnets with their +keepers. It may be remarked that a magnet never should +be knocked or allowed to fall, as rough usage of this kind +causes it to lose a considerable amount of its magnetism. +A magnet is injured also by allowing the keeper to slam on +to it; but pulling the keeper off vigorously does good +instead of harm.</p> + +<p>If a magnetized needle is suspended so that it is free to +swing either horizontally or vertically, it not only comes +to rest in a north and south direction, but also it tilts with +its north-pointing end downwards. If the needle were +taken to a place south of the equator it would still tilt, but +the south-pointing end would be downwards. In both +cases the angle the needle makes with the horizontal is +called the <em>magnetic dip</em>.</p> + +<figure id="plate_III" class="figcenter" style="max-width: 26em;"> + <p class="caption b1">PLATE III.</p> + <p class="caption smaller">(<i>a</i>) LINES OF MAGNETIC FORCE OF TWO OPPOSITE POLES.</p> + <img src="images/i_065.jpg" width="2080" height="3126" alt=" "> + <figcaption class="caption"> + <p class="smaller">(<i>b</i>) LINES OF MAGNETIC FORCE OF TWO SIMILAR POLES.</p> +</figcaption></figure> + +<p>It is evident that a suspended magnetized needle would +not invariably come to rest pointing north and south unless +it were compelled to do so, and a little consideration shows<span class="pagenum" id="Page_51">51</span> +that the needle acts as if it were under the influence of a +magnet. Dr. Gilbert of Colchester, of whom we spoke in +<a href="#chapter_I">Chapter I</a>., gave a great deal of time to the study of +magnetic phenomena, and in 1600 he announced what may +be regarded as his greatest discovery: <em>The terrestrial +globe itself is a great magnet</em>. Here, then, is the explanation +of the behaviour of the magnetized needle. The Earth +itself is a great magnet, having its poles near to the +geographical north and south poles. But a question at +once suggests itself: “Since similar poles repel one another, +how is it that the north pole of a magnet turns towards the +north magnetic pole of the earth?” This apparent difficulty +is caused by a confusion in terms. If the Earth’s +north magnetic pole really has north magnetism, then the +north-pointing end of a magnet must be a south pole; and +on the other hand, if the north-pointing end of a magnet +has north magnetism, then the Earth’s north magnetic pole +must be really a south pole. It is a troublesome matter to +settle, but it is now customary to regard the Earth’s north +magnetic pole as possessing south magnetism, and the +south magnetic pole as possessing north magnetism. In +this way the north-pointing pole of a magnet may be looked +upon as a true north pole, and the south-pointing pole as a +true south pole.</p> + +<p>Magnetic dip also is seen to be a natural result of the +Earth’s magnetic influence. Here in England, for instance, +the north magnetic pole is much nearer than the south +magnetic pole, and consequently its influence is the +stronger. Therefore a magnetized needle, if free to do +so, dips downwards towards the north. At any place +where the south magnetic pole is the nearer the direction +of the dip of course is reversed. If placed immediately +over either magnetic pole the needle would take up a +vertical position, and at the magnetic equator it would not<span class="pagenum" id="Page_52">52</span> +dip at all, for the influence of the two magnetic poles would +be equal. A little study of <a href="#fig_14">Fig. 14</a>, which represents a +dipping needle at different parts of the earth, will make +this matter clearer. N and S represent the Earth’s north +and south magnetic poles, and the arrow heads are the +north poles of the needles.</p> + +<figure id="fig_14" class="figleft" style="max-width: 16em;"> + <img src="images/i_068.png" width="1265" height="927" alt=" "> + <figcaption class="caption"><span class="smcap">Fig. 14.</span>—Diagram to illustrate Magnetic Dip. +</figcaption></figure> + +<p>Since the Earth is a magnet, we should expect it to +be able to induce magnetism in a bar of iron, just as our +artificial magnets do, and we can show that this is actually +the case. If a steel poker is held pointing to and dipping +down towards the +north, and struck +sharply with a piece +of wood while in this +position, it acquires +magnetic properties +which can be tested +by means of a small +compass needle. It +is an interesting fact +that iron pillars and +railings which have +been standing for a +long time in one position are found to be magnetized. +In the northern hemisphere the bases of upright iron +pillars are north poles, and their upper ends south poles, +and in the southern hemisphere the polarity is reversed.</p> + +<p>The most valuable application of the magnetic needle +is in the compass. An ordinary pocket compass for inland +use consists simply of a single magnetized needle pivoted +so as to swing freely over a card on which are marked the +thirty-two points of the compass. Ships’ compasses are +much more elaborate. As a rule a compound needle is +used, consisting of eight slender strips of steel, magnetized<span class="pagenum" id="Page_53">53</span> +separately, and suspended side by side. A compound +needle of this kind is very much more reliable than a +single needle. The material of which the card is made +depends upon whether the illumination for night work is +to come from above or below. If the latter, the card must +be transparent, and it is often made of thin sheet mica; +but if the light comes from above, the card is made of +some opaque material, such as very stout paper. The +needle and card are contained in a sort of bowl made of +copper. In order to keep this bowl in a horizontal position, +however the ship may be pitching and rolling, it is supported +on gimbals, which are two concentric rings attached +to horizontal pivots, and moving in axes at right angles +to one another. Further stability may be obtained by +weighting the bottom of the bowl with lead. There are +also liquid compasses, in which the card is floated on the +surface of dilute alcohol, and many modern ships’ compasses +have their movements regulated by a gyrostat.</p> + +<p>The large amount of iron and steel used in the construction +of modern vessels has a considerable effect upon +the compass needle, and unless the compass is protected +from this influence its readings are liable to serious errors. +The most satisfactory way of giving this protection is by +placing on each side of the compass a large globe of soft +iron, twelve or more inches in diameter.</p> + +<p>On account of the fact that the magnetic poles of the +Earth do not coincide with the geographical north and +south poles, a compass needle seldom points exactly north +and south, and the angle between the magnetic meridian +and the geographical meridian is called the <em>declination</em>. +The discovery that the declination varies in different parts +of the world was made by Columbus in 1492. For purposes +of navigation it is obviously very important that the +declination at all points of the Earth’s surface should be<span class="pagenum" id="Page_54">54</span> +known, and special magnetic maps are prepared in which +all places having the same declination are joined by a +line.</p> + +<p>It is an interesting fact that the Earth’s magnetism is +subject to variation. The declination and the dip slowly +change through long periods of years, and there are also +slight annual and even daily variations.</p> + +<p>At one time magnets were credited with extraordinary +effects upon the human body. Small doses of lodestone, +ground to powder and mixed with water, were supposed +to prolong life, and Paracelsus, a famous alchemist and +physician, born in Switzerland in 1493, believed in the +potency of lodestone ointment for wounds made with steel +weapons. Baron Reichenbach, 1788–1860, believed that +he had discovered the existence of a peculiar physical force +closely connected with magnetism, and he gave this force +the name <em>Od</em>. It was supposed to exist everywhere, +and, like magnetism, to have two poles, positive and +negative; the left side of the body being od-positive, and +the right side od-negative. Certain individuals, known as +“sensitives,” were said to be specially open to its influence. +These people stated that they saw strange flickering lights +at the poles of magnets, and that they experienced peculiar +sensations when a magnet was passed over them. Some +of them indeed were unable to sleep on the left side, because +the north pole of the Earth, being od-negative, had +a bad effect on the od-negative left side. The pretended +revelations of these “sensitives” created a great stir at the +time, but now nobody believes in the existence of <em>Od</em>.</p> + +<p>Professor Tyndall was once invited to a seance, with +the object of convincing him of the genuineness of spiritualism. +He sat beside a young lady who claimed to have +spiritualistic powers, and his record of his conversation with +her is amusing. The Reichenbach craze was in full swing<span class="pagenum" id="Page_55">55</span> +at the time, and Tyndall asked if the lady could see any of +the weird lights supposed to be visible to “sensitives.”</p> + +<div class="blockquot"> + +<p>“<i>Medium.</i>—Oh yes; but I see the light around all +bodies.</p> + +<p><i>I.</i>—Even in perfect darkness?</p> + +<p><i>Medium.</i>—Yes; I see luminous atmospheres round +all people. The atmosphere which surrounds +Mr. R. C. would fill this room with light.</p> + +<p><i>I.</i>—You are aware of the effects ascribed by Baron +Reichenbach to magnets?</p> + +<p><i>Medium.</i>—Yes; but a magnet makes me terribly ill.</p> + +<p><i>I.</i>—Am I to understand that, if this room were +perfectly dark, you could tell whether it contained +a magnet, without being informed of +the fact?</p> + +<p><i>Medium.</i>—I should know of its presence on entering +the room.</p> + +<p><i>I.</i>—How?</p> + +<p><i>Medium.</i>—I should be rendered instantly ill.</p> + +<p><i>I.</i>—How do you feel to-day?</p> + +<p><i>Medium.</i>—Particularly well; I have not been so +well for months.</p> + +<p><i>I.</i>—Then, may I ask you whether there is, at the +present moment, a magnet in my possession?</p> + +<p>The young lady looked at me, blushed, and +stammered, ‘No; I am not <i>en rapport</i> with +you.’</p> + +<p><em>I sat at her right hand, and a left-hand pocket, +within six inches of her person, contained a +magnet.</em>”</p> +</div> + +<p>Tyndall adds, “Our host here deprecated discussion +as it ‘exhausted the medium.’”</p> +<hr class="chap x-ebookmaker-drop"> + +<div class="chapter"> +<p><span class="pagenum" id="Page_56">56</span></p> + +<h2 class="nobreak" id="toclink_56"><a id="chapter_VII"></a>CHAPTER VII<br> + +<span class="subhead">THE PRODUCTION OF MAGNETISM BY ELECTRICITY</span></h2> +</div> + +<figure id="fig_15" class="figcenter" style="max-width: 19em;"> + <img src="images/i_072.png" width="1519" height="866" alt=" "> + <figcaption class="caption"><span class="smcap">Fig. 15.</span>—Diagram to illustrate Magnetic effect of an Electric Current. +</figcaption></figure> + +<p class="in0"><span class="firstword">In</span> the previous chapter attention was drawn to the fact +that there are many close parallels between electric and +magnetic phenomena, and in this chapter it will be shown +that magnetism can be produced by electricity. In the +year 1819 Professor Oersted, of the University of Copenhagen, +discovered that a freely swinging magnetized needle, +such as a compass needle, was deflected by a current of +electricity flowing through a wire. In <a href="#fig_15">Fig. 15</a>, A, a +magnetic needle is shown at rest in its usual north and +south direction, and over it is held a copper wire, also +pointing north and south. A current of electricity is now +sent through the wire, and the needle is at once deflected, +<a href="#fig_15">Fig. 15</a>, B. The direction of the current is indicated by<span class="pagenum" id="Page_57">57</span> +an arrow, and the direction in which the needle has moved +is shown by the two small arrows. If the direction of the +current is reversed, the needle will be deflected in the +opposite direction. From this experiment we see that the +current has brought magnetic influences into play, or in +other words has produced magnetism. If iron filings are +brought near the wire while the current is flowing, they +are at once attracted and cling to the wire, but as soon as +the current is stopped +they drop off. This +shows us that the wire +itself becomes a magnet +during the passage of +the current, and that it +loses its magnetism +when the current ceases +to flow.</p> + +<figure id="fig_16" class="figright" style="max-width: 15em;"> + <img src="images/i_073.jpg" width="1180" height="1174" alt=" "> + <figcaption class="caption"><span class="smcap">Fig. 16.</span>—Magnetic Field round wire +conveying a Current. +</figcaption></figure> + +<p>Further, it can be +shown that two freely +moving parallel wires +conveying currents attract +or repel one +another according to +the direction of the currents. +If both currents are flowing in the same direction +the wires attract one another, but if the currents flow in +opposite directions the wires repel each other. <a href="#fig_16">Fig. 16</a> +shows the direction of the lines of force of a wire conveying +a current and passed through a horizontal piece of cardboard +covered with a thin layer of iron filings; and from this +figure it is evident that the passage of the current produces +what we may call magnetic whirls round the wire.</p> + +<p>A spiral of insulated wire through which a current is +flowing shows all the properties of a magnet, and if free to<span class="pagenum" id="Page_58">58</span> +move it comes to rest pointing north and south. It is +attracted or repelled by an ordinary magnet according to +the pole presented to it and the direction of the current, +and two such spirals show mutual attraction and repulsion. +A spiral of this kind is called a <em>solenoid</em>, and in addition +to the properties already mentioned it has the peculiar +power of drawing or sucking into its interior a rod of +iron. Solenoids have various practical applications, and in +later chapters we shall refer to them again.</p> + +<p>If several turns of cotton-covered wire are wound round +an iron rod, the passing of a current through the wire +makes the rod into a magnet (<a href="#plate_IIa">Plate II.<i>b</i></a>), but the magnetism +disappears as soon as the current ceases to flow. A +magnet made by the passage of an electric current is called +an <em>electro-magnet</em>, and it has all the properties of the +magnets mentioned in the previous chapter. A bar of steel +may be magnetized in the same way, but unlike the iron +rod it retains its magnetism after the current is interrupted. +This provides us with a means of magnetizing a piece of +steel much more strongly than is possible by rubbing with +another magnet. Steel magnets, which retain their +magnetism, are called <em>permanent</em> magnets, as distinguished +from electro-magnets in which soft iron is used, so +that their magnetism lasts only as long as the current +flows.</p> + +<p>Electro-magnets play an extremely important part in +the harnessing of electricity; in fact they are used in one +form or another in almost every kind of electrical +mechanism. In later chapters many of these uses will be +described, and here we will mention only the use of +electro-magnets for lifting purposes. In large engineering +works powerful electro-magnets, suspended from some +sort of travelling crane, are most useful for picking up and +carrying about heavy masses of metal, such as large<span class="pagenum" id="Page_59">59</span> +castings. No time is lost in attaching the casting to the +crane; the magnet picks it up directly the current is +switched on, and lets it go the instant the current is +stopped. In any large steel works the amount of scrap +material produced is astonishingly great, hundreds of tons +of turnings and similar scrap accumulating in a very short +time. A huge mound of turnings is awkward to deal with +by ordinary manual labour, but a combination of electro-magnet +and crane solves the difficulty completely, lifting +and loading the scrap into carts or trucks at considerable +speed, and without requiring much attention.</p> + +<p>Some time ago a disastrous fire occurred at an +engineering works in the Midlands, the place being almost +entirely burnt out. Amongst the débris was, of course, a +large amount of metal, and as this was too valuable to be +wasted, an electro-magnet was set to work on the wreckage. +The larger pieces of metal were picked up in the ordinary +way, and then the remaining rubbish was shovelled against +the face of the magnet, which held on to the metal but +dropped everything else, and in this way some tons of +metal were recovered.</p> + +<p>The effect produced upon a magnetized needle by a +current of electricity affords a simple means of detecting the +existence of such a current. An ordinary pocket compass +can be made to show the presence of a moderate current, but +for the detection of extremely small currents a much more +sensitive apparatus is employed. This is called a <em>galvanometer</em>, +and in its simplest form it consists essentially of +a delicately poised magnetic needle placed in the middle of +a coil of several turns of wire. The current thus passes +many times round the needle, and this has the effect of +greatly increasing the deflection of the needle, and hence +the sensitiveness of the instrument. Although such an +arrangement is generally called a galvanometer, it is really<span class="pagenum" id="Page_60">60</span> +a galvanoscope, for it does not measure the current but only +shows its presence.</p> + +<p>We have seen that electro-motive force is measured in +volts, and that the definition of a volt is that electro-motive +force which will cause a current of one ampere to flow +through a conductor having a resistance of one ohm. If we +make a galvanometer with a long coil of very thin wire +having a high resistance, the amount of current that will +flow through it will be proportionate to the electro-motive +force. Such a galvanometer, fitted with a carefully +graduated scale, in this way will indicate the number of +volts, and it is called a <em>voltmeter</em>. If we have a galvanometer +with a short coil of very thick wire, the resistance put +in the way of the current is so small that it may be left out +of account, and by means of a graduated scale the number +of amperes may be shown; such an instrument being called +an <em>amperemeter</em>, or <em>ammeter</em>.</p> + +<p>For making exact measurements of electric currents the +instruments just described are not suitable, as they are not +sufficiently accurate; but their working shows the principle +upon which currents are measured. The actual instruments +used in electrical engineering and in scientific work are +unfortunately too complicated to be described here.</p> +<hr class="chap x-ebookmaker-drop"> + +<div class="chapter"> +<p><span class="pagenum" id="Page_61">61</span></p> + +<h2 class="nobreak" id="toclink_61"><a id="chapter_VIII"></a>CHAPTER VIII<br> + +<span class="subhead">THE INDUCTION COIL</span></h2> +</div> + +<p class="in0"><span class="firstword">The</span> voltaic cell and the accumulator provide us with +currents of electricity of considerable volume, but at low +pressure or voltage. For many purposes, however, we require +a comparatively small amount of current at very high +pressure, and in such cases we use an apparatus called +the <em>induction coil</em>. Just as an electrified body and a +magnet will induce electrification and magnetism respectively, +so a current of electricity will induce another current; +and an induction coil is simply an arrangement by which a +current in one coil of wire is made to induce a current in +another coil.</p> + +<p>Suppose we have two coils of wire placed close together, +one connected to a battery of voltaic cells, with +some arrangement for starting and stopping the current +suddenly, and the other to a galvanometer. As soon as +we send the current through the first coil, the needle of +the galvanometer moves, showing that there is a current +flowing through the second coil; but the needle quickly +comes back to its original position, showing that this +current was only momentary. So long as we keep the +current flowing through the first coil the galvanometer +shows no further movement, but as soon as we stop the +current the needle again shows by its movements that +another momentary current has been produced in the +second coil. This experiment shows us that a current<span class="pagenum" id="Page_62">62</span> +induces another current only at the instant it is started or +stopped, or, as we say, at the instant of making or breaking +the circuit.</p> + +<p>The coil through which we send the battery current is +called the “primary coil,” and the one in which a current is +induced is called the “secondary coil.” The two momentary +currents in the secondary coil do not both flow in the same +direction. The current induced on making the circuit +flows in a direction opposite to that of the current in the +primary coil; and the current induced on breaking the +circuit flows in the same direction as that in the primary +coil. If the two coils are exactly alike, the induced current +will have the same voltage as the primary current; but +if the secondary coil has twice as many turns of wire +as the primary coil, the induced current will have twice +the voltage of the primary current. In this way, by +multiplying the turns of wire in the secondary coil, we +can go on increasing the voltage of the induced current, +and this is the principle upon which the induction coil +works.</p> + +<p>We may now describe the construction of such a coil. +The primary coil is made of a few turns of thick copper +wire carefully insulated, and inside it is placed a core consisting +of a bundle of separate wires of soft iron. Upon +this coil, but carefully insulated from it, is wound the +secondary coil, consisting of a great number of turns of +very fine wire. In large induction coils the secondary coil +has thousands of times as many turns as the primary, and +the wire forming it may be more than a hundred miles in +length. The ends of the secondary coil are brought to +terminals so that they can be connected up to any apparatus +as desired.</p> + +<figure id="fig_17" class="figright" style="max-width: 13em;"> + <img src="images/i_079.png" width="1008" height="920" alt=" "> + <figcaption class="caption hang"><span class="smcap">Fig. 17.</span>—Diagram showing working of +Contact-Breaker for Induction Coil. +</figcaption></figure> + +<p>In order that the induced currents shall follow each +other in quick succession, some means of rapidly making<span class="pagenum" id="Page_63">63</span> +and breaking the circuit is required, and this is provided +by an automatic contact breaker. It consists of a small +piece of soft iron, A, <a href="#fig_17">Fig. 17</a>, fixed to a spring, B, having +a platinum tip at C. The adjustable screw, D, also has a +platinum tip, E. Normally the two platinum tips are just +touching one another, and matters are arranged so that +their contact completes the circuit. When the apparatus +is connected to a suitable battery a current flows through +the primary coil, and the iron core, F, becomes an electro-magnet, +which draws A towards it. The platinum tips +are thus no longer in contact and the circuit is broken. +Immediately this occurs the +iron core loses its magnetism +and ceases to attract A, which +is then moved back again by +the spring B, so that the +platinum tips touch, the circuit +is once more completed, and +the process begins over again. +All this takes place with the +utmost rapidity, and the speed +at which the contact-breaker +works is so great as to produce +a musical note. There +are many other types of contact-breakers, but in every +case the purpose is the same, namely, to make and +break the primary circuit as rapidly as possible.</p> + +<p>The efficiency of the coil is greatly increased by a +condenser which is inserted in the primary circuit. It +consists of alternate layers of tinfoil and paraffined +paper, and its action is like that of a Leyden jar. +A switch is provided to turn the battery current on or +off, and there is also a reversing switch or commutator, +by means of which the direction of the current may be<span class="pagenum" id="Page_64">64</span> +reversed. The whole arrangement is mounted on a +suitable wooden base, and its general appearance is shown +in <a href="#fig_18">Fig. 18</a>.</p> + +<figure id="fig_18" class="figcenter clear" style="max-width: 19em;"> + <img src="images/i_080.jpg" width="1473" height="851" alt=" "> + <figcaption class="caption"> + +<p class="floatl"><i>By permission of</i>]</p> +<p class="floatr">[<i>Harry W. Cox, Ltd.</i></p> + +<p class="floatc"><span class="smcap">Fig. 18.</span>—Typical Induction Coil.</p> +</figcaption></figure> + +<p>By means of a large induction coil we can obtain a +voltage hundreds or even thousands of times greater than +that of the original battery current, but on account of the +great resistance of a very long, thin wire, the amperage is +much smaller. The induction coil produces a rapid +succession of sparks, similar to those obtained from a +Wimshurst machine. A coil has been constructed capable +of giving sparks 42½ inches in length, and having a +secondary coil with 340,000 turns of wire, the total +length of the wire being 280 miles. Induction coils are +largely employed for scientific purposes, and they are +used in wireless telegraphy and in the production of +X-rays.</p> + +<p>The principle of the induction coil can be applied +also to the lowering of the voltage of a current. If +we make the secondary coil with less, instead of more +turns of wire than the primary coil, the induced current +will be of lower voltage than the primary current, but<span class="pagenum" id="Page_65">65</span> +its amperage will be correspondingly higher. This fact +is taken advantage of in cases where it is desirable +to transform a high voltage current from the public +mains down to a lower voltage current of greater +amperage.</p> +<hr class="chap x-ebookmaker-drop"> + +<div class="chapter"> +<p><span class="pagenum" id="Page_66">66</span></p> + +<h2 class="nobreak" id="toclink_66"><a id="chapter_IX"></a>CHAPTER IX<br> + +<span class="subhead">THE DYNAMO AND THE ELECTRIC MOTOR</span></h2> +</div> + +<p class="in0"><span class="firstword">Most</span> of my readers will have seen the small working +models of electric tramcars which can be bought at any +electrical supply stores. These usually require a current of +about one ampere at three or four volts. If we connect +such a car to the battery recommended for it, and keep it +running continuously, we find that the battery soon begins +to show signs of exhaustion. Now if we imagine our +little car increased to the size of an electric street car, and +further imagine, say, a hundred such cars carrying heavy +loads day after day from morning to night, we shall realize +that a battery of cells capable of supplying the current +necessary to run these cars would be so colossal as to be +utterly impracticable. We therefore must look beyond the +voltaic cell for a source of current for such a purpose, and +this source we find in a machine called the “dynamo,” +from the Greek word <em>dynamis</em>, meaning force.</p> + +<p>Oersted’s discovery of the production of magnetism by +electricity naturally suggested the possibility of producing +electricity from magnetism. In the year 1831 one of the +most brilliant of our British scientists, Michael Faraday, +discovered that a current of electricity could be induced in +a coil of wire either by moving the coil towards or away +from a magnet, or by moving a magnet towards or away +from the coil. This may be shown in a simple way by +connecting the ends of a coil of insulated wire to a galvanometer,<span class="pagenum" id="Page_67">67</span> +and moving a bar magnet in and out of the coil; +when the galvanometer shows that a current is induced in +the coil on the insertion of the magnet, and again on its +withdrawal. We have seen that a magnet is surrounded +by a field of magnetic force, and Faraday found that the +current was induced when the lines of force were cut across.</p> + +<p>Utilizing this discovery Faraday constructed the first +dynamo, which consisted of a copper plate or disc rotated +between the poles of a powerful horse-shoe magnet, so as +to cut the lines of force. The current flowed either from +the shaft to the rim, or <i lang="la">vice versa</i>, according to the direction +of rotation; and it was conducted away by means of two +wires with spring contacts, one pressing against the shaft, +and the other against the circumference of the disc. This +machine was miserably inefficient, but it was the very first +dynamo, and from it have been slowly evolved the mighty +dynamos used to-day in electric power stations throughout +the world. There is a little story told of Faraday +which is worth repeating even if it is not true. Speaking +of his discovery that a magnet could be made to produce +an electric current, a lady once said to him, “This is all +very interesting, but what is the use of it?” “Madam,” +replied Faraday, “what is the use of a baby?” In +Faraday’s “baby” dynamo, as in all others, some kind of +power must be used to produce the necessary motion, so +that all dynamos are really machines for converting +mechanical energy into electrical energy.</p> + +<p>The copper disc in this first dynamo did not prove +satisfactory, and Faraday soon substituted for it rotating +coils of wire. In 1832 a dynamo was constructed in which +a length of insulated wire was wound upon two bobbins +having soft iron cores, and a powerful horse-shoe magnet +was fixed to a rotating spindle in such a position that its +poles faced the cores of the bobbins. This machine gave<span class="pagenum" id="Page_68">68</span> +a fair current, but it was found that the magnet gradually +lost its magnetism on account of the vibration caused by +its rotation. The next step was to make the magnet a +fixture, and to rotate the bobbins of wire. This was a +great improvement, and the power of machines built on +this principle was much increased by having a number of +rotating coils and several magnets. One such machine +had 64 separate +coils rotating +between the +poles of 40 large +magnets. Finally, +permanent +magnets were +superseded by +electro-magnets, +which +gave a much +more powerful +field of force.</p> + +<figure id="fig_19" class="figcenter" style="max-width: 19em;"> + <img src="images/i_084.png" width="1519" height="1503" alt=" "> + <figcaption class="caption"><span class="smcap">Fig. 19.</span>—Diagram showing principle of Dynamo +producing Alternating Current. +</figcaption></figure> + +<p>Having seen +something of +the underlying +principle and of +the history of +the dynamo, we +must turn our attention to its actual working. <a href="#fig_19">Fig. 19</a> is a +rough representation of a dynamo in its simplest form. The +two poles of the magnet are shown marked north and south, +and between them revolves the coil of wire A¹ A², mounted +on a spindle SS. This revolving coil is called the armature. +To each of the insulated rings RR is fixed one end of the +coil, and BB are two brushes of copper or carbon, one +pressing on each ring. From these brushes the current is<span class="pagenum" id="Page_69">69</span> +led away into the main circuit, and in this case we may +suppose that the current is used to light a lamp.</p> + +<p>In speaking of the induction coil we saw that the +currents induced on making and on breaking the circuit +flowed in opposite directions, and similarly, Faraday found +that the currents induced in a coil of wire on inserting and +on withdrawing his magnet flowed in opposite directions. +In the present case the magnet is stationary and the coil +moves, but the effect is just the same. Now if we suppose +the armature to be revolving in a clockwise direction, then +A¹ is descending and entering the magnetic field in front of +the north pole, consequently a current is induced in the +coil, and of course in the main circuit also, in one direction. +Continuing its course, A¹ passes away from this portion of +the magnetic field, and thus a current is induced in the +opposite direction. In this way we get a current which +reverses its direction every half-revolution, and such a +current is called an alternating current. If, as in our +diagram, there are only two magnetic poles, the current +flows backwards and forwards once every revolution, but +by using a number of magnets, arranged so that the coil +passes in turn the poles of each, it can be made to flow +backwards and forwards several times. One complete +flow backwards and forwards is called a period, and the +number of periods per second is called the periodicity or +frequency of the current. A dynamo with one coil or set +of coils gives what is called “single-phase” current, that is, a +current having one wave which keeps flowing backwards +and forwards. If there are two distinct sets of coils we get +a two-phase current, in which there are two separate waves, +one rising as the other falls. Similarly, by using more +sets of coils, we may obtain three-phase or polyphase +currents.</p> + +<figure id="fig_20" class="figcenter" style="max-width: 19em;"> + <img src="images/i_086.png" width="1511" height="1651" alt=" "> + <figcaption class="caption"><span class="smcap">Fig. 20.</span>—Diagram showing principle of Dynamo +producing Continuous Current. +</figcaption></figure> + +<p>Alternating current is unsuitable for certain purposes,<span class="pagenum" id="Page_70">70</span> +such as electroplating; and by making a small alteration in +our dynamo we get a continuous or direct current, which +does not reverse its direction. <a href="#fig_20">Fig. 20</a> shows the new +arrangement. Instead of the two rings in <a href="#fig_19">Fig. 19</a>, we have +now a single ring divided into two parts, each half being +connected to one end of the revolving coil. Each brush, +therefore, remains on one portion of the ring for half a +revolution, and +then passes +over on to the +other portion. +During one +half-revolution +we will suppose +the current to +be flowing from +brush B¹ in the +direction of the +lamp. Then +during the next +half-revolution +the current +flows in the opposite +direction; +but brush B¹ +has passed on +to the other half +of the ring, and so the current is still leaving by it. +In this way the current must always flow in the same +direction in the main circuit, leaving by brush B¹ and +returning by brush B². This arrangement for making the +alternating current into a continuous current is called a +<em>commutator</em>.</p> + +<figure id="plate_IV" class="figcenter" style="max-width: 26em;"> + <p class="caption">PLATE IV.</p> + <img src="images/i_087.jpg" width="2024" height="2679" alt=" "> + <figcaption class="caption"> + +<p class="floatl"><i>By permission of</i></p> +<p class="floatr"><i>Lancashire Dynamo & Motor Co. Ltd.</i></p> + +<p class="floatc">A TYPICAL DYNAMO AND ITS PARTS.</p> +</figcaption></figure> + +<p>In actual practice a dynamo has a set of electro-magnets, +and the armature consists of many coils of wire mounted +on a core of iron, which has the effect of concentrating the +lines of force. The armature generally revolves in small +dynamos, but in large ones it is usually a fixture, while the +electro-magnets revolve. <a href="#plate_IV">Plate IV</a>. shows a typical dynamo +and its parts.</p> + +<p>As we saw in an earlier chapter, an electro-magnet has +magnetic powers only while a current is being passed +through its winding, and so some means of supplying +current to the electro-magnets in a dynamo must be provided. +It is a remarkable fact that it is almost impossible +to obtain a piece of iron which has not some traces of +magnetism, and so when a dynamo is first set up there is +often sufficient magnetism in the iron of the electro-magnets +to produce a very weak field. The rapid cutting of the +feeble lines of force of this field sets up a weak current, +which, acting upon the electro-magnets, gradually brings +them up to full strength. Once the dynamo is generating +current it keeps on feeding its magnets by sending either +the whole or a part of its current through them. After +it has once been set going the dynamo is always able to +start again, because the magnet cores retain enough +magnetism to set up a weak field. If there is not enough +magnetism in the cores to start a dynamo for the first +time, a current from some outside source is sent round the +magnets.</p> + +<p>The foregoing remarks apply to continuous current +dynamos only. Alternating current can be used for exciting +electro-magnets, but in this case the magnetic field produced +is alternating also, so that each pole of the magnet has +north and south magnetism alternately. This will not do +for dynamo field magnets, and therefore an alternating +current dynamo cannot feed its own magnets. The electro-magnets +in such dynamos are supplied with current from a<span class="pagenum" id="Page_72">72</span> +separate continuous current dynamo, which may be of quite +small size.</p> + +<p>It is a very interesting fact that electric current can +be generated by a dynamo in which the earth itself is +used to provide the magnetic field, no permanent or electro-magnets +being used at all. A simple form of dynamo +of this kind consists of a rectangular loop of copper wire +rotating about an axis pointing east and west, so that +the loop cuts the lines of force of the Earth’s magnetic +field.</p> + +<p>The dynamo provides us with a constant supply of +electric current, but this current is no use unless we can +make it do work for us. If we reverse the usual order of +things in regard to a dynamo, and supply the machine with +current instead of mechanical power, we find that the +armature begins to revolve rapidly, and the machine is +no longer a dynamo, but has become an electric motor. +This shows us that an electric motor is simply a dynamo +reversed. Let us suppose that we wish to use the +dynamo in <a href="#fig_20">Fig. 20</a> as a motor. In order to supply the +current we will take away the lamp and substitute a +second continuous-current dynamo. We know from +<a href="#chapter_VII">Chapter VII</a>. that when a current is sent through a coil +of wire the coil becomes a magnet with a north and a +south pole. The coil in our dynamo becomes a magnet +as soon as the current is switched on, and the attraction +between its poles and the opposite poles of the magnet +causes it to make half a revolution. At this point the +commutator reverses the current, and consequently the +polarity of the coil, so that there is now repulsion where +previously there was attraction, and the coil makes another +half-revolution. So the process goes on until the armature +attains a very high speed. In general construction there is +practically no difference between a dynamo and a motor,<span class="pagenum" id="Page_73">73</span> +but there are differences in detail which adapt each to its +own particular work. By making certain alterations in +their construction electric motors can be run with alternating +current.</p> + +<p>The fact that a dynamo could be reversed and run as a +motor was known probably as early as 1838, but the great +value of this reversibility does not seem to have been +realized until 1873. At an industrial exhibition held at +Vienna in that year, it so happened that a workman or +machinery attendant connected two cables to a dynamo +which was standing idle, and he was much surprised to +find that it at once began to revolve at a great speed. It +was then seen that the cables led to another dynamo which +was running, and that the current from this source had +made the first dynamo into a motor. There are many +versions of this story, but the important point in all +is that this was the first occasion on which general +attention was drawn to the possibilities of the electric +motor.</p> + +<p>The practical advantages afforded by the electric motor +are many and great. Once we have installed a sufficiently +powerful dynamo and a steam or other engine to drive it, +we can place motors just where they are required, either +close to the dynamo or miles away, driving them simply by +means of a connecting cable. In factories, motors can be +placed close to the machines they are required to drive, +anywhere in the building, thus doing away with all complicated +and dangerous systems of shafting and belts. In +many cases where it would be either utterly impossible or +at least extremely inconvenient to use any form of steam, +gas, or oil engine, electric motors can be employed without +the slightest difficulty. In order to realize this, one only +has to think of the positions in which electrically-driven +ventilating fans are placed, or of the unpleasantly familiar<span class="pagenum" id="Page_74">74</span> +electric drill of the dentist. An electric motor is small and +compact, gives off no fumes and practically no heat, makes +very little noise, is capable of running for very long periods +at high speed and with the utmost steadiness, and requires +extremely little attention.</p> +<hr class="chap x-ebookmaker-drop"> + +<div class="chapter"> +<p><span class="pagenum" id="Page_75">75</span></p> + +<h2 class="nobreak" id="toclink_75"><a id="chapter_X"></a>CHAPTER X<br> + +<span class="subhead">ELECTRIC POWER STATIONS</span></h2> +</div> + +<p class="in0"><span class="firstword">It</span> is apparently a very simple matter to fit up a power +station with a number of very large dynamos driven by +powerful engines, and to distribute the current produced by +these dynamos to all parts of a town or district by means +of cables, but as a matter of fact it is a fairly complicated +engineering problem. First of all the source of power for +driving the dynamos has to be considered. In private and +other small power plants, gas, petrol or oil engines are +generally used, but for large stations the choice lies between +steam and water power. In this country steam power is +used almost exclusively. Formerly the ordinary reciprocating +steam engines were always employed, and though these +are still in very extensive use, they are being superseded in +many cases by steam turbines. The turbine is capable of +running at higher speeds than the reciprocating engine, and +at the greatest speeds it runs with a great deal less noise, +and with practically no vibration at all. More than this, +turbines take up much less room, and require less oil and +attendance. The turbines are coupled directly to the +dynamos, so that the two machines appear almost as one. +In the power station shown on <a href="#plate_V">Plate V</a>. a number of alternating +current dynamos coupled to steam turbines are seen.</p> + +<p>A large power station consumes enormous quantities of +coal, and for convenience of supply it is situated on the +bank of a river or canal, or, if neither of these is available,<span class="pagenum" id="Page_76">76</span> +as close to the railway as possible. The unloading of the +coal barges or trucks is done mechanically, the coal passing +into a large receiving hopper. From here it is taken to +another hopper close to the furnaces by means of coal +elevators and conveyors, which consist of a number of +buckets fixed at short intervals on an endless travelling +chain. From the furnace hopper the coal is fed into the +furnaces by mechanical stokers, and the resulting ash and +clinker falls into a pit below the furnaces, from which it is +carted away.</p> + +<p>The heat produced in the furnaces is used to generate +steam, and from the boilers the steam passes to the engines +along a steam pipe. After doing its work in the engines, +the steam generally passes to a condenser, in which it is +cooled to water, freed from oil and grease, and returned to +the boilers to be transformed once more into steam. As +this water from the condenser is quite warm, less heat is +required to raise steam from it than would be the case if +the boiler supply were kept up with cold water. The +power generated by the engines is used to drive the +dynamos, and stout copper cables convey the current from +these to what are called “bus” bars. There are two of +these, one receiving the positive cable from the dynamos, +and the other the negative cable, and the bars run from end +to end of a large main switchboard. From this switchboard +the current is distributed by other cables known as feeders.</p> + +<p>The nature of the current generated at a power station +is determined to a great extent by the size of the district to +be supplied. Generally speaking, where the current is not +to be transmitted beyond a radius of about two miles from +the station, continuous current is generated; while alternating +current is employed for the supply of larger areas. In +some cases both kinds of current are generated at one +station.</p> + +<figure id="plate_V" class="figcenter" style="max-width: 40em;"> + <p class="caption">PLATE V.</p> + <img src="images/i_095.jpg" width="3146" height="2015" alt=" "> + <figcaption class="caption"> + +<p class="floatl"><i>By permission of</i></p> +<p class="floatr"><i>C. A. Parsons & Co.</i></p> + +<p class="floatc">LOTS ROAD ELECTRIC POWER STATION, CHELSEA.</p> +</figcaption></figure> + +<p>If continuous current is to be used, it is generated +usually at a pressure of from 400 to 500 volts, the average +being about 440 volts; and the supply is generally on what +is known as the three-wire system. Three separate wires +are employed. The two outer wires are connected +respectively to the positive and the negative bus bars +running along the main switchboard, these bars receiving +positive or negative current directly from the dynamos. +The outer wires therefore carry current at the full voltage +of the system. Between them is a third and smaller wire, +connected to a third bar, much smaller than the outer bars, +and known as the mid-wire bar. This bar is not connected +to the dynamos, but to earth, by means of a large plate of +copper sunk into the ground. Connexion between the +mid-wire bar and the outer bars is made by two machines +called “balancers,” one connecting the mid-wire bar and the +positive bus bar, and the other the mid-wire bar and the +negative bus bar. If the pressure between the outer bars +is 440 volts, then the pressure between the mid-wire bar and +either of the outer bars will be 220 volts, that is just half.</p> + +<p>The balancers serve the purpose of balancing the +voltage on each side, and they are machines capable of +acting either as motors or dynamos. In order to comply +with Board of Trade regulations, electric appliances of all +kinds intended for ordinary domestic purposes, including +lamps, and heating and cooking apparatus, are supplied +with current at a pressure not exceeding 250 volts. In a +system such as we are describing, all these appliances are +connected between the mid-wire and one or other of the +outer wires, thus receiving current at 220 volts. In +practice it is impossible to arrange matters so that the +lamps and other appliances connected with the positive side +of the system shall always take the same amount of current +as those connected with the negative side, and there is<span class="pagenum" id="Page_78">78</span> +always liable to be a much greater load on one side or the +other. If, for instance, a heavy load is thrown on the +negative side, the voltage on that side will drop. The +balancer on the positive side then acts as an electric motor, +drives the balancer on the negative side as a dynamo, and +thus provides the current required to raise the voltage on +the negative side until the balance is restored. The working +of the balancers, which need not be described in further +detail, is practically automatic. Electric motors, for driving +electric trams or machinery of any kind, are connected +between the outer wires, so that they receive the full 440 +volts of the system.</p> + +<p>In any electric supply system the demand for current +does not remain constant, but fluctuates more or less. For +instance, in a system including an electric tramway, if a car +breaks down and remains a fixture for a short time, all cars +behind it are held up, and a long line of cars is quickly +formed. When the breakdown is repaired, all the cars +start practically at the same instant, and consequently a +sudden and tremendous demand for current is made. In a +very large tramway system in a fairly level city, the +fluctuations in the demand for current, apart from accidents, +are not very serious, for they tend to average themselves; +but in a small system, and particularly if the district is hilly, +the fluctuations are very great, and the current demand +may vary as much as from 400 to 2000 amperes. Again, +in a system supplying power and light, the current demand +rises rapidly as the daylight fails on winter afternoons, +because, while workshop and other motors are still in full +swing, thousands of electric lamps are switched on more or +less at the same time. The power station must be able to +deal with any exceptional demands which are likely to +occur, and consequently more current must be available +than is actually required under average conditions. Instead<span class="pagenum" id="Page_79">79</span> +of having generating machinery large enough to meet all +unusual demands, the generators at a station using continuous +current may be only of sufficient size to supply a little +more than the average demand, any current beyond this +being supplied by a battery of storage cells. The battery +is charged during periods when the demand for current is +small, and when a heavy load comes on, the current from +the battery relieves the generators of the sudden strain. +To be of any service for such a purpose the storage battery +of course must be very large. <a href="#plate_VI">Plate VI</a>. shows a large +battery of no cells, and some idea of the size of the +individual cells may be obtained from the fact that each +weighs about 3900 lb.</p> + +<p>Alternating current is produced at almost all power +stations supplying large districts. It is generated at high +pressure, from 2000 volts upwards, the highest pressure +employed in this country being about 11,000 volts. Such +pressures are of course very much too high for electric +lamps or motors, and the object of generating current of +this kind is to secure the greatest economy in transmission +through the long cables. Electric energy is measured in +watts, the watts being obtained by multiplying together +the pressure or voltage of the current, and its rate of flow +or amperage. From this it will be seen that, providing the +product of voltage and amperage remains the same, it +makes no difference, so far as electric energy is concerned, +whether the current be of high voltage and low amperage, +or of low voltage and high amperage. Now in transmitting +a current through a long cable, there is a certain +amount of loss due to the heating of the conductor. This +heating is caused by the current flow, not by the pressure; +and the heavier the current, the greater the heating, and +the greater the loss. This being so, it is clear that by +decreasing the current flow, and correspondingly increasing<span class="pagenum" id="Page_80">80</span> +the pressure, the loss in transmission will be reduced; and +this is why alternating current is generated at high pressure +when it is to be transmitted to a distance.</p> + +<p>The kind of alternating current generated is usually +that known as three-phase current. Formerly single-phase +current was in general use, but it has been superseded by +three-phase current because the latter is more economical +to generate and to distribute, and also more satisfactory for +electric motors. The actual voltage of the current sent out +from the station varies according to the distance to which +the current is to be conveyed. In the United States and +in other countries where current has to be conveyed to +places a hundred or even more miles from the station, +pressures as high as 120,000 volts are in use. It is possible +to produce alternating current at such pressures directly +from the dynamos, but in practice this is never done, on +account of the great liability to breakdown of the insulation. +Instead, the current is generated at from 2000 to 10,000 or +11,000 volts, and raised to the required pressure, before +leaving the station, by means of a step-up transformer. +We have seen that an induction coil raises, or steps up, the +voltage of the current supplied to it. A step-up transformer +works on the same principle as the induction coil, and in +passing through it the current is raised in voltage, but +correspondingly lowered in amperage. Of course, if the +pressure of the current generated by the dynamos is already +sufficiently high to meet the local requirements, the transformer +is not used.</p> + +<figure id="plate_VI" class="figcenter" style="max-width: 42em;"> + <p class="caption">PLATE VI.</p> + <img src="images/i_101.jpg" width="3301" height="2190" alt=" "> + <figcaption class="caption"> + +<p class="floatl"><i>By permission of</i></p> +<p class="floatr"><i>Chloride Electrical Storage Co. Ltd.</i></p> + +<p class="floatc">POWER STATION BATTERY OF ACCUMULATORS.</p> +</figcaption></figure> + +<p>For town supply the current from the power station is +led along underground cables to a number of sub-stations, +situated in different parts of the town, and generally underground. +At each sub-station the current passes through a +step-down transformer, which also acts on the principle of the +induction coil, but in the reverse way, so that the voltage is +lowered instead of being raised. From the transformer the +current emerges at the pressure required for use, but it is +still alternating current; and if it is desired to have a +continuous-current supply this alternating current must be +converted. One of the simplest arrangements for this +purpose consists of an electric motor and a dynamo, the +two being coupled together. The motor is constructed to +run on the alternating current from the transformer, and +it drives the dynamo, which is arranged to generate continuous +current. There is also a machine called a “rotary +converter,” which is largely used instead of the motor +generator. This machine does the work of both motor and +dynamo, but its action is too complicated to be described +here. From the sub-stations the current, whether converted +or not, is distributed as required by a network of +underground cables.</p> + +<p>In many parts of the world, especially in America, +water power is utilized to a considerable extent instead of +steam for the generation of electric current. The immense +volume of water passing over the Falls of Niagara develops +energy equal to about seven million horse-power, and a +small amount of this energy, roughly about three-quarters +of a million horse-power, has been harnessed and made to +produce electric current for light and power. The water +passes down a number of penstocks, which are tubes or +tunnels about 7 feet in diameter, lined with brick and +concrete; and at the bottom of these tubes are placed +powerful water turbines. The falling water presses upon +the vanes of the turbines, setting them revolving at great +speed, and the power produced in this way is used to drive +a series of very large alternating current dynamos. The +current is conveyed at a pressure of about 60,000 volts +to various towns within a radius of 200 or 300 miles, +and it is anticipated that before very long the supply will<span class="pagenum" id="Page_82">82</span> +be extended to towns still more distant. Many other +American rivers have been harnessed in a similar way, +though not to the same extent; and Switzerland and +Norway are utilizing their water power on a rapidly +increasing scale. In England, owing to the abundance of +coal, little has been done in this direction. Scotland is +well favoured in the matter of water power, and it is +estimated that the total power available is considerably +more than enough to run the whole of the railways of that +country. Very little of this power has been utilized however, +and the only large hydro-electric installation is the +one at Kinlochleven, in Argyllshire. It is a mistake to +suppose that water power means power for nothing, but +taking things all round the cost of water power is considerably +lower than that of steam.</p> +<hr class="chap x-ebookmaker-drop"> + +<div class="chapter"> +<p><span class="pagenum" id="Page_83">83</span></p> + +<h2 class="nobreak" id="toclink_83"><a id="chapter_XI"></a>CHAPTER XI<br> + +<span class="subhead">ELECTRICITY IN LOCOMOTION</span></h2> +</div> + +<p class="in0"><span class="firstword">The</span> electric tramcar has become such a necessary feature +of our everyday life that it is very difficult to realize how +short a time it has been with us. To most of us a horse-drawn +tramcar looks like a relic of prehistoric times, and +yet it is not so many years since the horse tram was in full +possession of our streets. Strikes of tramway employees +are fortunately rare events, but a few have occurred during +the past two or three years in Leeds and in other towns, +and they have brought home to us our great dependence +upon the electric tram. During the Leeds strike the streets +presented a most curious appearance, and the city seemed +to have made a jump backward to fifty years ago. Every +available article on wheels was pressed into service to bring +business men into the city from the outlying districts, and +many worthy citizens were seen trying to look dignified +and unconcerned as they jogged along in conveyances +which might have come out of the Ark. On such an +occasion as this, if we imagine the electric light supply +stopped also, we can form some little idea of our indebtedness +to those who have harnessed electricity and made it +the greatest power of the twentieth century.</p> + +<p>There are three distinct electric tramway systems; the +trolley or overhead system, the surface contact system, +and the conduit system. The trolley system has almost +driven the other two from the field, and it is used almost<span class="pagenum" id="Page_84">84</span> +exclusively throughout Great Britain and Ireland. On the +Continent and in the United States the conduit system still +survives, but probably it will not be long before the trolley +system is universally employed.</p> + +<p>The superiority of the trolley system lies in the fact that +it is cheaper to construct and to maintain than the other +two, and also in its much greater reliability under all +working conditions. The overhead wire is not one continuous +cable, but is divided into sections of about half a +mile in length, each section being supplied with current +from a separate main. At each point where the current +is fed to the trolley wire a sort of metal box may be seen +at the side of the street. These boxes are called “feeder +pillars,” and each contains a switch by means of which the +current can be cut off from that particular section, for +repairing or other purposes. Above the car is fixed an arm +provided with a trolley wheel which runs along the wire, +and this wheel takes the current from the wire. From the +wheel the current passes down the trolley arm to the +controller, which is operated by the driver, and from there +to the motors beneath the car. Leaving the motors it +passes to the wheels and then to the rails, from which it +is led off at intervals by cables and so returned to the +generating station. The current carried by the rails is at +a pressure of only a few volts, so that there is not the +slightest danger of shock from them. There are generally +two electric motors beneath the car, and the horse-power +of each varies from about fifteen to twenty-five.</p> + +<p>The controller consists mainly of a number of graduated +resistances. To start the car the driver moves a handle +forward notch by notch, thus gradually cutting out the +resistance, and so the motors receive more and more +current until they are running at full speed. The movement +of the controller handle also alters the connexion of<span class="pagenum" id="Page_85">85</span> +the motors. When the car is started the motors are +connected in series, so that the full current passes through +each, while the pressure is divided between them; but +when the car is well on the move the controller connects +the motors in parallel, so that each receives the full pressure +of the current.</p> + +<p>The conduit and surface contact systems are much the +same as the trolley system except in the method of supplying +the current to the cars. In the conduit system two +conductors conveying the current are placed in an underground +channel or conduit of concrete strengthened by iron +yokes. The top of the conduit is almost closed in so as to +leave only a narrow slot, through which passes the current +collector of the car. This current collector, or “plough” as +it is called, carries two slippers which make contact with the +conductors, and thus take current from them. In this +system the current returns along one of the conductors, so +that no current passes along the track rails. This is the +most expensive of the three systems, both in construction +and maintenance.</p> + +<p>The surface contact or stud system is like the conduit +system in having conductors placed in a sort of underground +trough, but in this case contact with the conductors +is made by means of metal studs fixed at intervals in the +middle of the track. The studs are really the tops of +underground boxes each containing a switch, which, when +drawn up to a certain position, connects the stud to the +conductors. These switches are arranged to be moved by +magnets fixed beneath the car, and thus when the car +passes over a stud the magnets work the switch and connect +the stud to the conductors, so that the stud is then +“alive.” The current is taken from the studs by means of +sliding brushes or skates which are carried by the car. +The studs are thus alive only when the car is passing over<span class="pagenum" id="Page_86">86</span> +them, and at all other times they are dead, and not in any +way dangerous.</p> + +<p>The weight and speed of electric cars make it important +to have a thoroughly reliable system of brakes. First of +all there are ordinary mechanical brakes, which press +against the wheels. Then there are electro-magnetic +slipper brakes which press on the rails instead of on the +wheels of the car. These brakes are operated by electro-magnets +of great power, the current necessary to excite the +magnets being taken from the motors. Finally there is +a most interesting and ingenious method of regenerative +control. Before a car can be stopped after it has attained +considerable speed a certain amount of energy has to be +got rid of in some way. With the ordinary mechanical or +electro-magnetic brakes this energy is wasted, but in the +regenerative method it is turned into electric current, which +is sent back into the circuit. If an electric motor is supplied +with mechanical power instead of electric current it becomes +a dynamo, and generates current instead of using it. In +the regenerative system, when a car is “coasting” down a +hill it drives the wheels, and the wheels drive the motors, +so that the latter become dynamos and generate current +which is sent back to the power station. In this way some +of the abnormal amount of current taken by a car in climbing +a hill is returned when the car descends the hill. The +regenerative system limits the speed of the car, so that it +cannot possibly get beyond control.</p> + +<figure id="plate_VII" class="figcenter" style="max-width: 39em;"> + <p class="caption">PLATE VII.</p> + <img src="images/i_109.jpg" width="3105" height="1980" alt=" "> + <figcaption class="caption"> + +<p class="floatl"><i>By permission of</i></p> +<p class="floatr"><i>Siemens Brothers Dynamo Works Ltd.</i></p> + +<p class="floatc">ELECTRIC COLLIERY RAILWAY.</p> +</figcaption></figure> + +<p>A large tramway system spreads outwards from the +centre of a city to the suburbs, and usually terminates at +various points on the outskirts of these suburbs. It often +happens that there are villages lying some distance beyond +these terminal points, and it is very desirable that there +should be some means of transport between these villages +and the city. An extension of the existing tramway is not +practicable in many cases, because the traffic would not be +sufficient to pay for the heavy outlay, and also because the +road may not be of sufficient width to admit of cars running +on a fixed track. The difficulty may be overcome satisfactorily +by the use of trackless trolley cars. With these +cars the costly business of laying a rail track is altogether +avoided, only a system of overhead wires being necessary. +As there is no rail to take the return current, a second +overhead wire is required. The car is fitted with two +trolley arms, and the current is taken from one wire by the +first arm, sent through the controller and the motors, and +returned by the second arm to the other wire, and so back +to the generating station. The trolley poles are so arranged +that they allow the car to be steered round obstructions +or slow traffic, and the car wheels are usually fitted +with solid rubber tyres. Trackless cars are not capable of +dealing with a large traffic, but they are specially suitable +where an infrequent service, say a half-hourly one, is enough +to meet requirements.</p> + +<p>We come now to electric railways. These may be +divided into two classes, those with separate locomotives +and those without. The separate locomotive method is +largely used for haulage purposes in collieries and large +works of various kinds. In <a href="#plate_VII">Plate VII</a>. is seen an electric +locomotive hauling a train of coal waggons in a colliery +near the Tyne, and it will be seen that the overhead +system is used, the trolley arm and wheel being replaced +by sliding bows. In a colliery railway it is generally +impossible to select the most favourable track from the +railway constructor’s point of view, as the line must be +arranged to serve certain points. This often means taking +the line sometimes through low tunnels or bridges where +the overhead wire must be low, and sometimes over public +roads where the wire must be high; and the sliding bow<span class="pagenum" id="Page_88">88</span> +is better able than the trolley arm and wheel to adapt +itself to these variations. In the colliery where this locomotive +is used the height of the overhead wire ranges +from 10 feet 6 inches through tunnels or bridges, to 21 feet +where the public road is crossed. The locomotive weighs +33½ tons, and has four electric motors each developing +50 horse-power with the current employed. It will be +noticed that the locomotive has two sets of buffers. This +is because it has to deal with both main line waggons and +the smaller colliery waggons, the upper set of buffers being +for the former, and the lower and narrower set for the +latter. <a href="#plate_VIII">Plate VIII</a>. shows a 50-ton locomotive on the +British Columbia Electric Railway, and a powerful locomotive +in use in South America. In each case it will be +seen that the trolley wheel is used.</p> + +<p>In this country electric railways for passenger traffic +are mostly worked on what is known as the multiple-unit +system, in which no separate locomotives are used, the +motors and driving mechanism being placed on the cars +themselves. There are also other cars without this equipment, +so that a train consists of a single motor-car with or +without trailer, or of two motor-cars with trailer between, +or in fact of any other combination. When a train +contains two or more motor-cars all the controllers, which +are very similar to those on electric tramcars, are electrically +connected so as to be worked together from one +master controller. This system allows the length of the +train to be adjusted to the number of passengers, so that +no power is wasted in running empty cars during periods +of small traffic. In suburban railways, where the stopping-places +are many and close together, the efficiency of the +service depends to a large extent upon the time occupied +in bringing the trains from rest to full speed. In this +respect the electric train has a great advantage over the<span class="pagenum" id="Page_89">89</span> +ordinary train hauled by a steam locomotive, for it can +pick up speed at three or more times the rate of the latter, +thus enabling greater average speeds and a more frequent +service to be maintained.</p> + +<p>Electric trains are supplied with current from a central +generating station, just as in the case of electric tramcars, +but on passenger lines the overhead wire is in most cases +replaced by a third rail. This live rail is placed upon +insulators just outside the track rail, and the current is +collected from it by sliding metal slippers which are carried +by the cars. The return current may pass along the +track rails as in the case of trolley tramcars, or be conveyed +by another insulated conducting rail running along +the middle of the track.</p> + +<p>The electric railways already described are run on continuous +current, but there are also railways run on alternating +current. A section of the London, Brighton, and South +Coast Railway is electrically operated by alternating +current, the kind of current used being that known as +single-phase. The overhead system is used, and the +current is led to the wire at a pressure of about 6000 volts. +This current is collected by sliding bows and conveyed to +transformers carried on the trains, from which it emerges +at a pressure of about 300 volts, and is then sent through +the motors. The overhead wires are not fixed directly to +the supports as in the case of overhead tramway wires, but +instead two steel cables are carried by the supports, and +the live wires are hung from these. The effect of this +arrangement is to make the sliding bows run steadily and +evenly along the wires without jumping or jolting. If ever +electricity takes the place of steam for long distance railway +traffic, this system, or some modification of it, probably +will be employed.</p> + +<p>Mention must be made also of the Kearney high speed<span class="pagenum" id="Page_90">90</span> +electric mono-railway. In this system the cars, which are +electrically driven, are fitted above and below with grooved +wheels. The lower wheels run on a single central rail +fixed to sleepers resting on the ground, and the upper +wheels run on an overhead guide rail. It is claimed that +speeds of 150 miles an hour are attainable with safety and +economy in working. This system is yet only just out of +the experimental stage, but its working appears to be +exceedingly satisfactory.</p> + +<p>A self-contained electric locomotive has been constructed +by the North British Locomotive Company. It +is fitted with a steam turbine which drives a dynamo generating +continuous current, and the current is used to drive +four electric motors. This locomotive has undergone extensive +trials, but its practical value as compared with the +ordinary type of electric locomotive supplied with current +from an outside source is not yet definitely established.</p> + +<p>At first sight it appears as though the electric storage +cell or accumulator ought to provide an almost perfect +means of supplying power for self-propelled electric vehicles +of all kinds. In practice, however, it has been found that +against the advantages of the accumulator there are to be +set certain great drawbacks, which have not yet been +overcome. Many attempts have been made to apply +accumulator traction to electric tramway systems, but they +have all failed, and the idea has been abandoned. There +are many reasons for the failure of these attempts. The +weight of a battery of accumulators large enough to run a +car with a load of passengers is tremendous, and this is +of course so much dead weight to be hauled along, and it +becomes a very serious matter when steep hills have to be +negotiated. When a car is started on a steep up-gradient +a sudden and heavy demand for current is made, and this +puts upon the accumulators a strain which they are not<span class="pagenum" id="Page_91">91</span> +able to bear without injury. Another great drawback is +the comparatively short time for which accumulators can +give a heavy current, for this necessitates the frequent +return of the cars to the central station in order to have +the batteries re-charged. Finally, accumulators are sensitive +things, and the continuous heavy vibration of a tramcar +is ruinous to them.</p> + +<p>The application of accumulators to automobiles is much +more feasible, and within certain limits the electric motor-car +may be considered a practical success. The electric +automobile is superior to the petrol-driven car in its delightfully +easy and silent running, and its freedom from all +objectionable smells. On the other hand high speeds +cannot be attained, and there is the trouble of having the +accumulators re-charged, but for city work this is not a +serious matter. Two sets of accumulators are used, so that +one can be left at the garage to be charged while the other +is in use, the replacing of the exhausted set by the freshly +charged one being a matter of only a few minutes. The +petrol-driven car is undoubtedly superior in every way for +touring purposes. Petrol can now be obtained practically +anywhere, whereas accumulator charging stations are comparatively +few and far between, especially in country +districts; and there is no comparison as regards convenience +between the filling of a petrol tank and the charging of a +set of accumulators, for one process takes a few minutes +and the other a few hours.</p> + +<p>Accumulator-driven locomotives are not in general use, +but for certain special purposes they have proved very +satisfactory. A large locomotive of this kind was used for +removing excavated material and for taking in the iron +segments, sleepers, rails, and other materials in the construction +of the Great Northern, Piccadilly, and Brompton +Tube Railway. This locomotive is 50 feet 6 inches long,<span class="pagenum" id="Page_92">92</span> +and it carries a battery of eighty large “chloride” cells, the +total weight of locomotive and battery being about 64 +tons. It is capable of hauling a load of 60 tons at a +rate of from 7 to 9 miles an hour on the level.</p> + +<p>Amongst the latest developments of accumulator traction +is a complete train to take the place of a steam locomotive +hauling a single coach on the United Railways of Cuba. +According to the <cite>Scientific American</cite> the train consists +of three cars, each having a battery of 216 cells, supplying +current at 200 volts to the motors. Each car has accommodation +for forty-two passengers, and the three are arranged +to work on the multiple-unit system from one master controller. +The batteries will run from 60 to 100 miles for +each charging of seven hours.</p> +<hr class="chap x-ebookmaker-drop"> + +<div class="chapter"> +<p><span class="pagenum" id="Page_93">93</span></p> + +<h2 class="nobreak" id="toclink_93"><a id="chapter_XII"></a>CHAPTER XII<br> + +<span class="subhead">ELECTRIC LIGHTING</span></h2> +</div> + +<p class="in0"><span class="firstword">In</span> the first year of the nineteenth century one of the +greatest of England’s scientists, Sir Humphry Davy, +became lecturer on chemistry to the Royal Institution, +where his brilliant lectures attracted large and enthusiastic +audiences. He was an indefatigable experimenter, and in +order to help on his work the Institution placed at his +disposal a very large voltaic battery consisting of 2000 +cells. In 1802 he found that if two rods of carbon, one +connected to each terminal of his great battery, were +first made to touch one another and then gradually +separated, a brilliant arch of light was formed between +them. The intense brilliance of this electric arch, or <em>arc</em> +as it came to be called, naturally suggested the possibility +of utilizing Davy’s discovery for lighting purposes, but the +maintaining of the necessary current proved a serious +obstacle. The first cost of a battery of the required size +was considerable, but this was a small matter compared +with the expense of keeping the cells in good working +order. Several very ingenious and more or less efficient +arc lamps fed by battery current were produced by various +inventors, but for the above reason they were of little use +except for experimental purposes, and the commercial +success of the arc lamp was an impossibility until the +dynamo came to be a really reliable source of current. +Since that time innumerable shapes and forms of arc lamps<span class="pagenum" id="Page_94">94</span> +have been devised, while the use of such lamps has increased +by leaps and bounds. To-day, wherever artificial +illumination on a large scale is required, there the arc lamp +is to be found.</p> + +<p>When the carbon rods are brought into contact and +then slightly separated, a spark passes between them. +Particles of carbon are torn off by the spark and volatilized, +and these incandescent particles form a sort of bridge which +is a sufficiently good conductor for the current to pass +across it from one rod to the other. When the carbons +are placed horizontally, the glowing mass is carried upwards +by the ascending currents of heated air, and it assumes the +arch-like form from which it gets its name. If the carbons +are vertical the curve is not produced, a more or less +straight line being formed instead. The electric arc may be +formed between any conducting substances, but for practical +lighting purposes carbon is found to be most suitable.</p> + +<p>Either continuous or alternating currents may be used +to form the arc. With continuous current, if the carbon +rods are fully exposed to the air, they gradually consume +away, and minute particles of carbon are carried across +from the positive rod to the negative rod, so that the former +wastes at about twice the rate of the latter. The end of +the positive rod becomes hollowed out so as to resemble a +little crater, and the end of the negative rod becomes more +or less pointed. The fact that with continuous current the +positive rod consumes away twice as fast as the negative +rod, may be taken advantage of to decrease the cost of new +carbons, by replacing the wasted positive rod with a new +one, and using the unconsumed portion of the old positive +rod as a new negative rod.<a id="FNanchor_1" href="#Footnote_1" class="fnanchor">1</a> If alternating current is used, +each rod in turn becomes the positive rod, so that no crater<span class="pagenum" id="Page_95">95</span> +is formed, and both the carbons have the same shape and +are consumed at the same rate. A humming noise is liable +to be produced by the alternating current arc, but by careful +construction of the lamp this noise is reduced to the +minimum.</p> + +<div class="footnote"> + +<p><a id="Footnote_1" href="#FNanchor_1" class="label">1</a> In actual practice the positive carbon is made double the thickness of the +negative, so that the two consume at about the same rate.</p> + +</div> + +<p>If the carbons are enclosed in a suitable globe the +rate of wasting is very much less. The oxygen inside the +globe becomes rapidly consumed, and although the globe +is not air-tight, the heated gases produced inside it check +the entrance of further supplies of fresh air as long as the +lamp is kept burning. When the light is extinguished, +and the lamp cools down, fresh air enters again freely.</p> + +<p>Arc lamp carbons may be either solid or cored. The +solid form is made entirely of very hard carbon, while the +cored form consists of a narrow tube of carbon filled up with +soft graphite. Cored carbons usually burn more steadily +than the solid form. In what are known as flame arc +lamps the carbons are impregnated with certain metallic +salts, such as calcium. These lamps give more light for +the same amount of current. The arc is long and flame-like, +and usually of a striking yellow colour, but it is not so +steady as the ordinary arc.</p> + +<figure id="fig_21" class="figright" style="max-width: 13em;"> + <img src="images/i_120.png" width="1027" height="1092" alt=" "> + <figcaption class="caption hang"><span class="smcap">Fig. 21.</span>—Diagram showing simple +method of carbon regulation for Arc Lamps. +</figcaption></figure> + +<p>As the carbon rods waste away, the length of the arc +increases, and if this increase goes beyond a certain limit +the arc breaks and the current ceases. If the arc is to be +kept going for any length of time some arrangement for +pushing the rods closer together must be provided, in order +to counteract the waste. In arc lamps this pushing together, +or “feeding” as it is called, is done automatically, as +is also the first bringing together and separating of the rods +to start or strike the arc. <a href="#fig_21">Fig. 21</a> shows a simple arrangement +for this purpose. A is the positive carbon, and B +the negative. C is the holder for the positive carbon, and +this is connected to the rod D, which is made of soft iron.<span class="pagenum" id="Page_96">96</span> +This rod is wound with two separate coils of wire as shown, +coil E having a low resistance, and coil F a high one. +These two coils are solenoids, and D is the core, +(<a href="#chapter_VII">Chapter VII</a>.). When the lamp is not in use, the weight of +the holder keeps the positive carbon in contact with the +negative carbon. When switched on, the current flows +along the cable to the point H. Here it has two paths +open to it, one through coil E to the positive carbon, and +the other through coil F and back to the source of supply. +But coil E has a much lower +resistance than coil F, and +so most of the current +chooses the easier path +through E, only a small +amount of current taking +the path through the other +coil. Both coils are now +magnetized, and E tends to +draw the rod D upwards, +while F tends to pull it +downwards. Coil E, however, +has much greater power +than coil F, because a much +larger amount of current is +passing through it; and so it overcomes the feeble pull of F, +and draws up the rod. The raising of D lifts the positive +carbon away from the negative carbon, and the arc is struck. +The carbons now begin to waste away, and very slowly the +distance between them increases. The path of the current +passing through coil E is from carbon A to carbon B by +way of the arc, and as the length of the gap between A +and B increases, the resistance of this path also increases. +The way through coil E thus becomes less easy, and as +time goes on more and more current takes the alternative<span class="pagenum" id="Page_97">97</span> +path through coil F. This results in a decrease in the +magnetism of E, and an increase in that of F, and at a +certain point F becomes the more powerful of the two, and +pulls down the rod. In this way the positive carbon is +lowered and brought nearer to the negative carbon. +Directly the diminishing distance between A and B reaches +a certain limit, coil E once more asserts its superiority, and +by overcoming the pull of F it stops the further approach +of the carbons. So, by the opposing forces of the two +coils, the carbons are maintained between safe limits, in +spite of their wasting away.</p> + +<figure id="plate_IXa" class="figcenter" style="max-width: 29em;"> + <p class="caption">PLATE IX.</p> + <img src="images/i_121.jpg" width="2246" height="1522" alt=" "> +</figure> + +<figure id="plate_IXb" class="figcenter" style="max-width: 22em;"> + <img src="images/i_121b.jpg" width="1689" height="2160" alt=" "> + <figcaption class="caption"> + +<p class="floatl"><i>By permission of</i></p> +<p class="floatr"><i>Union Electric Co. Ltd.</i></p> + +<p class="floatc">NIGHT PHOTOGRAPHS, TAKEN BY THE LIGHT OF THE ARC LAMPS.</p> +</figcaption></figure> + +<p>The arc lamp is largely used for the illumination +of wide streets, public squares, railway stations, and the +exteriors of theatres, music-halls, picture houses, and large +shops. The intense brilliancy of the light produced may be +judged from the accompanying photographs (<a href="#plate_IXa">Plate IX</a>.), +which were taken entirely by the light of the arc lamps. +Still more powerful arc lamps are constructed for use in +lighthouses. The illuminating power of some of these +lamps is equal to that of hundreds of thousands of +candles, and the light, concentrated by large reflectors, is +visible at distances varying from thirty to one hundred +miles.</p> + +<p>Arc lamps are also largely used for lighting interiors, +such as large showrooms, factories or workshops. For +this kind of lighting the dazzling glare of the outdoor +lamp would be very objectionable and harmful to the eyes, +so methods of indirect lighting are employed to give a soft +and pleasant light. Most of the light in the arc lamp comes +from the positive carbon, and for ordinary outdoor lighting +this carbon is placed above the negative carbon. In lamps +for interior lighting the arrangement is frequently reversed, +so that the positive carbon is below. Most of the light is +thus directed upwards, and if the ceiling is fairly low and<span class="pagenum" id="Page_98">98</span> +of a white colour the rays are reflected by it, and a soft and +evenly diffused lighting is the result. Some light comes +also from the negative carbon, and those downward rays +are reflected to the ceiling by a reflector placed beneath the +lamp. Where the ceiling is very high or of an unsuitable +colour, a sort of artificial ceiling in the shape of a large +white reflector is placed above the lamp to produce the +same effect. Sometimes the lamp is arranged so that part +of the light is reflected to the ceiling, and part transmitted +directly through a semi-transparent reflector below the +lamp. The composition of the light of the arc lamp is very +similar to that of sunlight, and by the use of such lamps the +well-known difficulty of judging and matching colours by +artificial light is greatly reduced. This fact is of great +value in drapery establishments, and the arc lamp has +proved a great success for lighting rooms used for night +painting classes.</p> + +<p>The powerful searchlights used by warships are arc +lamps provided with special arrangements for projecting +the light in any direction. A reflector behind the arc concentrates +the light and sends it out as a bundle of parallel +rays, and the illuminating power is such that a good searchlight +has a working range of nearly two miles in clear +weather. According to the size of the projector, the +illumination varies from about 3000 to 30,000 or 40,000 +candle-power. For some purposes, such as the illuminating +of narrow stretches of water, a wider beam is required, and +this is obtained by a diverging lens placed in front of the +arc. In passing through this lens the light is dispersed or +spread out to a greater or less extent according to the +nature of the lens. Searchlights are used in navigating +the Suez Canal by night, for lighting up the buoys along +the sides of the canal. The ordinary form of searchlight +does this quite well, but at the same time it illuminates<span class="pagenum" id="Page_99">99</span> +equally an approaching vessel, so that the pilot on this +vessel is dazzled by the blinding glare. To avoid this +dangerous state of things a split reflector is used, which +produces two separate beams with a dark space between +them. In this way the sides of the canal are illuminated, +but the light is not thrown upon oncoming vessels, so that +the pilots can see clearly.</p> + +<p>Glass reflectors are much more efficient than metallic +ones, but they have the disadvantage of being easily put +out of action by gunfire. This defect is remedied by protecting +the glass reflector by a screen of wire netting. +This is secured at the back of the reflector, and even if the +glass is shattered to a considerable extent, as by a rifle +bullet, the netting holds it together, and keeps it quite +serviceable. Reflectors protected in this way are not put +out of action by even two or three shots fired through +them. Searchlight arcs and reflectors are enclosed in metal +cylinders, which can be moved in any direction, vertically +or horizontally.</p> + +<p>In the arc lamps already described, a large proportion +of the light comes from the incandescent carbon electrodes. +About the year 1901 an American electrician, Mr. P. C. +Hewitt, brought out an arc lamp in which the electrodes +took no part in producing the light, the whole of which +came from a glowing stream of mercury vapour. This +lamp, under the name of the Cooper-Hewitt mercury +vapour lamp, has certain advantages over other electric +illuminants, and it has come into extensive use.</p> + +<figure id="fig_22" class="figcenter" style="max-width: 23em;"> + <img src="images/i_126.png" width="1775" height="1198" alt=" "> + <figcaption class="caption"><span class="smcap">Fig. 22.</span>—Sketch of Mercury Vapour Lamp. +</figcaption></figure> + +<p>It consists of a long glass tube, exhausted of air, and +containing a small quantity of mercury. Platinum wires to +take the current from the source of supply are sealed in at +each end. The tube is attached to a light tubular framework +of metal suspended from the ceiling, and this frame +is arranged so that it can be tilted slightly downwards by<span class="pagenum" id="Page_100">100</span> +pulling a chain. As shown in <a href="#fig_22">Fig. 22</a>, the normal position +of the lamp is not quite horizontal, but tilted slightly downwards +towards the end of the tube having the bulb containing +the mercury. The platinum wire at this end dips +into the mercury, so making a metallic contact with it. +The lamp is lighted by switching on the current and pulling +down the chain. The altered angle makes the mercury +flow along the tube towards the other platinum electrode, +and as soon as it touches this a conducting path for the +current is formed from end to end of the tube. The lamp +is now allowed to fall back to its original angle, so that the +mercury returns to its bulb. There is now no metallic connexion +between the electrodes, but the current continues +to pass through the tube as a vacuum discharge. Some +of the mercury is immediately vaporized and rendered +brilliantly incandescent, and so the light is produced. The +trouble of pulling down the chain is avoided in the +automatic mercury vapour lamp, which is tilted by an +electro-magnet. This magnet is automatically cut out of<span class="pagenum" id="Page_101">101</span> +circuit as soon as the tilting is completed and the arc +struck.</p> + +<p>The average length of the tube in the ordinary form of +mercury vapour lamp is about 30 inches, and a light of +from 500 to 3000 candle-power is produced, according to +the current used. Another form, known as the “Silica” +lamp, is enclosed in a globe like that of an ordinary electric +arc lamp. The tube is only about 5 or 6 inches in +length, and it is made of quartz instead of glass, the +arrangements for automatically tilting the tube being +similar to those in the ordinary form of lamp.</p> + +<p>The light of the mercury vapour lamp is different from +that of all other lamps. Its peculiarity is that it contains +practically no red rays, most of the light being yellow, with +a certain proportion of green and blue. The result is a +light of a peacock-blue colour. The absence of red rays +alters colour-values greatly, scarlet objects appearing +black; and on this account it is impossible to match colours +by this light. In many respects, however, the deficiency +in red rays is a great positive advantage. Every one who +has worked by mercury vapour light must have noticed +that it enables very fine details to be seen with remarkable +distinctness. This property is due to an interesting fact. +Daylight and ordinary artificial light is a compound or +mixture of rays of different colours. It is a well-known +optical fact that a simple lens is unable to bring all these +rays to the same focus; so that if we sharply focus an image +by red light, it is out of focus or blurred by blue light. This +defect of the lens is called “chromatic aberration.” The +eye too suffers from chromatic aberration, so that it cannot +focus sharply all the different rays at the same time. The +violet rays are brought to a focus considerably in front of +the red rays, and the green and the yellow rays come in +between the two. The eye therefore automatically and<span class="pagenum" id="Page_102">102</span> +unconsciously effects a compromise, and focuses for the +greenish-yellow rays. The mercury vapour light consists +very largely of these rays, and consequently it enables the +image to be focused with greater sharpness; or, in other +words, it increases the acuteness of vision. Experiments +carried out by Dr. Louis Bell and Dr. C. H. Williams +demonstrated this increase in visual sharpness very conclusively. +Type, all of exactly the same size, was examined +by mercury vapour light, and by the light from an electric +incandescent lamp with tungsten filament. The feeling of +sharper definition produced by the mercury vapour light +was so strong that many observers were certain that the +type was larger, and they were convinced that it was +exactly the same only after careful personal examination.</p> + +<p>Mercury vapour light apparently imposes less strain +upon the eyes than ordinary artificial light, and this +desirable feature is the result of the absence of the red rays, +which, besides having little effect in producing vision, are +tiring to the eyes on account of their heating action. The +light is very highly actinic, and for this reason it is largely +used for studio and other interior photographic work. In +cases where true daylight colour effects are necessary, a +special fluorescent reflector is used with the lamp. By +transforming the frequency of the light waves, this reflector +supplies the missing red and orange rays, the result being +a light giving normal colour effects.</p> + +<p>Another interesting vapour lamp may be mentioned +briefly. This has a highly exhausted glass tube containing +neon, a rare gas discovered by Sir William Ramsay. The +light of this lamp contains no blue rays, and it is of a +striking red colour. Neon lamps are used chiefly for +advertising purposes, and they are most effective for +illuminated designs and announcements, the peculiar and +distinctive colour of the light attracting the eye at once.</p> + +<p><span class="pagenum" id="Page_103">103</span></p> + +<p>An electric current meets with some resistance in +passing through any substance, and if the substance is a +bad conductor the resistance is very great. As the current +forces its way through the resistance, heat is produced, and +a very thin wire, which offers a high resistance, may be +raised to a white heat by an electric current, and it then +glows with a brilliant light. This fact forms the basis of +the electric incandescent or glow lamp.</p> + +<p>In the year 1878, Thomas A. Edison set himself the +task of producing a perfect electric incandescent lamp, +which should be capable of superseding gas for household +and other interior lighting. The first and the greatest +difficulty was that of finding a substance which could be +formed into a fine filament, and which could be kept +in a state of incandescence without melting or burning +away. Platinum was first chosen, on account of its very +high melting-point, and the fact that it was not acted +upon by the gases of the air. Edison’s earliest lamps +consisted of a piece of very thin platinum wire in the +shape of a spiral, and enclosed in a glass bulb from which +the air was exhausted. The ends of the spiral were +connected to outside wires sealed into the bulb. It was +found, however, that keeping platinum continuously at a +high temperature caused it to disintegrate slowly, so that +the lamps had only a short life. Fine threads or filaments +of carbon were then tried, and found to be much more +durable, besides being a great deal cheaper. The carbon +filament lamp quickly became a commercial success, and +up to quite recent years it was the only form of electric +incandescent lamp in general use.</p> + +<p>In 1903 a German scientist, Dr. Auer von Welsbach, +of incandescent gas mantle fame, produced an electric lamp +in which the filament was made of the metal osmium, and +this was followed by a lamp using the metal tantalum for<span class="pagenum" id="Page_104">104</span> +the filament, the invention of Siemens and Halske. For a +while the tantalum lamp was very successful, but more +recently it has been superseded in popularity by lamps +having a filament of the metal tungsten. The success of +these lamps has caused the carbon lamp to decline in +favour. The metal filaments become incandescent much +more easily than the carbon filament, and for the same +candle-power the metal filament lamp consumes much less +current than the carbon lamp.</p> + +<p>The construction of tungsten lamps is very interesting. +Tungsten is a very brittle metal, and at first the lamps +were fitted with a number of separate filaments. These +were made by mixing tungsten powder with a sort of paste, +and then squirting the mixture through very small +apertures, so that it formed hair-like threads. Early in +1911 lamps having a filament consisting of a single continuous +piece of drawn tungsten wire were produced. It +had been known for some time that although tungsten was +so brittle at ordinary temperatures, it became quite soft +and flexible when heated to incandescence in the lamp, and +that it lost this quality again as soon as it cooled down. +A process was discovered by which the metal could be +made permanently ductile, by mechanical treatment while +in the heated state. In this process pure tungsten powder +is pressed into rods and then made coherent by heating. +While still hot it is hammered, and finally drawn out into +fine wires through diamond dies. The wire is no thicker +than a fine hair, and it varies in size from about 0·012 mm. +to about 0·375 mm., according to the amount of current it +is intended to take. It is mounted by winding it continuously +zigzag shape round a glass carrier, which has at +the top and the bottom a number of metal supports +arranged in the form of a star, and insulated by a central +rod of glass. One star is made of strong, stiff material,<span class="pagenum" id="Page_105">105</span> +and the other consists of fine wires of some refractory +metal, molybdenum being used in the Osram lamps. +These supports act as springs, and keep the wire securely +in its original shape, no matter in what position the lamp +is used. The whole is placed in a glass bulb, which is +exhausted of air and sealed up.</p> + +<p>For some purposes lamps with specially small bulbs are +required, and in these the tungsten wire is made in the +shape of fine spirals, instead of in straight pieces, so that +it takes up much less room. In the “Axial” lamp the +spiral is mounted in such a position that most of the light +is sent out in one particular direction.</p> + +<p>The latest development in electric incandescent lamps is +the “half-watt” lamp. The watt is the standard of electrical +energy, and it is the rate of work represented by a current +of one ampere at a pressure of 1 volt. With continuous +currents the watts are found very simply by multiplying +together the volts and the amperes. For instance, a +dynamo giving a current of 20 amperes at a pressure +of 50 volts would be called a 1000-watt dynamo. +With alternating currents the calculation is more complicated, +but the final result is the same. The ordinary form +of tungsten lamp gives about one candle-power for every +watt, and is known as a one-watt lamp. As its name +suggests, the half-watt lamp requires only half this amount +of energy to give the same candle-power, so that it is very +much more economical in current. In this lamp the +tungsten filament is wound in a spiral, but instead of being +placed in the usual exhausted bulb, it is sealed into a bulb +containing nitrogen gas. The increased efficiency is +obtained by running the filament at a temperature from +400° to 600° C. higher than that at which the filament in +the ordinary lamp is used.</p> + +<p>In spite of the great advances in artificial lighting<span class="pagenum" id="Page_106">106</span> +made during recent years, no one has yet succeeded in +producing light without heat. This heat is not wanted, +and it represents so much waste energy. It has often been +said that the glow-worm is the most expert of all illuminating +engineers, for it has the power of producing at will a +light which is absolutely without heat. Perhaps the nearest +approach to light without heat is the so-called “cold light” +invented by M. Dussaud, a French scientist. His device +consists of a revolving ring of exactly similar tungsten +lamps. Each of these lamps has current passed through it +in turn, and the duration of the current in each is so short, +being only a fraction of a second, that the lamp has not +sufficient time to develop any appreciable amount of heat. +The light from the ring of lamps is brought to a focus, and +passed through a lens to wherever it is required. Electric +incandescent lamps are made in a variety of sizes, each one +being intended for a certain definite voltage. If a lamp +designed for, say, 8 volts, is used on a circuit of +32 volts, its candle-power is greatly increased, while the +amount of current consumed is not increased in proportion. +In this way the lamp becomes a more efficient source +of light, but the “over-running,” as it is called, has a +destructive effect on the filament, so that the life of the +lamp is greatly shortened. In the Dussaud system however +the time during which each lamp has current passing +through it is so short, followed by a period of rest, that +the destructive effect of over-running is reduced to the +minimum; so that by using very high voltages an extremely +brilliant light is safely obtained with a comparatively +small consumption of current. It might be thought +that the constant interchange of lamps would result in an +unsteady effect, but the substitution of one lamp for another +is carried out so rapidly that the eye gets the impression of +perfect steadiness. The Dussaud system is of little use<span class="pagenum" id="Page_107">107</span> +for ordinary lighting purposes, but for lighthouse illumination, +photographic studio work, and the projection of +lantern slides and cinematograph films, it appears to be of +considerable value.</p> + +<p>Electric light has many advantages over all other +illuminants. It gives off very little heat, and does not use +up the oxygen in the air of a room as gas does; while by +means of flexible wires the lamps can be put practically +anywhere, so that the light may be had just where it is +wanted. Another great advantage is that the light may be +switched on without any trouble about matches, and there +is none of the danger from fire which always exists with +a flame.</p> + +<p>The current for electric lamps is generally taken from +the public mains, but in isolated country houses a dynamo +has to be installed on the premises. This is usually driven +by a small engine running on petrol or paraffin. In order +to avoid having to run the engine and dynamo continually, +the current is not taken directly from the dynamo, but from +a battery of accumulators. During the day the dynamo is +used to charge the accumulators, and these supply the +current at night without requiring any attention.</p> + +<p>Electric lighting from primary cells is out of the +question if a good light is wanted continuously for long +periods, for the process is far too costly and troublesome. +If a light of small candle-power is required for periods of +from a few minutes to about an hour, with fairly long +intervals of rest, primary cells may be made a success. +Large dry cells are useful for this purpose, but probably +the most satisfactory cell is the sack Leclanché. This is +similar in working to the ordinary Leclanché cell used for +bells, but the carbon mixture is placed in a canvas bag or +sack, instead of in a porous pot, and the zinc rod is replaced +by a sheet of zinc surrounding the sack. These cells give<span class="pagenum" id="Page_108">108</span> +about 1½ volt each, so that four, connected in series, +are required to light a 6-volt lamp. The lamps must +take only a very small current, or the cells will fail +quickly. Small metal filament lamps taking from a third +to half an ampere are made specially for this purpose, and +these always should be used. A battery of sack Leclanché +cells with a miniature lamp of this kind forms a convenient +outfit for use as a night-light, or for lighting a dark cupboard, +passage or staircase. Lamps with ruby glass, or +with a ruby cap to slip over the bulb, may be obtained for +photographic purposes. If the outfit is wanted for use as +a reading-lamp it is better to have two separate batteries, +and to use them alternately for short periods. With this +arrangement each battery has a short spell of work followed +by a rest, and the light may be kept on for longer periods +without overworking the cells.</p> +<hr class="chap x-ebookmaker-drop"> + +<div class="chapter"> +<p><span class="pagenum" id="Page_109">109</span></p> + +<h2 class="nobreak" id="toclink_109"><a id="chapter_XIII"></a>CHAPTER XIII<br> + +<span class="subhead">ELECTRIC HEATING</span></h2> +</div> + +<p class="in0"><span class="firstword">The</span> light of the electric incandescent lamp is produced by +the heating to incandescence of a thin filament of metal or +carbon, and the heat itself is produced by the electric +current forcing its way through the great resistance opposed +to it by the filament. In such lamps the amount of heat +produced is too small to be of much practical use, but by +applying the same principle on a larger scale we get an +effective electric heater.</p> + +<p>The most familiar and the most attractive of all electric +heaters is the luminous radiator. This consists of two or +more large incandescent lamps, having filaments of carbon. +The lamps are made in the form of long cylinders, the +glass being frosted, and they are set, generally in a vertical +position, in an ornamental case or frame of metal. This +case is open at the front, and has a metal reflector behind. +The carbon filaments are raised to an orange-red heat by +the passage of the current, and they then radiate heat rays +which warm the bulbs and any other objects in their path. +The air in contact with these heated bodies is warmed, +and gradually fills the room. This form of heater, with +its bright glowing lamps, gives a room a very cheerful +appearance.</p> + +<p>In the non-luminous heaters, or “convectors” as they are +called, the heating elements consist of strips of metal or +wires having a very high resistance. These are placed in<span class="pagenum" id="Page_110">110</span> +a frame and made red-hot by the current. Cold air enters +at the bottom of the frame, becomes warm by passing over +the heating elements, and rises out at top and into the +room. More cold air enters the frame and is heated in +the same way, and in a very short time the whole of the +air of the room becomes warmed. The full power of the +heater is used in the preliminary warming of the room, but +afterwards the temperature may be kept up with a much +smaller consumption of current, and special regulating +switches are provided to give different degrees of heat. +Although these heaters are more powerful than the +luminous radiators, they are not cheerful looking; but in +some forms the appearance is improved by an incandescent +lamp with a ruby glass bulb, which shines through the +perforated front of the frame.</p> + +<p>The Bastian, or red glow heater, has thin wires wound +in a spiral and enclosed in tubes made of quartz. These +tubes are transparent both to light and heat, and so the +pleasant glow of the red-hot wire is visible. A different +type of heater, the hot oil radiator, is very suitable for +large rooms. This has a wire of high resistance immersed +in oil, which becomes hot and maintains a steady +temperature.</p> + +<p>Electric cooking appliances, like the heaters just described, +depend upon the heating of resistance wires or +strips of metal. The familiar electric kettle has a double +bottom, and in the cavity thus formed is placed the resistance +material, protected by strips of mica, a mineral +substance very largely used in electrical appliances of all +kinds on account of its splendid insulating qualities. +Electric irons are constructed in much the same way as +kettles, and sometimes they are used with stands which +cut off the current automatically when the iron is laid down +upon them, so that waste and overheating are prevented.<span class="pagenum" id="Page_111">111</span> +There are also a great many varieties of electric ovens, +grillers, hot-plates, water-heaters, glue-pots, and foot and +bed warmers. These of course differ greatly in construction, +but as they all work on the same principle there +is no need to describe them.</p> + +<p>Electric hot-plates are used in an interesting way in +Glasgow, to enable the police on night duty to have a hot +supper. The plates are fitted to street telephone signal +boxes situated at points where a number of beats join. By +switching on current from the +public mains the policemen +are able to warm their food +and tea, and a supper interval +of twenty minutes is allowed. +Even policemen are sometimes +absent-minded, and to +avoid the waste of current and +overheating of the plate that +would result if a “bobby” +forgot to switch off, an arrangement +is provided which +automatically switches off the +current when the plate is not +in use.</p> + +<figure id="fig_23" class="figright" style="max-width: 13em;"> + <img src="images/i_137.jpg" width="1025" height="1188" alt=" "> + <figcaption class="caption"><span class="smcap">Fig. 23.</span>—Diagram to illustrate principle +of Electric Furnace. +</figcaption></figure> + +<p>We must turn now to +electric heating on a much larger scale, in the electric +furnaces used for industrial purposes. The dazzling +brilliance of the light from the electric arc lamp is due +to the intense heat of the stream of vaporized carbon +particles between the carbon rods, the temperature of this +stream being roughly about 5400° F. This great heat +is made use of in various industries in the electric arc +furnace. <a href="#fig_23">Fig. 23</a> is a diagram of a simple furnace of this +kind. A is a vertical carbon rod which can be raised or<span class="pagenum" id="Page_112">112</span> +lowered, and B is a bed of carbon, forming the bottom of +the furnace, and acting as a second rod. A is lowered +until it touches B, the current, either continuous or alternating, +is switched on, and A is then raised. The arc is +thus struck between A and B, and the material contained +in the furnace is subjected to intense heat. When the +proper stage is reached the contents of the furnace are +drawn off at C, and fresh material is fed in from above, +so that if desired the process may be kept going continuously. +Besides the electric arc furnace there are also +resistance furnaces, in which the heat is produced by +the resistance of a conductor to a current passing +through it. This conductor may be the actual substance +to be heated, or some other resisting material placed close +to it.</p> + +<p>It will be of interest to mention now one or two of +the uses of electric furnaces. The well-known substance +calcium carbide, so much used for producing acetylene gas +for lighting purposes, is a compound of calcium and +carbon; it is made by raising a mixture of lime and coke +to an intense heat in an electric furnace. The manufacture +of calcium carbide is carried on on a very large scale at +Niagara, with electric power obtained from the Falls, and +at Odda in Norway, where the power is supplied by the +river Tysse. Carborundum, a substance almost as hard +as the diamond, is largely used for grinding and polishing +purposes. It is manufactured by sending a strong current +through a furnace containing a core of coke surrounded by +a mixture of sand, sawdust, and carbon. The core becomes +incandescent, and the heating is continued until the +carbon combines with the sand, the process taking about +a day. Graphite, a kind of carbon, occurs naturally in the +form of plumbago, which is used for making black lead +pencils. It is obtained by mining, but many of the mines<span class="pagenum" id="Page_113">113</span> +are already worked out, and others will be exhausted +before long. By means of the electric furnace, graphite +can now be made artificially, by heating anthracite +coal, and at Niagara a quantity running into thousands +of tons is produced every year. Electric furnaces are +now largely employed, particularly in France, in the +production of the various alloys of iron which are used +in making special kinds of steel; and they are used also +to a considerable extent in the manufacture of quartz +glass.</p> + +<p>For many years past a great deal of time and money +has been spent in the attempt to make artificial diamonds. +Quite apart from its use in articles of jewellery, the +diamond has many very important industrial applications, +its value lying in its extreme hardness, which is not +equalled by any other substance. The very high price +of diamonds however is at present a serious obstacle to +their general use. If they could be made artificially on a +commercial scale they would become much cheaper, and +this would be of the greatest importance to many industries, +in which various more or less unsatisfactory substitutes are +now used on account of their much smaller cost. Recent +experiments seem to show that electricity will solve the +problem of diamond making. Small diamonds, one-tenth +of an inch long, have been made in Paris by means of the +electric arc furnace. The furnace contains calcium carbide, +surrounded by a mixture of carbon and lime, and the arc, +maintained by a very powerful current, is kept in operation +for several hours. A black substance, something like coke, +is formed round the negative carbon, and in this are found +tiny diamonds. The diamonds continue to increase slowly +in size during the time that the arc is at work, and it is estimated +that they grow at the rate of about one-hundredth +of an inch per hour. So far only small diamonds have<span class="pagenum" id="Page_114">114</span> +been made, but there seems to be no reason why large ones +should not be produced, by continuing the process for three +or four days.</p> + +<p>A chapter on electric heating would not be complete +without some mention of electric welding. Welding is the +process of uniting two pieces of metal by means of a combination +of heat and pressure, so that a strong and permanent +joint is produced. The chief difficulty in welding is +that of securing and keeping up the proper temperature, +and some metals are much more troublesome than others +in this respect. Platinum, iron, and steel are fairly easy to +weld, but most of the other metals, and alloys of different +metals, require very exact regulation of temperature. It +is almost impossible to obtain this exact regulation by +ordinary methods of heating, but the electric current makes +it a comparatively easy matter. The principle of ordinary +electric welding is very simple. The ends of the two +pieces of metal are placed together, and a powerful current +is passed through them. This current meets with a high +resistance at the point of contact of the two pieces, and so +heat is produced. When the proper welding temperature +is reached, and the metal is in a sort of pasty condition, +the two pieces are pressed strongly together, and the +current is switched off. The pieces are now firmly united +together. The process may be carried out by hand, the +welding smith switching the current on and off, and applying +pressure at the right moment by means of hydraulic +power. There are also automatic welders, which perform +the same operations without requiring any manual control. +Alternating current is used, of low voltage but very high +amperage.</p> + +<p>Steel castings are sometimes found to have small +defects, such as cracks or blow-holes. These are not +discarded as useless, but are made quite sound by welding<span class="pagenum" id="Page_115">115</span> +additional metal into the defective places by means of the +electric arc. The arc is formed between the casting and a +carbon rod, and the tremendous heat reduces the surface of +the metal to a molten condition. Small pieces or rods of +metal are then welded in where required.</p> +<hr class="chap x-ebookmaker-drop"> + +<div class="chapter"> +<p><span class="pagenum" id="Page_116">116</span></p> + +<h2 class="nobreak" id="toclink_116"><a id="chapter_XIV"></a>CHAPTER XIV<br> + +<span class="subhead">ELECTRIC BELLS AND ALARMS</span></h2> +</div> + +<p class="in0"><span class="firstword">The</span> most familiar of all electrically worked appliances is +probably the electric bell, which in some form or other is +in use practically all over the world. Electric bells are +operated by means of a current of electricity sent through +the coils of an electro-magnet, and one of the very simplest +forms is that known as the single-stroke bell. In this bell +an armature or piece of soft iron is placed across, but at a +little distance from, the poles of an electro-magnet, and to +this piece of iron is fixed a lever terminating in a sort of +knob which lies close to a bell or gong. When a current +is sent round the electro-magnet the armature is attracted, +so that the lever moves forward and strikes a sharp blow +upon the gong. Before the gong can be sounded a second +time the current must be interrupted in order to make the +magnet release the armature, so that the lever may fall +back to its original position. Thus the bell gives only one +ring each time the circuit is closed. Bells of this kind may +be used for signalling in exactly the same way as the Morse +sounder, and sometimes they are made with two gongs of +different tones, which are arranged so as to be sounded +alternately.</p> + +<figure id="fig_24" class="figright" style="max-width: 11em;"> + <img src="images/i_143.png" width="853" height="1534" alt=" "> + <figcaption class="caption"><span class="smcap">Fig. 24.</span>—Mechanism of +Electric Bell. +</figcaption></figure> + +<figure id="fig_25" class="figleft" style="max-width: 14em;"> + <img src="images/i_143b.png" width="1113" height="415" alt=" "> + <figcaption class="caption"><span class="smcap">Fig. 25.</span>—Diagram showing principle +of Bell-push. +</figcaption></figure> + +<p>For most purposes however another form called the +trembler bell is much more convenient. <a href="#fig_24">Fig. 24</a> is a rough +diagram of the usual arrangement of the essential parts of +a trembler bell. When the circuit is closed by pressing the<span class="pagenum" id="Page_117">117</span> +bell-push, a current flows from the battery to the electro-magnet +EE, by way of terminal T. The electro-magnet +then attracts the soft iron armature +A, thus causing the hammer H to +strike the gong. But immediately +the armature is pulled away from +the terminal T¹ the circuit is +broken and the magnet loses its +attraction for the armature, which +is moved back again into contact +with T¹ by the spring S. The +circuit is thus again closed, the +armature is again attracted, and +the hammer strikes the gong a +second time. This process goes +on over and over again at a great +speed as long as the bell-push is +kept pressed down, resulting in +an extremely rapid succession of +strokes upon the gong. It will +be noticed that the working of +this bell is very similar to that of the automatic contact-breaker +used for induction coils (<a href="#chapter_VIII">Chapter VIII</a>.). For +household purposes this +form of bell has completely +driven out the once popular +wire-pulled bell. Bell-pushes +are made in a +number of shapes and +forms, and <a href="#fig_25">Fig. 25</a> will +make clear the working +principle of the familiar form which greets us from almost +every doorway with the invitation, “Press.” In private +offices and elsewhere the rather aggressive sound of an<span class="pagenum" id="Page_118">118</span> +ordinary trembler bell is apt to become a nuisance, and +in such cases a modified form which gives a quiet buzzing +sound is often employed.</p> + +<p>It is frequently necessary to have an electric bell which, +when once started, will continue ringing until it is stopped. +Such bells are used for fire and burglar alarms and for +many other similar purposes, and they are called continuous-ringing +bells as distinguished from the ordinary +trembler bells. In one common form of continuous-ringing +bell two separate batteries are used, one to start the bell +and the other to keep it ringing. When a momentary +current from the first battery is sent over the bell lines the +armature is attracted by the electro-magnet, and its movement +allows a lever to fall upon a metal contact piece. +This closes the circuit of the second battery, which keeps +the bell ringing until the lever is replaced by pulling a cord +or pressing a knob. Continuous-ringing bells are often +fitted to alarm clocks. The alarm is set in the usual way, +and at the appointed hour the bell begins to ring, and goes +on ringing until its owner, able to stand the noise no longer, +gets out of bed to stop it.</p> + +<p>There is another form of electric bell which has been +devised to do away with the annoyance of bells suddenly +ceasing to work on account of the failure of the battery. +In this form the battery is entirely dispensed with, and +the current for ringing the bell is taken from a very small +dynamo fitted with a permanent steel horse-shoe magnet. +The armature is connected to a little handle, and current +is generated by twisting the handle rapidly to and fro +between the thumb and finger. A special form of bell is +required for this arrangement, which is not in general use.</p> + +<p>In the days of wire-pulled bells it was necessary to have +quite a battery of bells of different tones for different rooms, +but a single electric bell can be rung from bell-pushes<span class="pagenum" id="Page_119">119</span> +placed in any part of a house or hotel. An indicator is +used to show which push has been pressed, and, this like +the bell itself, depends upon the attraction of an armature +by an electro-magnet. Before reaching the bell the wire +from each bell-push passes round a separate small electro-magnet, +which is thus magnetized by the current at the +same time that the bell is rung. In the simplest form of +indicator the attraction of the magnet causes a little flag to +swing backwards and forwards over its number. Another +form is the drop indicator, in which the movement of the +armature when attracted by the magnet allows a little flag +to drop, thus exposing the number of the room from which +the bell was rung. The dropped flag has to be replaced, +either by means of a knob fixed to a rod which pushes the +flag up again, or by pressing a push which sends the +current through another little electro-magnet so arranged +as to re-set the flag.</p> + +<p>The electric current is used to operate an almost endless +variety of automatic alarms for special purposes. Houses +may be thoroughly protected from undesired nocturnal +visitors by means of a carefully arranged system of burglar +alarms. Doors and windows are fitted with spring contacts +so that the slightest opening of them closes a battery circuit +and causes an alarm to sound, and even if the burglar +succeeds in getting inside without moving a door or +window, say by cutting out a pane of glass, his troubles are +not by any means at an end. Other contacts are concealed +under the doormats, and under the carpets in passages and +stairways, so that the burglar is practically certain to tread +on one or other of them and so rouse the house. A window +may be further guarded by a blind contact. The blind is +left down, and is secured at the bottom to a hook, and the +slightest pressure upon it, such as would be given by a +burglar trying to get through the window, sets off the alarm.<span class="pagenum" id="Page_120">120</span> +Safes also may be protected in similar ways, and a camera +and flashlight apparatus may be provided, so that when the +burglar closes the circuit by tampering with the safe he +takes his own photograph.</p> + +<p>The modern professional burglar is a bit of a scientist +in his way, and he is wily enough to find and cut the wires +leading to the contacts, so that he can open a door or +window at his leisure without setting off the alarm. In +order to circumvent this little game, burglar alarms are +often arranged on the closed-circuit principle, so that the +alarm is sounded by the breaking of the circuit. A burglar +who deftly cut the wires of an alarm worked on this +principle would not be particularly pleased with the results +of his handiwork. The bells of burglar alarms may be +arranged to ring in a bedroom or in the street, and in the +United States, where burglar and in fact all electric +alarms are in more general use than in England, large +houses are sometimes connected to a police station, so that +the alarm is given there by bell or otherwise.</p> + +<figure id="plate_X" class="figcenter" style="max-width: 37em;"> + <p class="caption">PLATE X.</p> + <img src="images/i_147.jpg" width="2885" height="2046" alt=" "> + <figcaption class="caption"> + +<p class="floatl"><i>By permission of</i></p> +<p class="floatr"><i>Vickers Limited.</i></p> + +<p class="floatc">WHERE ELECTRICAL MACHINERY IS MADE.</p> +</figcaption></figure> + +<p>When an outbreak of fire is discovered it is of the +utmost importance that the nearest fire-station should be +notified instantly, for fire spreads with such rapidity that a +delay of even a few minutes in getting the fire-engines to +the spot may result in the total destruction of a building +which otherwise might have been saved. In almost all +large towns some system of public fire alarms is now in +use. The signal boxes are placed in conspicuous positions +in the streets, and sometimes also in very large buildings. +The alarm is generally given by the starting of a clockwork +mechanism which automatically makes and breaks a circuit +a certain number of times. When this occurs an alarm +bell rings at the fire-station, and the number of strokes on +the bell, which depends upon the number of times the +alarm mechanism makes and breaks the circuit, tells the +attendant from which box the alarm has been given. One +well-known form of call box has a glass front, and the +breaking of the glass automatically closes the circuit. In +other forms turning a handle or pulling a knob serves the +same purpose.</p> + +<p>It is often required to maintain a room at one particular +temperature, and electricity may be employed to give an +alarm whenever the temperature rises above or falls below +a certain point. One arrangement for this purpose consists +of an ordinary thermometer having the top of the mercury +tube fitted with an air-tight stopper, through which a wire +is passed down into the tube as far as the mark indicating +the temperature at which the alarm is desired to sound. +Another wire is connected with the mercury in the bulb, +and the free ends of both wires are taken to a suitable +battery, a continuous-ringing bell being inserted in the +circuit at some convenient point. If a rise in temperature +takes place the mercury expands and moves up the tube, +and at the critical temperature it touches the wire, thus +completing the circuit and sounding the alarm. This +arrangement only announces a rise in temperature, but by +making the thermometer tube in the shape of a letter <span class="sans bold">U</span> an +alarm may be given also when the temperature falls below +a certain degree. A device known as a “thermostat” is also +used for the same purpose. This consists of two thin strips +of unlike metals, such as brass and steel, riveted together +and suspended between two contact pieces. The two +metals expand and contract at different rates, so that an +increase in temperature makes the compound strip bend in +one direction, and a decrease in temperature makes it bend +in the opposite direction. When the temperature rises or +falls beyond a certain limit the strip bends so far as to touch +one or other of the contact pieces, and the alarm is then +given. Either of the preceding arrangements can be used<span class="pagenum" id="Page_122">122</span> +also as an automatic fire alarm, or if desired matters may be +arranged so that the closing of the circuit, instead of ringing +a bell, turns on or off a lamp, or adjusts a stove, and +in this way automatically keeps the room at a constant +temperature.</p> + +<p>Electric alarms operated by ball floats are used to some +extent for announcing the rise or fall beyond a pre-arranged +limit of water or other liquids, and there is a very ingenious +electrical device by which the level of the water in a tank +or reservoir can be ascertained at any time by indicators +placed in convenient positions any distance away.</p> + +<p>In factories and other large buildings a watchman is +frequently employed to make a certain number of rounds +every night. Being human, a night-watchman would much +rather sit and snooze over his fire than tramp round a dark +and silent factory on a cold winter night; and in order to +make sure that he pays regular visits to every point +electricity is called in to keep an eye on him. A good +eight-day clock is fitted with a second dial which is rotated +by the clockwork mechanism, and a sheet of paper, which +can be renewed when required, is placed over this dial. +On the paper are marked divisions representing hours and +minutes, and other divisions representing the various places +the watchman is required to visit. A press-button is fixed +at each point to be visited, and connected by wires with +the clock and with a battery. As the watchman reaches +each point on his rounds he presses the button, which is +usually locked up so that no one else can interfere with it, +and the current passes round an electro-magnet inside the +clock case. The magnet then attracts an armature which +operates a sort of fine-pointed hammer, and a perforation +is made in the paper, thus recording the exact time at +which the watchman visited that particular place.</p> + +<p>The current for ordinary electric bells is generally supplied<span class="pagenum" id="Page_123">123</span> +by Leclanché cells, which require little attention, and +keep in good working order for a very long time. As we +saw in <a href="#chapter_IV">Chapter IV</a>., these bells soon polarize if used continuously, +but as in bell work they are required to give +current for short periods only, with fairly long intervals of +rest, no trouble is caused on this account. These cells +cannot be used for burglar or other alarms worked on the +closed-circuit principle, and in such cases some form of +Daniell cell is usually employed.</p> +<hr class="chap x-ebookmaker-drop"> + +<div class="chapter"> +<p><span class="pagenum" id="Page_124">124</span></p> + +<h2 class="nobreak" id="toclink_124"><a id="chapter_XV"></a>CHAPTER XV<br> + +<span class="subhead">ELECTRIC CLOCKS</span></h2> +</div> + +<p class="in0"><span class="firstword">Amongst</span> the many little worries of domestic life is the +keeping in order of the various clocks. It ought to be a +very simple matter to remember to wind up a clock, but +curiously enough almost everybody forgets to do so now +and then. We gaze meditatively at the solemn-looking +machine ticking away on the mantelpiece, wondering +whether we wound it up last week or not; and we wish +the wretched thing would go without winding, instead of +causing us all this mental effort.</p> + +<p>There is usually a way of getting rid of little troubles +of this kind, and in this case the remedy is to be found in +an electrically-driven clock. The peculiar feature about +clocks driven by electricity is that they reverse the order +of things in key-wound clocks, the pendulum being made +to drive the clockwork instead of the clockwork driving +the pendulum. No driving spring is required, and the +motive power is supplied by a small electro-magnet.</p> + +<p>The actual mechanism varies considerably in different +makes of clock. In one of the simplest arrangements there +is a pendulum with an armature of soft iron fixed to the +extremity of its bob. Below the pendulum is an electro-magnet, +and this is supplied with current from a small +battery of dry cells. A short piece of metal, called a “pallet,” +is attached to the rod of the pendulum by means of a +pivot; and as the pendulum swings it trails this pallet<span class="pagenum" id="Page_125">125</span> +backwards and forwards along a horizontal spring. In this +spring are cut two small notches, one on each side of the +centre of the swing. As long as the pendulum is swinging +sufficiently vigorously, the pallet slides over these notches; +but when the swing has diminished to a certain point the +pallet catches in one or other of the notches. This has +the effect of pressing down the spring so that it touches a +contact piece just below, and the battery circuit is then +completed. The electro-magnet now comes into action +and attracts the armature, thus giving the pendulum a pull +which sets it swinging vigorously again. The spring is +then freed from the pressure of the pallet, and it rises to its +original position, so that the circuit is broken. This puts +out of action the electro-magnet, and the latter does no +further work until the pendulum requires another pull. +The movement of the pendulum drives the wheelwork, +which is similar to that of an ordinary clock, and the wheelwork +moves the hands in the usual way. A clock of this +kind will run without attention for several months, and +then the battery requires to be renewed. As time-keepers, +electrically-driven clocks are quite as good as, and often +very much better than key-wound clocks.</p> + +<p>Everybody must have noticed that the numerous public +clocks in a large town do not often agree exactly with one +another, the differences sometimes being quite large; while +even in one building, such as a large hotel, the different +clocks vary more or less. This state of things is very +unsatisfactory, for it is difficult to know which of the clocks +is exactly right. Although large clocks are made with the +utmost care by skilled workmen, they cannot possibly be +made to maintain anything like the accuracy of a high-class +chronometer, such as is used by navigators; and the only +way to keep a number of such clocks in perfect agreement +is to control their movements from one central or master<span class="pagenum" id="Page_126">126</span> +clock. This can be done quite satisfactorily by electricity. +The master-clock and the various sub-clocks are connected +electrically, so that a current can be sent from the master-clock +to all the others. Each sub-clock is fitted with an +electro-magnet placed behind the figure XII at the top of +the dial. At the instant when the master-clock reaches the +hour, the circuit is closed automatically, and the current +energizes these magnets. The minute hands of all the +sub-clocks are gripped by the action of the magnets, and +pulled exactly to the hour; the pulling being backward or +forward according to whether the clocks are fast or slow. +In this way all the clocks in the system are in exact agreement +at each hour. The same result may be attained by +adjusting all the sub-clocks so that they gain a little, say a +few seconds in the hour. In this case the circuit is closed +about half a minute before the hour. As each sub-clock +reaches the hour, its electro-magnet comes into action, and +holds the hands so that they cannot proceed. When the +master-clock arrives at the hour the circuit is broken, the +magnets release their captives, and all the clocks move +forward together.</p> + +<p>It is possible to control sub-clocks so that their pendulums +actually beat exactly with the pendulum of the master-clock; +but only a small number of clocks can be controlled +in this way, and they must be of the best quality. The +method is similar to that used for hourly corrections, the +main difference being that the circuit is closed by the +pendulum of the master-clock at each end of its swing, so +that the pendulums of the sub-clocks are accelerated or +held back as may be required.</p> + +<p>In the correcting systems already described the sub-clocks +are complete in themselves, so that they work quite +independently, except at the instant of correction. For +hotels, schools, and other large buildings requiring clocks<span class="pagenum" id="Page_127">127</span> +at a number of different points, a simpler arrangement is +adopted. Only one complete clock is used, this being the +master-clock, which may be wound either electrically or by +key. The sub-clocks are dummies, having only a dial with +its hands, and an electro-magnetic arrangement behind the +dial for moving the hands. The sub-clocks are electrically +connected with the master-clock, and the mechanism of this +clock is arranged to close the circuit automatically every +half-minute. Each time this occurs the magnet of each +sub-clock moves forward the hands half a minute, and in +this way the dummy clocks are made to travel on together +by half-minute steps, exactly in unison with the master-clock.</p> +<hr class="chap x-ebookmaker-drop"> + +<div class="chapter"> +<p><span class="pagenum" id="Page_128">128</span></p> + +<h2 class="nobreak" id="toclink_128"><a id="chapter_XVI"></a>CHAPTER XVI<br> + +<span class="subhead">THE TELEGRAPH</span></h2> +</div> + +<p class="in0"><span class="firstword">We</span> come now to one of the most important inventions of +the nineteenth century, the electric telegraph. From very +early times men have felt the necessity for some means of +rapidly communicating between two distant points. The +first really practical method of signalling was that of lighting +beacon fires on the tops of hills, to spread some important +tidings, such as the approach of an enemy. From this +simple beginning arose more complicated systems of signalling +by semaphore, flags, or flashing lights. All these +methods proved incapable of dealing with the rapidly +growing requirements of commerce, for they were far too +slow in action, and in foggy weather they were of no use +at all. We are so accustomed to walking into a telegraph +office, filling up a form, and paying our sixpence or more, +that it is very difficult for us to realize the immense importance +of the electric telegraph; and probably the best +way of doing this is to try to imagine the state of things +which would result if the world’s telegraphic instruments +were put out of action for a week or two.</p> + +<p>The earliest attempts at the construction of an electric +telegraph date back to a time long before the discovery of +the electric current. As early as 1727 it was known that +an electric discharge could be transmitted to a considerable +distance through a conducting substance such as a moistened +thread or a wire, and this fact suggested the possibility of<span class="pagenum" id="Page_129">129</span> +a method of electric signalling. In 1753 a writer in <cite>Scott’s +Magazine</cite> brought forward an ingenious scheme based upon +the attraction between an electrified body and any light +substance. His telegraph was worked by an electric +machine, and it consisted of twenty-six separate parallel +wires, every wire having a metal ball suspended from it at +each end. Close to each ball was placed a small piece of +paper upon which was written a letter of the alphabet. +When any wire was charged, the paper letters at each end +of it were attracted towards the metal balls, and in this +way words and sentences were spelled out. Many other +systems more or less on the same lines were suggested +during the next fifty years, but although some of them had +considerable success in an experimental way, they were all +far too unreliable to have any commercial success.</p> + +<p>With the invention of the voltaic cell, inventors’ ideas +took a new direction. In 1812 a telegraph based upon the +power of an electric current to decompose water was +devised by a German named Sömmering. He used a +number of separate wires, each connected to a gold pin +projecting from below into a glass vessel filled with +acidulated water. There were thirty-five wires in all, for +letters and numbers, and when a current was sent along +any wire bubbles of gas formed at the pin at the end of it, +and so the letters or numbers were indicated. This telegraph, +like its predecessors, never came into practical use. +Oersted’s discovery in 1829 of the production of magnetism +by electricity laid the foundation of the first really practical +electric telegraphs, but little progress was made until the +appearance of the Daniell cell, in 1836. The earlier forms +of voltaic cells polarized so rapidly that it was impossible +to obtain a constant current from them, but the non-polarizing +Daniell cell at once removed all difficulty in this +respect. In the year 1837 three separate practical telegraphs<span class="pagenum" id="Page_130">130</span> +were invented: by Morse in the United States, by +Wheatstone and Cooke in England, and by Steinheil in +Munich.</p> + +<figure id="fig_26" class="figleft" style="max-width: 16em;"> + <img src="images/i_158.png" width="1215" height="2021" alt=" "> + <figcaption class="caption"><span class="smcap">Fig. 26.</span>—Dial of Five-Needle Telegraph. +</figcaption></figure> + +<p>The first telegraph of Wheatstone and Cooke consisted +of five magnetic needles +pivoted on a vertical +dial. The letters of the +alphabet were marked +on the dial, and the +needles were deflected +by currents made to +pass through wires by +the depression of keys, +so that two needles +would point towards +the required letter. +<a href="#fig_26">Fig. 26</a> is a sketch of +the dial of this apparatus. +This telegraph was +tried successfully on the +London and North-Western +Railway, over +a wire a mile and a half +in length. Wheatstone +and Cooke afterwards +invented a single-needle +telegraph in which the +letters were indicated +by movements of the needle to the right or to the left, +according to the direction of a current sent through a coil +of wire. Wheatstone subsequently produced an apparatus +which printed the letters on paper.</p> + +<p>In the United States, Morse had thought out a scheme +of telegraphy in 1832, but it was not until 1837 that he got<span class="pagenum" id="Page_131">131</span> +his apparatus into working order. He was an artist by +profession, and for a long time he was unable to develop +his ideas for lack of money. After many efforts he succeeded +in obtaining a State grant of £6000 for the +construction of a telegraph line between Baltimore and +Washington, and the first message over this line was sent +in 1844, the line being thrown open to the public in the +following year. Amongst the features of this telegraph +were a receiving instrument which automatically recorded +the messages on a moving paper ribbon, by means of a +pencil actuated by an electro-magnet; and an apparatus +called a relay, which enabled the recording instrument to +be worked when the current was enfeebled by the resistance +of a very long wire. Morse also devised a telegraphic +code which is practically the same as that in use to-day.</p> + +<p>The great discovery of the German Steinheil was that +a second wire for the return of the current was not necessary, +and that the earth could be used for this part of the circuit.</p> + +<p>In reading the early history of great inventions one is +continually struck with the indifference or even hostility +shown by the general public. In England the electric +telegraph was practically ignored until the capture of a +murderer by means of it literally forced the public to see +its value. The murder was committed near Slough, and +the murderer succeeded in taking train for London. +Fortunately the Great Western Railway had a telegraph +line between Slough and London, and a description telegraphed +to Paddington enabled the police to arrest the +murderer on his arrival. In the United States too there +was just the same indifference. The rate for messages on +the line between Baltimore and Washington was one cent +for four words, and the total amount taken during the first +four days was one cent!</p> + +<p>One of the simplest forms of telegraph is the single-needle<span class="pagenum" id="Page_132">132</span> +instrument. This consists of a magnetic needle +fixed to a spindle at the back of an upright board through +which the spindle is passed. On the same spindle, but in +front of the board, is fixed a dial needle, which, of course, +moves along with the magnetic needle. A coil of wire is +passed round the magnetic needle, and connected to a +commutator for reversing +the direction of the +current. By turning a +handle to the left a +current is made to flow +through the coil, and +the magnetic needle +moves to one side; but +if the handle is turned +to the right the current +flows through the coil +in the opposite direction, +and the needle +moves to the other +side. Instead of a +handle, two keys may +be used, the movement +of the needle varying +according to which key +is pressed. A good +operator can transmit +at the rate of about twenty words a minute with this +instrument. The Morse code, which consists of combinations +of dots and dashes, is used, a movement of the +dial needle to the left meaning a dot, and one to the right +a dash. The code as used in the single-needle instrument +is shown in <a href="#fig_27">Fig. 27</a>.</p> + +<figure id="fig_27" class="figcenter" style="max-width: 16em;"> + <img src="images/i_160.png" width="1228" height="1747" alt=" "> + <figcaption class="caption"><span class="smcap">Fig. 27.</span>—Code for Single-Needle Telegraph. +</figcaption></figure> + +<p>Needle instruments are largely used in railway signal<span class="pagenum" id="Page_133">133</span> +cabins, but for general telegraphic work an instrument +called the Morse sounder is employed. This consists of +an electro-magnet which, when a current is passed through +it, attracts a small piece of iron fixed to one end of a +pivoted lever. The other end of this lever moves between +two stops. At the transmitting station the operator closes +a battery circuit by pressing a key, when the electro-magnet +of the sounder at the receiving station attracts the iron, +and the lever flies from one stop to the other with a sharp +click, returning again as soon as the circuit is broken. A +dot is signalled when the lever falls back immediately after +the click, and a dash when it makes a short stay before +returning. <a href="#fig_28">Fig. 28</a> shows the code of signals for the +Morse telegraph.</p> + +<figure id="fig_28" class="figcenter" style="max-width: 26em;"> + <img src="images/i_161.png" width="2064" height="875" alt=" "> + <figcaption class="caption"><span class="smcap">Fig. 28.</span>—The Morse Code. +</figcaption></figure> + +<p>In passing through a very long wire an electric current +becomes greatly reduced in strength owing to the resistance +of the wire. If two telegraph stations are a great distance +apart the energy of the current thus may be unequal to the +task of making the electro-magnet move the lever of the +sounder so as to produce a click, but this difficulty is overcome +by the use of an ingenious arrangement called a +“relay.” It consists of a very small electro-magnet which<span class="pagenum" id="Page_134">134</span> +attracts a light bar, the movement of the bar being made +to close the circuit of another battery at the receiving +station. The feeble current works the relay, and the +current in the local circuit operates the sounder.</p> + +<p>The word “telegraph,” which is derived from the Greek +<em>tele</em>, far off, and <em>grapho</em>, I write, strictly signifies writing at +a distance. The needle instrument and the sounder do not +write in any way, but by modifying the construction of the +sounder it can be made to record the messages it receives. +A small wheel is fitted to the free end of the lever of the +sounder, and an ink-well is placed so that the wheel dips +into it when the lever is in the normal position. When +the circuit is closed the lever moves just as in the ordinary +sounder, but instead of clicking against a stop it presses +the inked wheel against a paper ribbon which is kept +slowly moving forward by clockwork. In this way the +wheel continues to mark a line along the paper as long as +the circuit remains closed, and according to the time the +transmitting key is kept down a short mark or dot, or a +long mark or dash, is produced. The clockwork which +moves the paper ribbon is started automatically by the +current, and it continues working until the message is +finished.</p> + +<figure id="fig_29" class="figcenter" style="max-width: 27em;"> + <img src="images/i_163.jpg" width="2101" height="2229" alt=" "> + <figcaption class="caption"><p><span class="smcap">Fig. 29.</span>—A Morse Message.</p> + +<p>(<i>a</i>) Perforated Tape. <span class="in4">(<i>b</i>) Printed Tape.</span></p> + +<p class="p1">TRANSLATION.</p> + +<p class="justify"><i>Series of alternate dots and dashes indicating commencement of message.</i></p> + +<p class="justify">Sec (<i>section</i>) A. D. T. (<i>Daily Telegraph</i>) Fm (<i>from</i>) Berri, Antivari.</p> + +<p class="justify"><i>Then follow the letters</i> G. Q., <i>signifying fresh line</i>.</p> + +<p class="justify">They hd (<i>had</i>) bn (<i>been</i>) seen advancing in t (<i>the</i>) distance and wr (<i>were</i>) +recognised by thr (<i>their</i>) usual uniform wh (<i>which</i>) consists o (<i>of</i>) a white fez.</p> + +<p class="justify"><em>Finally double dots indicating full stop.</em></p> +</figcaption></figure> + +<p>A good Morse operator can maintain a speed of about +thirty words a minute, but this is far too slow for certain +kinds of telegraphic work, such as the transmission of press +news, and for such work the Wheatstone automatic transmitter +is used. First of all the messages are punched on +a paper ribbon. This is done by passing the ribbon from +right to left by clockwork through a punching machine +which is provided with three keys, one for dots, one for +dashes, and the other for spaces. If the left-hand key is +pressed, two holes opposite to one another are made, +representing a dot; and if the right-hand key is pressed,<span class="pagenum" id="Page_136">136</span> +two diagonal holes are punched, representing a dash. In +<a href="#fig_29">Fig. 29</a>, which shows a piece of ribbon punched in this +way, a third line of holes will be noticed between the outside +holes representing the dots and dashes. These holes +are for the purpose of guiding the paper ribbon steadily +along through the transmitting machine. The punched +ribbon is then drawn by clockwork through a Wheatstone +transmitter. In this machine two oscillating needles, connected +with one pole of a battery, are placed below the +moving ribbon. Each time a hole passes, these needles +make contact with a piece of metal connected with the +other pole of the battery, thus making and breaking the +circuit with much greater rapidity than is possible with +the Morse key. At the receiving station the messages are +recorded by a form of Morse inker, coming out in dots and +dashes as though sent by hand. Below the punched ribbon +in <a href="#fig_29">Fig. 29</a> is shown the corresponding arrangement of dots +and dashes. The same punched ribbon may be used +repeatedly when the message has to be sent on a number +of different lines. The Wheatstone automatic machine is +capable of transmitting at the rate of from 250 to 400 +words a minute. <a href="#fig_29">Fig. 29</a> is a fragment of a <cite>Daily +Telegraph</cite> Balkan War special, as transmitted to the +<cite>Yorkshire Post</cite> over the latter’s private wire from London +to Leeds. In the translation it will be seen that many +common words are abbreviated.</p> + +<p>One weak point of telegraphy with Wheatstone instruments +is that the messages are received in Morse +code, and have to be translated. During recent years +telegraphs have been invented which actually produce +their messages in ordinary written or printed characters. +A very ingenious instrument is the Hughes printing +telegraph, which turns out messages in typewritten form. +Its mechanism is too complicated to be described here,<span class="pagenum" id="Page_137">137</span> +but in general it consists of a transmitter having a keyboard +something like that of a typewriter, by means of +which currents of electricity are made to press a sheet of +paper at the right instant against a revolving type-wheel +bearing the various characters. This telegraph has been +modified and brought to considerable perfection, and in +one form or another it is used in European countries and +in the United States.</p> + +<p>In the Pollak-Virag system of telegraphy the action of +light upon sensitized photographic paper is utilized. An +operator punches special groupings of holes on a paper +ribbon about 1 inch wide, by means of a perforating +machine resembling a typewriter, and the ribbon is then +passed through a machine which transmits by brush contacts. +The receiver consists of a very small mirror +connected to two vibrating diaphragms, which control its +movements according to the currents received, one diaphragm +moving the mirror in a vertical direction, and the +other in a horizontal direction. The mirror reflects a ray +of light on to photographic bromide paper in the form of a +moving band about 3 inches in width, and the combined +action of the two diaphragms makes the mirror move so +that the ray of light traces out the messages in ordinary +alphabetical characters. As it moves forward after being +acted upon by the light, the paper is automatically developed +and fixed, and then passed through drying rollers. Although +the writing is rather imperfect in formation it is quite legible +enough for most messages, but trouble occasionally occurs +with messages containing figures, owing to confusion arising +from the similarity of the figures, 3, 5, and 8. The whole +process is carried out with such rapidity that 40,000 or +even more words can be transmitted easily in an hour.</p> + +<p>One of the most remarkable of present-day telegraphs +is the Creed high-speed automatic printing telegraph.<span class="pagenum" id="Page_138">138</span> +This has been devised to do away with hand working as +far as possible, and to substitute quicker and more accurate +automatic methods. In this system a perforated paper +tape is produced by a keyboard perforator at the sending +station. This tape is just ordinary Wheatstone tape, its +perforations representing in the Morse code the message to +be transmitted; and the main advantage of the Creed perforator +over the three-key punching machine already +described lies in the ease and speed with which it can be +worked. The keyboard contains a separate key for each +letter or signal of the Morse code, and the pressing of any +key brings into operation certain punches which make the +perforations corresponding to that particular letter. The +perforator can be worked by any one who understands how +to use an ordinary typewriter, and a speed of about 60 +words a minute can be maintained by a fairly skilful +operator. If desired a number of tapes may be perforated +at the same time.</p> + +<p>The tape prepared in this way is passed through a +Wheatstone transmitter, and long or short currents, according +to the arrangement of the perforations, are sent out +along the telegraph line. At the receiving station these +signals operate a receiving perforator. This machine +produces another perforated tape, which is an exact copy +of the tape at the sending station, and it turns out this +duplicate tape at the rate of from 150 to 200 words a +minute. There are two forms of this receiving perforator, +one worked entirely by electricity, and the other by a combination +of electricity and compressed air, both forms +serving the same purpose. The duplicate tape is then +passed through an automatic printer, which reproduces the +message in large Roman characters on a paper tape. The +printer works at a speed of from 80 to about 100 words a +minute, and the printed tape is pasted on a telegraphic<span class="pagenum" id="Page_139">139</span> +form by a semi-automatic process, and the message is then +ready for delivery. <a href="#plate_XI">Plate XI</a>. shows a specimen of the +tape from the receiving perforator, and the corresponding +translation as turned out by the printer. This message +formed part of a leading article in the <cite>Daily Mail</cite>. Some +idea of the wonderful capabilities of the Creed system may +be gained from the fact that by means of it practically the +whole contents of the <cite>Daily Mail</cite> are telegraphed every +night from London to Manchester and Paris, for publication +next morning.</p> + +<p>One of the most remarkable features about present-day +telegraphy is the ease with which two or more messages +can be sent simultaneously over one line. Duplex telegraphy, +or the simultaneous transmission of two separate +messages in opposite directions over one wire, is now +practised on almost every line of any importance. At first +sight duplex telegraphy seems to be an impossibility, for if +we have two stations, one at each end of a single wire, and +each station fitted with a transmitter and a receiver, it +appears as if each transmitter would affect not only the +receiver at the opposite end of the wire, but also the +receiver at its own end, thus causing hopeless confusion +when both transmitters were in use at the same time. +This actually would be the case with ordinary telegraphic +methods, but by the use of a special arrangement all confusion +in working is avoided.</p> + +<figure id="plate_XI" class="figcenter" style="max-width: 35em;"> + <p class="caption">PLATE XI.</p> + <img src="images/i_169.jpg" width="2745" height="2077" alt=" "> + <figcaption class="caption"> + +<p class="floatl"><i>By permission of</i></p> +<p class="floatr"><i>Creed, Bille & Co. Ltd.</i></p> + +<p class="floatc">SPECIMEN OF THE WORK OF THE CREED HIGH-SPEED PRINTING TELEGRAPH.</p> +</figcaption></figure> + +<p>We have seen that a magnetic needle is deflected by a +current passing through a coil of wire placed round it, and +that the direction in which the needle is deflected depends +upon the direction of the current in the coil. Now suppose +we place round the needle two coils of wire, wound so that +the current in one flows in a direction opposite to that of +the current in the other. Then, if we pass two equal +currents, one through each coil, it is evident that they will<span class="pagenum" id="Page_140">140</span> +neutralize one another, so that the needle will not be +deflected at all. In a duplex system one end of one of +these coils is connected to earth, say to a copper plate +buried in the ground, and one end of the other to the line +wire. The two remaining ends are arranged as branches +leading off from a single wire connected with the transmitting +key. The whole arrangement of coils and needle +is repeated at the other end of the line. If now the +transmitting key at station A is pressed, the circuit is closed +and a current flows along the single wire, and then divides +into two where the wire branches, half of it taking the path +through one coil and half the path through the other. +Equal currents thus flow through the oppositely wound +coils, and the needle at station A is not deflected. Leaving +the coils, one of these equal currents flows away to earth, +while the other passes out along the line wire. On its +arrival at station B the current is able to pass through only +one of the coils round the needle, and consequently the +needle is deflected and the signal given. In this way the +transmitting operator at station A is able to signal to station +B without affecting the receiver at his own end, and +similarly the operator at station B can transmit to A without +affecting the B receiver. Thus there can be no confusion +whether the transmitters are worked at different times or +simultaneously, for each transmitter affects only the +receiver at the opposite end of the line. The diagram in +<a href="#fig_30">Fig. 30</a> will help to make clearer the general principle. +K and K¹ are the two transmitting keys which close the +circuit, and C and C¹ are the points at which the current +divides into two. Instead of coils and needles, electro-magnets +operating sounders may be used, such magnets +having two separate and oppositely wound coils, acting in +exactly the same way as the coils round the needles. The +above description is of course only a rough outline of the<span class="pagenum" id="Page_141">141</span> +method, and in practice matters are more complicated, +owing to the necessity for carefully adjusted resistances and +for condensers. There is also another and different method +of duplexing a line, but we have not space to describe it. +Duplex telegraphy requires two operators at each end of +the line, one to send and the other to receive.</p> + +<p>Diplex telegraphy is the simultaneous transmission of +two separate messages in the same direction over one line. +Without going into details it may be said that for this +purpose two different transmitting keys are required, one of +which alters the direction, and the other the strength of the +current though the line wire. The receivers are arranged +so that one responds only to a strong current, and the other +only to a current in one particular direction. A line also +may be quadruplexed, so that it is possible to transmit +simultaneously two messages from each end, four operators +being required at each station, two to transmit and two +to receive. Systems of multiplex telegraphy have been +devised by which very large numbers of messages can be +sent at once over a single wire, and the Baudot multiplex +telegraph has proved very successful.</p> + +<figure id="fig_30" class="figcenter" style="max-width: 23em;"> + <img src="images/i_171.png" width="1775" height="826" alt=" "> + <figcaption class="caption"><span class="smcap">Fig. 30.</span>—Diagram to illustrate principle of Duplex Telegraphy. +</figcaption></figure> + +<p>The wires for telegraphic purposes may be conveyed +either above or below the ground. Overground wires are<span class="pagenum" id="Page_142">142</span> +carried on poles by means of insulators of porcelain or other +non-conducting material, protected by a sort of overhanging +screen. The wires are left bare, and they are generally +made of copper, but iron is used in some cases. In underground +lines the wires formerly were insulated by a covering +of gutta-percha, but now paper is generally used. Several +wires, each covered loosely with thoroughly dry paper, are +laid together in a bundle, the whole bundle or cable being +enclosed in a strong lead pipe. The paper coverings are +made to fit loosely so that the wires are surrounded by an +insulating layer of dry air. As many as 1200 separate +wires are sometimes enclosed in one pipe. In order to +keep telegraph lines in working order frequent tests are +necessary, and the most important British Postal Telegraph +lines are tested once a week between 7.30 and 7.45 a.m. +The earth is generally used for the return circuit in +telegraphy, and the ends of the return wires are connected +either to metal plates buried in the ground to a depth at +which the earth is permanently moist, or to iron gas or +water pipes. The current for telegraph working on a small +scale is usually supplied by primary cells, the Daniell cell +being a favourite for this purpose. In large offices the +current is generally taken from a battery of storage cells.</p> + +<p>During the early days of telegraphy, overhead lines +were a source of considerable danger when thunderstorms +were taking place. Lightning flashes often completely +wrecked the instruments, giving severe shocks to those in +the vicinity, and in a few cases operators were killed at +their posts. Danger of this kind is now obviated by the +use of contrivances known as lightning arresters. There +are several forms of these, but only one need be mentioned. +The main features of this are two metal plates separated +slightly from one another, so that there is a small air gap +between them. One plate is connected to the line wire,<span class="pagenum" id="Page_143">143</span> +and the other to earth. Almost all lightning flashes consist +of an oscillatory discharge, that is one which passes a +number of times backwards and forwards between a cloud +and the earth. A very rapidly alternating discharge of +this kind finds difficulty in passing along the line wire, +being greatly impeded by the coils of wire in the various +pieces of apparatus; and although the resistance of this +air gap is very high, the lightning discharge will cross the +gap sooner than struggle along the line wire. In this way, +when a flash affects the line, the discharge jumps the gap +between the plates of the arrester and passes away harmlessly +to earth, without entering the telegraph office at all. +As was mentioned in <a href="#chapter_III">Chapter III</a>., the prevalence of +magnetic storms sometimes renders telegraph lines quite +unworkable for a time, but although such disturbances +cause great delay and general inconvenience, they are not +likely to be at all dangerous. It is often possible to +maintain telegraphic communication during magnetic disturbances +by using two lines to form a complete metallic +loop, so that there is no earth return.</p> +<hr class="chap x-ebookmaker-drop"> + +<div class="chapter"> +<p><span class="pagenum" id="Page_144">144</span></p> + +<h2 class="nobreak" id="toclink_144"><a id="chapter_XVII"></a>CHAPTER XVII<br> + +<span class="subhead">SUBMARINE TELEGRAPHY</span></h2> +</div> + +<p class="in0"><span class="firstword">The</span> story of submarine telegraphy is a wonderful record +of dogged perseverance in the face of tremendous obstacles +and disastrous failures. It would be of no interest to trace +the story to its very beginning, and so we will commence +with the laying of the first cable across the English +Channel from Dover to Calais, in 1850. A single copper +wire covered with a layer of gutta-percha half an inch thick +was used, and leaden weights were attached to it at intervals +of one hundred yards, the fixing of each weight necessitating +the stoppage of the cable-laying ship. The line was laid +successfully, but it failed after working for a single day, and +it afterwards turned out that a Boulogne fisherman had +hauled up the cable with his trawl. This line proved that +telegraphic communication between England and France +was possible, but the enterprise was assailed with every +imaginable kind of abuse and ridicule. It is said that some +people really believed that the cable was worked in the +style of the old-fashioned house bell, and that the signals +were given by pulling the wire! In the next year another +attempt was made by Mr. T. R. Crampton, a prominent +railway engineer, who himself contributed half of the +£15,000 required. The form of cable adopted by him +consisted of four copper wires, each covered with two layers +of gutta-percha, and the four enclosed in a covering formed +of ten galvanized iron wires wound spirally round them.<span class="pagenum" id="Page_145">145</span> +The line proved a permanent success, and this type of +cable, with certain modifications, is still in use. In 1852 +three attempts were made to connect England and Ireland, +but the first two failed owing to the employment of cables +too light to withstand the strong tidal currents, and the +third was somehow mismanaged as regards the paying-out, +so that there was not enough cable to reach across. A +heavier cable was tried in the next year, and this was a +lasting success.</p> + +<p>The success of these two cables led to the laying of +many other European cables over similar distances, but we +must now pass on to a very much bigger undertaking, the +laying of the Atlantic cable. In 1856 the Atlantic Telegraph +Company was formed, with the object of establishing +and working telegraphic communication between Ireland +and Newfoundland, the three projectors being Messrs. +J. W. Brett, C. T. Bright, and C. W. Field. The British +and the United States Governments granted a subsidy, in +return for which Government messages were to have +priority over all others, and were to be transmitted free. +The objections launched against the scheme were of course +many, some of them making very amusing reading. It is +however very strange to find so eminent a scientist as +Professor Airy, then Astronomer Royal, seriously stating +that it was a mathematical impossibility to submerge a +cable safely to such depths, and that even if this could be +done, messages could not be transmitted through such a +great length of cable.</p> + +<p>It was estimated that a length of about 2500 nautical +miles would be enough to allow for all contingencies, and +the construction of the cable was commenced in February +1857, and completed in June of that year. It is difficult to +realize the gigantic nature of the task of making a cable of +such dimensions. The length of copper wire used in<span class="pagenum" id="Page_146">146</span> +making the conductor was 20,500 miles, while the outer +sheathing took 367,500 miles of iron wire; the total length +of wire used being enough to go round the Earth thirteen +times. The cable was finally stowed away on board two +warships, one British and the other American.</p> + +<p>The real troubles began with the laying of the cable. +After landing the shore end in Valentia Bay, the paying-out +commenced, but scarcely had five miles been laid when +the cable caught in the paying-out machinery and parted. +By tracing it from the shore the lost end was picked up +and spliced, and the paying-out began again. Everything +went well for two or three days, and then, after 380 miles +had been laid, the cable snapped again, owing to some +mismanagement of the brakes, and was lost at a depth of +2000 fathoms. The cable had to be abandoned, and the +ships returned to Plymouth.</p> + +<p>In the next year, 1858, another attempt was made, with +new and improved machinery and 3000 miles of cable, and +this time it was decided that the two ships should start +paying-out from mid-ocean, proceeding in opposite directions +towards the two shores after splicing their cables. On the +voyage out the expedition encountered one of the most +fearful storms on record, which lasted over a week, and the +British man-of-war, encumbered with the dead weight of +the cable, came near to disaster. Part of the cable shifted, +and those on board feared that the whole of the huge mass +would break away and crash through the vessel’s side. +Sixteen days after leaving Plymouth the rendezvous was +reached, the cables were spliced and the ships started. +After the British ship had paid out 40 miles it was discovered +that the cable had parted at some distance from +the ship, and the vessels once more sought each other, and +spliced again ready for another effort. This time the cable +parted after each vessel had paid out a little more than<span class="pagenum" id="Page_147">147</span> +100 miles, and the ships were forced to abandon the +attempt.</p> + +<p>The failure of this second expedition naturally caused +great discouragement, and the general feeling was that the +whole enterprise would have to be given up. The chairman +of the company recommended that in order to make the +best of a bad job the remainder of the cable should be sold, +and the proceeds divided amongst the shareholders, but +after great efforts on the part of a dauntless few who refused +to admit defeat, it was finally decided to make one +more effort. No time was lost, and on 17th July 1858 +the vessels again sailed from Queenstown. As before, the +cables were spliced in mid-ocean, and this time, after many +anxious days, many false alarms, and one or two narrow +escapes from disaster through faulty pieces of cable discovered +almost too late, the cable was landed successfully +on both shores of the Atlantic early in August.</p> + +<p>The Atlantic cable was now an accomplished fact, and +dismal forebodings were turned into expressions of extravagant +joy. The first messages passed between Queen +Victoria and the President of the United States, and +amongst the more important communications was one +which prevented the sailing from Canada of two British +regiments which had been ordered to India during the +Mutiny. In the meantime the Indian Mutiny had been +suppressed, and therefore these regiments were not required. +The dispatch of this message saved a sum of about +£50,000. The prospects of the cable company seemed +bright, but after a short time the signals began to grow +weaker and weaker, and finally, after about seven hundred +messages had been transmitted, the cable failed altogether. +This was a great blow to the general public, and we can +imagine the bitter disappointment of the engineers and +electricians who had laboured so hard and so long to bring<span class="pagenum" id="Page_148">148</span> +the cable into being. It was a favourable opportunity for +the croakers, and amongst a certain section of the public +doubts were expressed as to whether any messages had +been transmitted at all.</p> + +<p>A great consultation of experts took place with the +object of determining the cause of the failure, and the +unanimous opinion was that the cable had been injured by +the use of currents of too great intensity. Some years +elapsed before another attempt could be made, but the +idea was never abandoned, and a great deal of study was +given to the problems involved. Mr. Field, the most +energetic of the original projectors, never relaxed his determination +that the cable should be made a success, and he +worked incessantly to achieve his ambition. It is said +that in pursuance of his object he made sixty-four crossings +of the Atlantic, and considering that he suffered greatly +from sea-sickness every time this shows remarkable pluck +and endurance.</p> + +<p>In 1865, new capital having been raised, preparations +were made for another expedition. It was now decided +to use only one vessel for laying the cable, and the <i>Great +Eastern</i> was chosen for the task. This vessel had been +lying idle for close on ten years, owing to her failure as a +cargo boat, but her great size and capacity made her most +suitable for carrying the enormous weight of the whole +cable. In July 1865 the <i>Great Eastern</i> set sail, under +the escort of two British warships. When 84 miles had +been paid out, a fault occurred, and after drawing up about +10½ miles it was found that a piece of iron wire had pierced +the coating of the cable. The trouble was put right, and +the paying-out continued successfully until over 700 miles +had been laid, when another fault appeared. The cable +was again drawn in until the fault was reached, and +another piece of iron was found piercing clean through.<span class="pagenum" id="Page_149">149</span> +It was evident that two such pieces of iron could not have +got there by accident, and there was no doubt that they +had been inserted intentionally by some malicious scoundrel, +most likely with the object of affecting the company’s +shares. A start was made once more, and all went well +until about two-thirds of the distance had been covered, +when the cable broke and had to be abandoned after +several nearly successful attempts to recover it.</p> + +<p>In spite of the loss, which amounted to £600,000, the +energetic promoters contrived to raise fresh capital, and in +1866 the <i>Great Eastern</i> started again. This effort was +completely successful, and on 28th July 1866 the cable +was landed amidst great rejoicing. The following extracts +from the diary of the engineer Sir Daniell Gooch, give us +some idea of the landing.</p> + +<p>“Is it wrong that I should have felt as though my +heart would burst when that end of our long line touched +the shore amid the booming of cannon, the wild, half-mad +cheers and shouts of the men?... I am given a never-dying +thought; that I aided in laying the Atlantic cable.... +The old cable hands seemed as though they could +eat the end; one man actually put it into his mouth and +sucked it. They held it up and danced round it, cheering +at the top of their voices. It was a strange sight, nay, a +sight that filled our eyes with tears.... I did cheer, but +I could better have silently cried.”</p> + +<p>This time the cable was destined to have a long and +useful life, and later in the same year the 1865 cable was +recovered, spliced to a new length, and safely brought to +land, so that there were now two links between the Old +World and the New. It was estimated that the total cost +of completing the great undertaking, including the cost of +the unsuccessful attempts, was nearly two and a half millions +sterling. Since 1866 cable-laying has proceeded very<span class="pagenum" id="Page_150">150</span> +rapidly, and to-day telegraphic communication exists between +almost all parts of the civilized world. According +to recent statistics, the North Atlantic Ocean is now +crossed by no less than 17 cables, the number of cables +all over the world being 2937, with a total length of +291,137 nautical miles.</p> + +<p>Before describing the actual working of a submarine +cable, a few words on cable-laying may be of interest. +Before the cable-ship starts, another vessel is sent over +the proposed course to make soundings. Galvanized steel +pianoforte wire is used for sounding, and it is wound in +lengths of 3 or 4 nautical miles on gun-metal drums. +The drums are worked by an engine, and the average +speed of working is somewhere about 100 fathoms a +minute in descending, and 70 fathoms a minute in picking +up. Some idea of the time occupied may be gained from +a sounding in the Atlantic Ocean which registered a depth +of 3233 fathoms, or nearly 3½ miles. The sinker took +thirty-three minutes fifty seconds in descending, and forty-five +minutes were taken in picking up. The heavy sinker +is not brought up with the line, but is detached from the +sounder by an ingenious contrivance and left at the bottom. +The sounder is fitted with an arrangement to bring up a +specimen of the bottom, and also a sample of water; and +the temperature at any depth is ascertained by self-registering +thermometers.</p> + +<p>When the soundings are complete the cable-ship takes +up her task. The cable is coiled in tanks on board, and +is kept constantly under water to prevent injury to the +gutta-percha insulation by overheating. As each section +is placed in the tank, the ends of it are led to a test-box, +and labelled so that they can be easily recognized. Insulated +wires run from the test-box to instruments in the +testing-room, so that the electrical condition of the whole<span class="pagenum" id="Page_151">151</span> +cable is constantly under observation. During the whole +time the cable is being laid its insulation is tested continuously, +and at intervals of five minutes signals are sent from +the shore end to the ship, so that a fault is instantly detected. +The cable in its tank is eased out by a number of +men, and mechanics are posted at the cable drums and +brakes, while constant streams of water cool the cable and +the bearings and surfaces of the brakes. The tension, as +shown by the dynamometer, is at all times under careful +observation. When it becomes necessary to wind back the +cable on account of some fault, cuts are made at intervals +of a quarter or half a mile, tests being made at each cutting +until the fault is localized in-board. As soon as the cable +out-board is found “O.K.,” the ends are spliced up and the +paying-out begins again. If the cable breaks from any +cause, a mark-buoy is lowered instantly on the spot, and +the cable is grappled for. This may take a day or two in +good weather, but a delay of weeks may be caused by bad +weather, which makes grappling impossible.</p> + +<p>The practical working of a submarine cable differs in +many respects from that of a land telegraph line. The +currents used in submarine telegraphy are extremely small, +contrary to the popular impression. An insulated cable +acts like a Leyden jar, in the sense that it accumulates +electricity and does not quickly part with it, as does a bare +overhead wire. In the case of a very long cable, such as +one across the Atlantic, a current continues to flow from it +for some time after the battery is disconnected. A second +signal cannot be sent until the electricity is dissipated and +the cable clear, and if a powerful current were employed +the time occupied in this clearing would be considerable, so +that the speed of signalling would be slow. Another +objection to a powerful current is that if any flaw +exists in the insulation of the cable, such a current is apt<span class="pagenum" id="Page_152">152</span> +to increase the flaw, and finally cause the breakdown of +the line.</p> + +<p>The feebleness of the currents in submarine telegraphy +makes it impossible to use the ordinary land telegraph +receiver, and a more sensitive instrument known as the +“mirror receiver” is used. This consists of a coil of very +fine wire, in the centre of which a tiny magnetic needle is +suspended by a fibre of unspun silk. A magnet placed close +by keeps the needle in one position when no current is +flowing. As the deflections of the needle are extremely +small, it is necessary to magnify them in some way, and +this is done by fixing to the needle a very small mirror, +upon which falls a ray of light from a lamp. The mirror +reflects this ray on to a sheet of white paper marked with +a scale, and as the mirror moves along with the needle the +point of light travels over the paper, a very small movement +of the needle causing the light to travel some inches. +The receiving operator sits in a darkened room and +watches the light, which moves to the right or to the left +according to the direction of the current. The signals +employed are the same as those for the single-needle +instrument, a movement to the left indicating a dot, and +one to the right a dash. In many instruments the total +weight of magnet and mirror is only two or three grains, +and the sensitiveness is such that the current from a voltaic +cell consisting of a lady’s silver thimble with a few drops of +acidulated water and a diminutive rod of zinc, is sufficient +to transmit a message across the Atlantic.</p> + +<p>The mirror receiver cannot write down its messages, +and for recording purposes an instrument invented by Lord +Kelvin, and called the “siphon recorder,” is used. In this +instrument a coil of wire is suspended between the poles of +an electro-magnet, and to it is connected by means of a silk +fibre a delicate glass tube or siphon. One end of the<span class="pagenum" id="Page_153">153</span> +siphon dips into an ink-well, and capillary attraction causes +the ink to fill the siphon. The other end of the siphon +almost touches a moving paper ribbon placed beneath it. +The ink and the paper are oppositely electrified, and the +attraction between the opposite charges causes the ink to +spurt out of the siphon in very minute drops, which fall on +to the paper. As long as no current is passing the siphon +remains stationary, but when a current flows from the cable +through the coil, the latter moves to one side or the other, +according to the direction of the current, and makes the +siphon move also. Consequently, instead of a straight line +along the middle of the paper ribbon, a wavy line with +little peaks on each side of the centre is produced by the +minute drops of ink. This recorder sometimes refuses to +work properly in damp weather, owing to the loss of the +opposite charges on ink and paper, but a later inventor, +named Cuttriss, has removed this trouble by using a siphon +kept constantly in vibration by electro-magnetism. The +ordinary single-needle code is used for the siphon recorder.</p> +<hr class="chap x-ebookmaker-drop"> + +<div class="chapter"> +<p><span class="pagenum" id="Page_154">154</span></p> + +<h2 class="nobreak" id="toclink_154"><a id="chapter_XVIII"></a>CHAPTER XVIII<br> + +<span class="subhead">THE TELEPHONE</span></h2> +</div> + +<p class="in0"><span class="firstword">In</span> our younger days most of us have amused ourselves +with a toy telephone consisting of a long piece of string +having each end passed through the bottom of a little cardboard +box, and secured by a knot. If the string is stretched +tightly this arrangement enables whispered words to be +heard at a distance of 20 or 30 yards. Simple as is +this little toy, yet it is probable that many people would +be rather nonplussed if asked suddenly to explain how the +sounds travel along the string from one box to the other. +If the toy had some complicated mechanism most likely +every one would want to know how it worked, but the whole +thing is so extremely simple that generally it is dismissed +without a thought.</p> + +<p>If we strike a tuning-fork and then hold it close to the +ear, we hear that it produces a sound, and at the same time, +from a slight sensation in the hand, we become aware that +the fork is in vibration. As the fork vibrates it disturbs +the tiny particles of air round it and sets them vibrating, +and these vibrations are communicated from one particle to +another until they reach the drum of the ear, when that also +begins to vibrate and we hear a sound. This is only +another way of saying that the disturbances of the air +caused by the vibrations of the tuning-fork are propagated +in a series of waves, which we call “sound waves.” Sound is +transmitted better through liquids than through the air, and<span class="pagenum" id="Page_155">155</span> +better still through solids, and this is why words spoken so +softly as to be inaudible through the air at a distance of, +say, 100 feet, can be heard fairly distinctly at that distance +by means of the string telephone. The sound reaches us +along the string in exactly the same way as through the air, +that is, by means of minute impulses passed on from particle +to particle.</p> + +<p>A more satisfactory arrangement than the string telephone +consists of two thin plates of metal connected by a +wire which is stretched very tightly. Words spoken close +to one plate are heard by a listener at the other plate up to +a considerable distance. Let us try to see exactly what +takes place when this apparatus is used. In the act of +speaking, vibrations are set up in the air, and these in turn +set up vibrations in the metal plate. The vibrations are +then communicated to the wire and to the metal plate at +the other end, and finally the vibrations of this plate produce +vibrations in the air between the plate and the listener, +and the sound reaches the ear.</p> + +<p>This simple experiment shows the remarkable fact that +a plate of metal is able to reproduce faithfully all the vibrations +communicated to it by the human voice, and from this +fact it follows that if we can communicate the vibrations set +up in one plate by the voice, to another plate at a distance +of 100 miles, we shall be able to speak to a listener +at the further plate just as if he were close to us. A +stretched string or wire transmits the vibrations fairly well +up to a certain distance, but beyond this distance the vibrations +become weaker and weaker until no sound at all +reaches the air. By the aid of electricity, however, we can +transmit the vibrations to a tremendous distance, the range +being limited only by the imperfections of our apparatus.</p> + +<p>The first attempt at the construction of an electric +telephone, that is an instrument by means of which the<span class="pagenum" id="Page_156">156</span> +vibrations set up by the voice or by a musical instrument +are transmitted by electricity, was made in 1860 by Johann +Philipp Reis, a teacher in a school at Friedrichsdorf, in +Germany. His transmitting apparatus consisted of a box +having a hole covered by a tightly stretched membrane, to +which was attached a little strip of platinum. When the +membrane was made to vibrate by sounds produced close +to the box, the strip of platinum moved to and fro against +a metal tip, which closed the circuit of a battery. The +receiver was a long needle of soft iron round which was +wound a coil of wire, and the ends of the needle rested on +two little bridges of a sounding box. The vibrations of +the membrane opened and closed the circuit at a great +speed, and the rapid magnetization of the needle produced +a tone of the same pitch as the one which set the membrane +vibrating. This apparatus transmitted musical sounds and +melodies with great accuracy, but there is considerable +difference of opinion as to whether it was able to transmit +speech. Professor Sylvanus Thompson distinctly states +that Reis’s telephone could and did transmit speech, but +other experts dispute the fact. We probably shall be quite +safe in concluding that this telephone did transmit speech, +but very imperfectly. In any case it is certain that the +receiver of this apparatus is not based on the same principle +as the modern telephone receiver.</p> + +<p>Some years later Graham Bell, Professor of Vocal +Physiology in the University of Boston, turned his attention +to the electric transmission of speech, probably being led to +do so from his experiments in teaching the deaf and dumb. +His apparatuses shown at an exhibition in Philadelphia in +1876, consisted of a tube having one end open for speaking +into, and the other closed by a tightly stretched membrane +to which was attached a very light steel bar magnet. The +vibrations set up in the membrane by the voice made the<span class="pagenum" id="Page_157">157</span> +little magnet move to and fro in front of the poles of an +electro-magnet, inserted in a battery circuit, thus inducing +currents of electricity in the coils of the latter magnet. +The currents produced in this way varied in direction and +strength according to the vibratory movements of the +membrane, and being transmitted along a wire they +produced similar variations in current in another electro-magnet +in the receiver. The currents produced in this +manner in the receiver set up vibrations in a metal +diaphragm in front of the magnet poles, and so the words +spoken into the transmitter were reproduced.</p> + +<p>Since the year 1876 the telephone has developed with +remarkable rapidity, and an attempt to trace its growth +would involve a series of detailed descriptions of closely +similar inventions which would be quite uninteresting to +most readers. Now, therefore, that we have introduced +the instruments, and seen something of its principle and +its early forms, it will be most satisfactory to omit the +intermediate stages and to go on to the telephone as used +in recent years. The first telephone to come into general +use was the invention of Graham Bell, and was an improved +form of his early instrument just described. A case or +tube of ebonite, which forms the handle of the instrument, +contains a steel bar magnet having a small coil of insulated +wire at the end nearest the mouthpiece of the tube, the +ends of the coil passing along the tube to be connected to +the line wires. Close to the coil end of the magnet, and +between it and the mouthpiece, is fixed a diaphragm of +thin sheet-iron. A complete outfit consists of two of these +instruments connected by wires, and it will be noticed that +no battery is employed.</p> + +<p>The air vibrations set up by the voice make the +diaphragm vibrate also, so that it moves backwards and +forwards. These movements are infinitesimally small, but<span class="pagenum" id="Page_158">158</span> +they are sufficient to affect the lines of force of the magnet +to such an extent that rapidly alternating currents of varying +degrees of strength are set up in the coil and sent along +the line wire. On arriving at the receiver these currents +pass through the coil and produce rapid variations in the +strength of the magnet, so that instead of exerting a +uniform attraction upon the iron diaphragm, the magnet +pulls it with constantly varying force, and thus sets it +vibrating. The air in front of the diaphragm now begins +to vibrate, and the listener hears a reproduction of the +words spoken into the transmitter. The way in which the +fluctuations of the current make the second diaphragm +vibrate exactly in accordance with the first is very remarkable, +and it is important to notice that the listener does not +hear the actual voice of the speaker, but a perfect reproduction +of it; in fact, the second diaphragm speaks.</p> + +<p>The reader probably will be surprised to be told that +the transmitter and the receiver of a magneto-electric +telephone are respectively a dynamo and electric motor of +minute proportions. We provide a dynamo with mechanical +motion and it gives us electric current, and by sending +this current through an electric motor we get mechanical +motion back again. In the transmitter of the telephone +just described, the mechanical motion is in the form of +vibrations of the metal diaphragm, which set up currents of +electricity in the coil of wire round the magnet, so that the +transmitter is really a tiny dynamo driven by the voice. +The receiver is provided with electric current from the +transmitter, and it converts this into mechanical motion in +the diaphragm, so that the receiver is a little electric motor.</p> + +<p>Transmitters of the type just described work well over +short distances, but the currents they produce are too feeble +for transmission over a very long wire, and on this account +they have been superseded by transmitters on the microphone<span class="pagenum" id="Page_159">159</span> +principle. A microphone is an instrument for +making extremely small sounds plainly audible. If a +current is passed through a box containing loose bits of +broken carbon, it meets with great resistance, but if the +bits of carbon are compressed their conducting power is +considerably increased. Even such slight differences in +pressure as are produced by vibrating the box will affect +the amount of current passing through the carbon. If this +current is led by wires to an ordinary telephone receiver +the arrangement becomes a simple form of microphone. +The vibrations of the box vary +the resistance of the carbon, +and the corresponding variations +in the current set up +vibrations in the receiver, but +in a magnified form. The +smallest sound vibrations alter +the resistance of the carbon, +and as these vibrations are +magnified in the receiver, the +reproduced sound is magnified +also. The footsteps of a fly +may be heard quite distinctly by means of a good microphone, +and the ticks of a watch sound like the strokes of a +hammer.</p> + +<figure id="fig_31" class="figright" style="max-width: 11em;"> + <img src="images/i_189.png" width="801" height="903" alt=" "> + <figcaption class="caption"><span class="smcap">Fig. 31.</span>—Diagram of Microphone Transmitter. +</figcaption></figure> + +<p>By means of this power of magnifying vibrations a +microphone transmitter can be used on a line of tremendous +length, where an ordinary Bell transmitter would be utterly +useless. The general features of this transmitter, <a href="#fig_31">Fig. 31</a>, +are a diaphragm and a block of carbon separated slightly +from one another, the intervening space being filled with +granules of carbon. These are enclosed in a case of +ebonite having a mouthpiece in front and two terminals +behind, one terminal being connected with the carbon block<span class="pagenum" id="Page_160">160</span> +and the other with the diaphragm. From these terminals +wires are led to a battery and to the receiver, which is of +the Bell type. The current has to pass through the carbon +granules, and the movements of the diaphragm when set in +vibration by the voice vary the pressure upon the granules, +and in this way set up variations in the current. Carbon +dust also may be used instead of granular carbon, and then the +instrument is called a “dust transmitter.”</p> + +<figure id="fig_32" class="figleft" style="max-width:7em;"> + <img src="images/i_190.png" width="528" height="1277" alt=" "> + <figcaption class="caption hang"><span class="smcap">Fig. 32.</span>—Combined Telephone Transmitter +and Receiver. +</figcaption></figure> + +<p>It is usual to have a transmitter and +a receiver on one handle for the greater +convenience of the user. The arrangement +is shown in <a href="#fig_32">Fig. 32</a>, and it will be +seen that when the user places the receiver +to his ear the transmitting mouthpiece is +in position for speaking. The microphone +with its carbon dust is placed at A, just +below the mouthpiece, and the earpiece +or receiver B contains a little magnet and +coil with a diaphragm in front, so that it +is really a Bell instrument. A little lever +will be noticed at C. This is a switch +which brings the transmitter into circuit +on being pressed with the finger.</p> + +<p>It is now time to see something of the +arrangement and working of telephone +systems. As soon as the telephone became a commercially +practicable instrument the necessity for some means of +inter-communication became evident, and the telephone +exchange was brought into being. The first exchange +was started in 1877, in Boston, but this was a very +small affair and it was run on very crude lines. When +one subscriber wished to communicate with another he +had to call up an operator, who received the message +and repeated it to the person for whom it was intended;<span class="pagenum" id="Page_161">161</span> +there was no direct communication between the various +subscribers’ instruments. As the number of users increased +it became necessary to devise some system +whereby each subscriber could call the attention of an +operator at the central station, and be put into direct +communication with any other subscriber without delay; +and the exchange system of to-day, which fulfils these +requirements almost to perfection, is the result of gradual +improvements in telephone methods extending over some +thirty-five years.</p> + +<p>When a subscriber wishes to telephone, he first must +call up the operator at the exchange. Until comparatively +recently this was done by turning a handle placed at the +side of the instrument. This handle operated a little +dynamo, and the current produced caused a shutter at the +exchange to drop and reveal a number, just as in the +electric bell indicator, so that the operator knew which +instrument was calling. As soon as the operator answered +the call, the shutter replaced itself automatically. The +signal to disconnect was given in the same way, but the +indicator was of a different colour in order to prevent confusion +with a call signal. These handle-operated telephones +are still in common use, but they are being replaced by +instruments which do away with handle-turning on the +part of the subscriber, and with dropping shutters at the +exchange. In this latest system all that the subscriber has +to do is to lift his telephone from its rest, when a little +electric lamp lights up at the exchange; and when he has +finished his conversation he merely replaces the telephone, +and again a little lamp glows.</p> + +<p>We must now see what happens at the exchange when +a call is made. Each operator has control of a number of +pairs of flexible cords terminating in plugs, the two cords +of each pair being electrically connected. The plugs rest<span class="pagenum" id="Page_162">162</span> +on a shelf in front of the operator, and the cords pass +through the shelf and hang down below it. If a plug is +lifted, the cord comes up through the shelf, and it is drawn +back again by a weight when the plug is not in use. Two +lamps are provided for each pair of cords, one being fixed +close to each cord. The two wires leading from each +subscriber’s instrument are connected to a little tube-shaped +switch called a “jack,” and each jack has a lamp of its own. +When a subscriber lifts his telephone from its rest a lamp +glows, and the operator inserts one plug of a pair into the +jack thus indicated, and the lamp goes out automatically. +She then switches on her telephone to the caller and asks +for the number of the subscriber to whom he wishes to +speak; and as soon as she gets this she inserts the other +plug of the pair into the jack belonging to this number. +By a simple movement she then rings up the required +person by switching on the current to his telephone bell.</p> + +<p>Here comes in the use of the two lamps connected with +the cords. As long as the subscribers’ telephones are on +their rests the lamps are lighted, but as soon as they are +lifted off the lamps go out. The caller’s telephone is of +course off its rest, and so the lamp connected with the first +cord is not lit; but until the subscriber rung up lifts his +instrument to answer the call, the lamp of the second cord +remains lit, having first lighted up when the plug was +inserted in the jack of his number. When the second +lamp goes out the operator knows that the call has been +responded to, and that the two subscribers are in communication +with each other. Having finished their conversation, +both subscribers replace their instruments on the +rests, whereupon both lamps light up, informing the operator +that she may disconnect by pulling out the plugs.</p> + +<p>It is manifestly impossible for one operator to attend +to the calls of all the subscribers in the exchange, and so a<span class="pagenum" id="Page_163">163</span> +number of operators are employed, each one having to +attend to the calls of a certain number of subscribers. At +the same time it is clear that each operator may be called +upon to connect one of her subscribers to any other subscriber +in the whole exchange. In order to make this +possible the switchboard is divided into sections, each +having as many jacks as there are lines in the exchange, so +that in this respect all the sections are multiples of each +other, and the whole arrangement is called a “multiple +switchboard,” the repeated jacks being called “multiple +jacks.” Then there are other jacks which it is not necessary +to duplicate. We have seen that when a subscriber calls the +exchange a lamp glows, and the operator inserts a plug into +the jack beside the lamp, in order to answer the call and +ascertain what number is required. These are called +“answering jacks,” and the lamp is the line signal. It is +usual to have three operators to each section of the switchboard, +and each operator has charge of so many answering +jacks, representing so many subscribers. At the same +time she has access to the whole section, so that she can +connect any of her subscribers to any other line in the +exchange.</p> + +<p>When a number is called for, the operator must be able +to tell at once whether the line is free or not. The jack +in her section may be unoccupied, but she must know also +whether all the multiple jacks belonging to that number +are free, for an operator at another section may have +connected the line to one of her subscribers. To enable +an operator to ascertain this quickly an electrical test is +provided. When two lines are connected, the whole of the +multiple jacks belonging to each are charged with electricity, +and if an operator at any section touches one of these jacks +with a plug, a current through her receiver makes a click, +and on hearing the click she knows that the line is engaged.<span class="pagenum" id="Page_164">164</span> +The testing takes an extremely short time, and this is why a +caller receives the reply, “Number engaged,” so promptly +that he feels inclined to doubt whether the operator has +made any attempt at all to connect him up to the number.</p> + +<p>In order that an operator may have both hands free to +manipulate the plugs, her telephone receiver is fixed over +one ear by a fastening passing over her head, and the +transmitter is hung from her shoulders so as to be close to +her mouth.</p> + +<p>In telegraphy it is the rule to employ the earth for the +return part of the circuit, but this is not customary in +telephony. The telephone is a much more sensitive +instrument than the telegraph, and a telephone having an +earth return is subject to all kinds of strange and weird +noises which greatly interfere with conversation. These +noises may be caused by natural electrical disturbances, or +by the proximity of telegraph and other wires conveying +electric currents. On this account telephone lines are +made with a complete metallic circuit. As in telegraphy, +protection from lightning flashes is afforded by lightning +arresters. The current for the working of a telephone +exchange is supplied from a central battery of accumulators, +and also from dynamos.</p> + +<figure id="plate_XII" class="figcenter" style="max-width: 40em;"> + <p class="caption">PLATE XII.</p> + <img src="images/i_195.jpg" width="3146" height="2012" alt=" "> + <figcaption class="caption"> + +<p class="floatl"><i>By permission of</i></p> +<p class="floatr"><i>Craven Brothers Ltd.</i></p> + +<p class="floatc">LARGE ELECTRIC TRAVELLING CRANE AT A RAILWAY WORKS.</p> +</figcaption></figure> + +<p>Although the manual exchange telephone system of +to-day works with remarkable efficiency, it has certain +weak points. For instance, if an operator cares to do so, +she can listen to conversations between subscribers, so +that privacy cannot be assured. As a matter of fact, the +operators have little time for this kind of thing, at any rate +during the busy hours of the day, and as a rule they are +not sufficiently interested in other people’s affairs to make +any attempt to listen to their remarks. The male operators +who work through the slack hours of the night are +occasionally guilty of listening. Some time ago the writer +had to ring up a friend in the very early morning, and +during the conversation this gentleman asked what time it +was. Before the writer had time to get a word out, a deep +bass voice from the exchange replied, “Half-past two.” +Little incidents of this sort remind one that it is not wise +to speak too freely by telephone. Then again operators are +liable to make wrong connexions through faulty hearing +of the number called for, and these are equally annoying to +the caller and to the person rung up in mistake. Many +other defects might be mentioned, but these are sufficient +to show that the manual system is not perfect.</p> + +<p>For a long time inventors have been striving to do +away with all such defects by abolishing the exchange +operators, and substituting mechanism to work the +exchanges automatically, and during the last few years the +system of the Automatic Electric Company, of Chicago, +has been brought to great perfection. This system is in +extensive use in the United States, and is employed in +two or three exchanges in this country. Unfortunately +the mechanism of this system is extremely complicated, so +that it is impossible to describe it fully in a book of this +kind; but some idea of the method of working may be +given without entering into technical details.</p> + +<p>Each subscriber’s telephone instrument is fitted with a +dial which turns round on a pivot at its centre. This dial +has a series of holes round its circumference, numbered +consecutively from 1 to 9, and 0. Suppose now a +subscriber wishes to speak to a friend whose telephone +number is 2583. He removes the receiver from its hook, +places his finger in the hole marked 2, and turns the dial +round in a clockwise direction until his finger comes in +contact with a stop. He then removes his finger, and the +dial automatically returns to its original position. He then +places his finger in the hole marked 5, and again turns the<span class="pagenum" id="Page_166">166</span> +dial as far as the stop, and when the dial has returned to +the normal position he repeats the process with his finger +placed successively in the holes marked 8 and 3. He now +places the receiver to his ear, and by the time he has done +this the automatic mechanism at the exchange has made +the necessary connexions, and has rung the bell of +subscriber number 2583. On completing the conversation +each subscriber returns his receiver to its hook, and the +exchange mechanism returns to its normal position.</p> + +<p>The turning of the dial by the finger coils up a spring, +and this spring, acting along with a speed governor, makes +the dial return to its first position at a certain definite +speed as soon as the finger is removed. During this +retrograde movement a switch automatically sends out into +the line a certain number of impulses, the number being +determined by the hole in which the finger is placed. In +the case supposed, groups of two, five, eight, and three +impulses respectively would be sent out, each group +separated from the next by an interval during which the +subscriber is turning the dial.</p> + +<p>Now let us see what takes place at the exchange. +The subscriber’s instrument is connected to a mechanical +arrangement known as a “line switch.” This switch +is brought into play by the act of removing the receiver +from its hook, and it then automatically connects the +subscriber’s line to what is called a “first selector” switch. +The group of two impulses sent out by the first turning of +the dial raises this first selector two steps, and it then +sweeps along a row of contacts connected to “trunks” +going to the 2000 section. Passing by occupied trunks, it +finds an idle one, and so connects the line to an idle +“second selector.” This selector is operated by the second +group of impulses, five in number, and after being raised +five steps it acts like the first selector, and finds an idle<span class="pagenum" id="Page_167">167</span> +trunk leading to the 2500 section. This places the caller’s +line in connexion with still another switch called a +“connector,” and this switch, operated by the remaining +groups of eight and three impulses, finds the required tens +section, and selects the third member of that section. If +the number 2583 is disengaged, the connector switch now +sends current from the central battery to this instrument, +thus ringing its bell, and it also supplies speaking current +to the two lines during the conversation, restores the +exchange mechanism to its original condition as soon as +the conversation is ended and the subscribers have hung +up their receivers, and registers the call on the calling +subscriber’s meter. If the connector finds the number +engaged, it sends out an intermittent buzzing sound, to +inform the caller of the fact. All these operations take +time to describe, even in outline, but in practice they are +carried out with the utmost rapidity, each step in the connecting-up +process taking only a small fraction of a second.</p> + +<p>For ordinary local calls the automatic system requires +no operators at all, but for the convenience of users there +are usually two clerks at the exchange, one to give +any information required by subscribers, and the other to +record complaints regarding faulty working. For trunk +calls, the subscriber places his finger in the hole marked 0, +and gives the dial one turn. This connects him to an +operator at the trunk switchboard, who makes the required +connexion and then calls him up in the usual way.</p> + +<p>It might be thought that the complex mechanism of an +automatic exchange would constantly be getting out of +order, but it is found to work with great smoothness. +Each automatic switchboard has a skilled electrician in +attendance, and he is informed instantly of any faulty +working by means of supervisory lamps and other signals. +Even without these signals the attendant would be quickly<span class="pagenum" id="Page_168">168</span> +aware of any breakdown, for his ear becomes so accustomed +to the sounds made by the apparatus during the connecting-up, +that any abnormal sound due to faulty connecting +attracts his attention at once. However detected, the +faults are put right immediately, and it often happens that +a defective line is noted and repaired before the subscriber +knows that anything is wrong.</p> + +<p>On account of its high speed in making connexions +and disconnexions, its absolute accuracy, and its privacy, +the automatic telephone system has proved most popular +wherever it has been given a fair trial. Its advantages are +most obvious in large city exchanges where the traffic +during business hours is tremendously heavy, and it is +probable that before very long the automatic system will +have replaced manual methods for all such exchanges.</p> + +<p>The telephone system is more highly developed in the +United States than in this country, and some of the +exchanges have been made to do a great deal more than +simply transmit messages. For instance, in Chicago there +is a system by which a subscriber, on connecting himself to +a special circuit, is automatically informed of the correct +time, by means of phonographs, between the hours of 8 +a.m. and 10 p.m. New York goes further than this however, +and has a regular system of news circulation by telephone. +According to <cite>Electricity</cite>, the daily programme is +as follows: “8 a.m., exact astronomical time; 8 to 9 a.m., +weather reports, London Stock Exchange news, special +news item; 9 to 9.30 a.m., sales, amusements, business +events; 9.45 to 10 a.m., personal news, small notices; 10 +to 10.30 a.m., New York Stock Exchange and market +news; 11.30 a.m. to 12 noon, local news, miscellaneous; +12 noon, exact astronomical time, latest telegrams, military +and parliamentary news; 2 to 2.15 p.m., European cables; +1.15 to 2.30 p.m., Washington news; 2.30 to 2.45 p.m.,<span class="pagenum" id="Page_169">169</span> +fashions, ladies’ news; 2.45 to 3.15 p.m., sporting and +theatrical news; 3.15 to 3.30 p.m., closing news from Wall +Street; 3.30 to 5 p.m., musical news, recitals, etc.; 5 to 6 +p.m., feuilleton sketches, literary news; 8 to 10.30 p.m., +selected evening performance—music, opera, recitations.” +Considering the elaborate nature of this scheme one might +imagine that the subscription would be high, but as a +matter of fact it is only six shillings per month.</p> + +<p>The telephone has proved of great value in mine rescue +work, in providing means of communication between the +rescue party and those in the rear. This end is achieved +by means of a portable telephone, but as the members of a +rescue party often wear oxygen helmets, the ordinary telephone +mouthpiece is of no use. To overcome this difficulty +the transmitter is fastened round the throat. The vibrations +of the vocal cords pass through the wall of the throat, +and thus operate the transmitter. The receiver is fixed +over one ear by means of suitable head-gear, and the connecting +wire is laid by the advancing rescuers. A case +containing some hundreds of feet of wire is strapped round +the waist, and as the wearer walks forward this wire pays +itself out automatically.</p> + +<p>By the time that the telephone came to be a really +practical instrument, capable of communicating over long +distances on land, the Atlantic telegraph cable was in +operation, and an attempt was made to telephone from one +continent to the other by means of it, but without success. +In speaking of submarine telegraphy in <a href="#chapter_XVII">Chapter XVII</a>. we +saw that the cable acts like a Leyden jar, and it was this +fact that made it impossible to telephone through more than +about 20 miles of cable, so that transatlantic telephony +was quite out of the question. It was evident that little +progress could be made in this direction unless some means +could be devised for neutralizing this capacity effect, as it<span class="pagenum" id="Page_170">170</span> +is called, of the cable, and finally it was discovered that +this could be done by inserting at intervals along the cable +a number of coils of wire. These coils are known as “loading +coils,” and a cable provided with them is called a “loaded +cable.” Such cables have been laid across various narrow +seas, such as between England and France, and England +and Ireland, and these have proved very successful for +telephonic communication. The problem of transatlantic +telephony however still remains to be solved. Experiments +have been made in submarine telephony over a bare +iron cable, instead of the usual insulated cable. Conversations +have been carried on in this way without difficulty +between Seattle, Washington, U.S.A., and Vashon +Island, a total distance of about 11 miles, and it is +possible that uninsulated cables may play an extremely +important part in the development of submarine telephony.</p> +<hr class="chap x-ebookmaker-drop"> + +<div class="chapter"> +<p><span class="pagenum" id="Page_171">171</span></p> + +<h2 class="nobreak" id="toclink_171"><a id="chapter_XIX"></a>CHAPTER XIX<br> + +<span class="subhead">SOME TELEGRAPHIC AND TELEPHONIC INVENTIONS</span></h2> +</div> + +<p class="in0"><span class="firstword">In</span> telegraphy messages not only may be received, but also +recorded, by the Morse printer or one of its modifications, +but in ordinary telephony there is no mechanical method of +recording messages. This means that we can communicate +by telephone only when we can call up somebody to receive +the message at the other end, and if no one happens to be +within hearing of the telephone bell we are quite helpless. +This is always annoying, and if the message is urgent the +delay may be serious. Several arrangements for overcoming +this difficulty by means of automatic recording +mechanism have been invented, but the only really successful +one is the telegraphone.</p> + +<p>This instrument is the invention of Waldemar Poulsen, +whose apparatus for wireless telegraphy we shall speak of +in the next chapter. The telegraphone performs at the +same time the work of a telephone and of a phonograph. +In the ordinary type of phonograph the record is made in +the form of depressions or indentations on the surface of a +cylinder of wax; these indentations being produced by a +stylus actuated by vibrations set up in a diaphragm by the +act of speaking. In the telegraphone the same result is +obtained entirely by electro-magnetic action. The wax +cylinder of the phonograph is replaced by a steel wire or +ribbon, and the recording stylus by an electro-magnet.<span class="pagenum" id="Page_172">172</span> +The steel ribbon is arranged to travel along over two +cylinders or reels kept in constant rotation, and a small +electro-magnet is fixed midway between the cylinders so +that the ribbon passes close above it. This magnet is +connected to the telephone line, so that its magnetism +fluctuates in accordance with the variations in the current +in the line. We have seen that steel retains magnetism +imparted to it. In passing over the electro-magnet the +steel ribbon is magnetized in constantly varying degrees, +corresponding exactly with the variations in the line current +set up by the speaker’s voice, and these magnetic impressions +are retained by the ribbon. When the speaker has +finished, the telephone line is disconnected, the ribbon is +carried back to the point at which it started, and the +apparatus is connected to the telephone receiver. The +ribbon now moves forward again, and this time it acts like +the speaker’s voice, the varying intensity of its magnetic +record producing corresponding variations in the strength +of the magnet, so causing the receiver diaphragm to reproduce +the sounds in the ordinary way.</p> + +<p>The magnetic record made in this manner is fairly +permanent, and if desired it may be reproduced over and +over again. In most cases, however, a permanent record +is of no value, and so the magnetic impressions are +obliterated in order that the ribbon may be used to take +a new record. This can be done by passing a permanent +magnet along the ribbon, but it is more convenient to have +an automatic obliterating arrangement. This consists of +another electro-magnet fixed close to the recording magnet, +so that the ribbon passes over it before reaching the latter. +The obliterating magnet is connected with a battery, and +its unvarying magnetism destroys all traces of the previous +record, and the ribbon passes forward to the recording +magnet ready to receive new impressions.</p> + +<p><span class="pagenum" id="Page_173">173</span></p> + +<p>For recording telephone messages the telegraphone is +attached to the telephone instrument, and by automatically +operated switches it is set working by a distant speaker. +It records all messages received during the absence of its +owner, who, on his return, connects it to his receiver, and +thus hears a faithful reproduction of every word. By +speaking into his instrument before going out, the owner +can leave a message stating the time at which he expects +to return, and this message will be repeated by the telegraphone +to anybody ringing up in the meantime. The +most recent forms of telegraphone are capable of recording +speeches over an hour in length, and their reproduction is +as clear as that of any phonograph, indeed in many respects +it is considerably more perfect.</p> + +<p>Another electrical apparatus for recording speech may +be mentioned. This rejoices in the uncouth name of the +Photographophone, and it is the invention of Ernst +Ruhmer, a German. Its working is based upon the fact +that the intensity of the light of the electric arc may be +varied by sound vibrations, each variation in the latter +producing a corresponding variation in the amount of light. +In the photographophone the light of an arc lamp is passed +through a lens which focuses it upon a moving photographic +film. By speaking or singing, the light is made to vary in +brilliance, and proportionate effects are produced in the +silver bromide of the film. On developing the film a +permanent record of the changes in the light intensity is +obtained, in the form of shadings of different degrees of +darkness. The film is now moved forward from end to +end in front of a fairly powerful lamp. The light passes +through the film, and falls upon a sort of plate made of +selenium. This is a non-metallic substance which possesses +the curious property of altering its resistance to an electric +current according to the amount of light falling upon it;<span class="pagenum" id="Page_174">174</span> +the greater the amount of light, the more current will the +selenium allow to pass. The selenium plate is connected +with a telephone receiver and with a battery. As the film +travels along, its varying shadings allow an ever-changing +amount of light to pass through and fall upon the selenium, +which varies its resistance accordingly. The resulting +variations in the current make the receiver diaphragm give +out a series of sounds, which are exact reproductions of the +original sounds made by the voice. The reproduction of +speech by the photographophone is quite good, but as a +rule it is not so perfect as with the telegraphone.</p> + +<p>About ten years ago a German inventor, Professor A. +Korn, brought out the first really practical method of +telegraphing drawings or photographs. This invention is +remarkable not only for what it accomplishes, but perhaps +still more for the ingenuity with which the many peculiar +difficulties of the process are overcome. Like the photographophone, +Korn’s photo-telegraphic apparatus utilizes +the power of selenium to alter its resistance with the amount +of light reaching it.</p> + +<p>Almost everybody is familiar with the terms “positive” +and “negative” as used in photography. The finished paper +print is a positive, with light and shade in the correct +positions; while the glass plate from which the print is made +is a negative, with light and shade reversed. The lantern +slide also is a positive, and it is exactly like the paper print, +except that it has a base of glass instead of paper, so that +it is transparent. Similarly, a positive may be made on a +piece of celluloid, and this, besides being transparent, is +flexible. The first step in transmitting on the Korn system +is to make from the photograph to be telegraphed a positive +of this kind, both transparent and flexible. This is bent +round a glass drum or cylinder, and fixed so that it cannot +possibly move. The cylinder is given a twofold movement.<span class="pagenum" id="Page_175">175</span> +It is rotated by means of an electric motor, and at +the same time it is made to travel slowly along in the +direction of its length. In fact its movement is very +similar to that of a screw, which turns round and moves +forward at the same time. A powerful beam of light is +concentrated upon the positive. This beam remains +stationary, but owing to the dual movement of the cylinder +it passes over every part of the positive, following a spiral +path. Exactly the same effect would be produced by +keeping the cylinder still and moving the beam spirally +round it, but this arrangement would be more difficult to +manipulate. The forward movement of the cylinder is +extremely small, so that the spiral is as fine as it is possible +to get it without having adjacent lines actually touching. +The light passes through the positive into the cylinder, and +is reflected towards a selenium cell; and as the positive +has an almost infinite number of gradations of tone, or +degrees of light and shade, the amount of light reaching +the cell varies constantly all the time. The selenium +therefore alters its resistance, and allows a constantly +varying current to pass through it, and so to the transmission +line.</p> + +<p>At the receiving end is another cylinder having the +same rotating and forward movement, and round this is +fixed a sensitive photographic film. This film is protected +by a screen having a small opening, and no light can reach +it except through this aperture. The incoming current is +made to control a beam of light focused to fall upon the +screen aperture, the amount of light varying according to +the amount of current. In this way the beam of light, like +the one at the transmitting end, traces a spiral from end to +end of the film, and on developing the film a reproduction +of the original photograph is obtained. The telegraphed +photograph is thus made up of an enormous number<span class="pagenum" id="Page_176">176</span> +of lines side by side, but these are so close to one +another that they are scarcely noticed, and the effect is +something like that of a rather coarse-grained ordinary +photograph.</p> + +<p>It is obvious that the success of this method depends +upon the maintaining of absolute uniformity in the motion +of the two cylinders, and this is managed in a very ingenious +way. It will be remembered that one method of securing +uniformity in a number of sub-clocks under the control of +a master-clock is that of adjusting the sub-clocks to go a +little faster than the master-clock. Then, when the sub-clocks +reach the hour, they are held back by electro-magnetic +action until the master-clock arrives at the hour, +when all proceed together.</p> + +<p>A similar method is employed for the cylinders. They +are driven by electric motors, and the motor at the receiving +end is adjusted so as to run very slightly faster +than the motor at the sending end. The result is that +the receiving cylinder completes one revolution a minute +fraction of a second before the transmitting cylinder. It +is then automatically held back until the sending cylinder +completes its revolution, and then both commence the next +revolution exactly together. The pause made by the +receiving cylinder is of extremely short duration, but in +order that there shall be no break in the spiral traced by +light upon the film, the pause takes place at the point +where the ends of the film come together. In actual +practice certain other details of adjustment are required +to ensure precision in working, but the main features of +the process are as described.</p> + +<p>Although the above photo-telegraphic process is very +satisfactory in working, it has been superseded to some +extent by another process of a quite different nature. By +copying the original photograph through a glass screen<span class="pagenum" id="Page_177">177</span> +covered with a multitude of very fine parallel lines, a half-tone +reproduction is made. This is formed of an immense +number of light and dark lines of varying breadth, and it +is printed in non-conducting ink on lead-foil, so that while +the dark lines are bare foil, the light ones are covered with +the ink. This half-tone is placed round a metal cylinder +having the same movement as the cylinders in the previous +processes, and a metal point, or “stylus” as it is called, is +made to rest lightly upon the foil picture, so that it travels +all over it, from one end to the other. An electrical circuit +is arranged so that when the stylus touches a piece of the +bare foil a current is sent out along the line wire. This +current is therefore intermittent, being interrupted each +time the stylus passes over a part of the half-tone picture +covered with the non-conducting ink, the succeeding +periods of current and no current varying with the breadth +of the conducting and the non-conducting lines. This +intermittent current goes to a similar arrangement of +stylus and cylinder at the receiving end, this cylinder +having round it a sheet of paper coated with a chemical +preparation. The coating is white all over to begin with, +but it turns black wherever the current passes through it. +The final result is that the intermittent current builds up +a reproduction in black-and-white of the original photograph. +In this process also the cylinders have to be +“synchronized,” or adjusted to run at the same speed. +Both this process and the foregoing one have been used +successfully for the transmission of press photographs, +notably by the <cite>Daily Mirror</cite>.</p> + +<p>Professor Korn has carried out some interesting and +fairly successful experiments in wireless transmission of +photographs, but as yet the wireless results are considerably +inferior to those obtained with a line conductor. For +transmitting black-and-white pictures, line drawings, or<span class="pagenum" id="Page_178">178</span> +autographs by wireless, a combination of the two methods +just mentioned is employed; the second method being +used for sending, and the first or selenium method for +receiving. For true half-tone pictures the selenium method +is used at each end.</p> +<hr class="chap x-ebookmaker-drop"> + +<div class="chapter"> +<p><span class="pagenum" id="Page_179">179</span></p> + +<h2 class="nobreak" id="toclink_179"><a id="chapter_XX"></a>CHAPTER XX<br> + +<span class="subhead">WIRELESS TELEGRAPHY AND TELEPHONY—PRINCIPLES AND APPARATUS</span></h2> +</div> + +<p class="in0"><span class="firstword">Wireless</span> telegraphy is probably the most remarkable and +at the same time the most interesting of all the varied +applications of electricity. The exceptional popular +interest in wireless communication, as compared with most +of the other daily tasks which electricity is called upon +to perform, is easy to understand. The average man does +not realize that although we are able to make electricity +come and go at our bidding, we have little certain knowledge +of its nature. He is so accustomed to hearing of +the electric current, and of the work it is made to do, that +he sees little to marvel at so long as there is a connecting +wire. Electricity is produced by batteries or by a dynamo, +sent along a wire, and made to drive the necessary +machinery; apparently it is all quite simple. But take +away the connecting wire, and the case is different. In +wireless telegraphy electricity is produced as usual, but +instantly it passes out into the unknown, and, as far as +our senses can tell, it is lost for ever. Yet at some +distant point, hundreds or even thousands of miles away, +the electrical influence reappears, emerging from the +unknown with its burden of words and sentences. There +is something uncanny about this, something suggesting +telepathy and the occult, and herein lies the fascination of +wireless telegraphy.</p> + +<p><span class="pagenum" id="Page_180">180</span></p> + +<p>The idea of communicating without any connecting +wires is an old one. About the year 1842, Morse, of telegraph +fame, succeeded in transmitting telegraphic signals +across rivers and canals without a connecting wire. His +method was to stretch along each bank of the river a wire +equal in length to three times the breadth of the river. +One of these wires was connected with the transmitter and +with a battery, and the other with a receiver, both wires +terminating in copper plates sunk in the water. In this +case the water took the place of a connecting wire, and +acted as the conducting medium. A few years later +another investigator, a Scotchman named Lindsay, succeeded +in telegraphing across the river Tay, at a point +where it is over a mile and a half wide, by similar methods. +Lindsay appears to have been the first to suggest the possibility +of telegraphing across the Atlantic, and although at +that time, 1845, the idea must have seemed a wild one, he +had the firmest faith in its ultimate accomplishment.</p> + +<p>Amongst those who followed Lindsay’s experiments +with keen interest was the late Sir William, then Mr. +Preece, but it was not until 1882, twenty years after +Lindsay’s death, that he commenced experiments on his +own account. In March of that year the cable across the +Solent failed, and Preece took the opportunity of trying to +signal across without a connecting wire. He used two +overhead wires, each terminating in large copper plates +sunk in the sea, one stretching from Southampton to +Southsea Pier, and the other from Ryde Pier to Sconce +Point. The experiment was successful, audible Morse +signals being received on each side. In this experiment, +as in those of Morse and Lindsay, the water acted as the +conducting medium; but a year or two later, Preece +turned his attention to a different method of wireless communication, +by means of induction. This method was<span class="pagenum" id="Page_181">181</span> +based upon the fact that at the instant of starting and +stopping a current in one wire, another current is induced +in a second wire placed parallel to it, even when the two +wires are a considerable distance apart. Many successful +experiments in this induction telegraphy were made, one +of the most striking being that between the Island of Mull +and the mainland, in 1895. The cable between the island +and the mainland had broken, and by means of induction +perfect telegraphic communication was maintained during +the time that the cable was being repaired. Although this +system of wireless telegraphy is quite successful for short +distances, it becomes impracticable when the distance is +increased, because the length of each of the two parallel +wires must be roughly equal to the distance between them. +These experiments of Preece are of great interest, but we +must leave them because they have little connexion with +present-day wireless telegraphy, in which utterly different +methods are used.</p> + +<p>All the commercial wireless systems of to-day depend +upon the production and transmission of electric waves. +About the year 1837 it was discovered that the discharge +of a Leyden jar did not consist of only one sudden rush of +electricity, but of a series of electric oscillations, which +surged backwards and forwards until electric equilibrium +was restored. This discovery was verified by later +experimenters, and it forms the foundation of our knowledge +of electric waves. At this point many readers probably +will ask, “What are electric waves?” It is impossible to +answer this question fully, for we still have a great deal to +learn about these waves, and we only can state the conclusions +at which our greatest scientists have arrived after +much thought and many experiments. It is believed that +all space is filled with a medium to which the name +“ether” has been given, and that this ether extends<span class="pagenum" id="Page_182">182</span> +throughout the matter. We do not know what the ether +is, but the important fact is that it can receive and transmit +vibrations in the form of ether waves. There are different +kinds of ether waves, and they produce entirely different +effects. Some of them produce the effect which we call +light, and these are called “light waves.” Others produce +the effect known as heat, and they are called “heat waves”; +and still others produce electricity, and these we call +“electric waves.” These waves travel through the ether at +the enormous speed of 186,000 miles per second, so that +they would cross the Atlantic Ocean in about 1/80 second. +The fact that light also travels at this speed suggested that +there might be some connexion between the two sets of +waves, and after much experiment it has been demonstrated +that the waves of light and electricity are identical except +in their length.</p> + +<p>Later on in this chapter we shall have occasion to refer +frequently to wave-length, and we may take this opportunity +of explaining what is understood by this term. Wave-length +is the distance measured from the crest of one wave +to the crest of the next, across the intervening trough or +hollow. From this it will be seen that the greater the +wave-length, the farther apart are the waves; and also that +if we have two sets of waves of different wave-lengths but +travelling at the same speed, then the number of waves +arriving at any point in one second will be greater in the +case of the shorter waves, because these are closer together.</p> + +<p>A tuning-fork in vibration disturbs the surrounding air, +and sets up air waves which produce the effect called sound +when they strike against the drums of our ears. In a +similar way the discharge of a Leyden jar disturbs the +surrounding ether, and sets up electric ether waves; but +these waves produce no effect upon us in the shape of sight, +sound, or feeling. There is however a very simple piece<span class="pagenum" id="Page_183">183</span> +of apparatus which acts as a sort of electric eye or ear, and +detects the waves for us. This consists of a glass tube +loosely filled with metal filings, and having a cork at each +end. A wire is passed through each cork so as to project +well into the tube, but so that the two ends do not touch +one another, and the outer ends of these wires are connected +to a battery of one or two cells, and to some kind of +electrically worked apparatus, such as an electric bell. So +long as the filings lie quite loosely in the tube they offer +a very high resistance, and no current passes. If now +electric waves are set up by the discharge of a Leyden jar, +these waves fall upon the tube and cause the resistance +of the filings to decrease greatly. The filings now form a +conducting path through which the current passes, and so +the bell rings. If no further discharge takes place the +electric waves cease, but the filings do not return to their +original highly resistant condition, but retain their conductivity, +and the current continues to pass, and the bell +goes on ringing. To stop the bell it is only necessary +to tap the tube gently, when the filings immediately fall +back into their first state, so that the current cannot pass +through them.</p> + +<p>Now let us see how the “coherer,” as the filings tube is +called, is used in actual wireless telegraphy. <a href="#fig_33">Fig. 33<i>a</i></a> +shows a simple arrangement for the purpose. A is an +induction coil, and B the battery supplying the current. +The coil is fitted with a spark gap, consisting of two +highly polished brass balls CC, one of these balls being +connected to a vertical wire supported by a pole, and the +other to earth. D is a Morse key for starting and stopping +the current. When the key is pressed down, current flows +from the battery to the coil, and in passing through the +coil it is raised to a very high voltage, as described in +<a href="#chapter_VIII">Chapter VIII</a>. This high tension current is sent into the<span class="pagenum" id="Page_184">184</span> +aerial wire, which quickly becomes charged up to its +utmost limits. But more current continues to arrive, and +so the electricity in the aerial, unable to bear any longer +the enormous pressure, takes the only path of escape and +bursts violently across the air gap separating the brass +balls. Surging oscillations are then produced in the aerial, +the ether is violently disturbed, and electric waves are +set in motion. This is the transmitting part of the +apparatus.</p> + +<figure id="fig_33" class="figcenter" style="max-width: 25em;"> + <img src="images/i_216.png" width="2000" height="1836" alt=" "> + <figcaption class="caption"><p><i>a.</i> Transmitting. + <span class="in4"><i>b.</i> Receiving.</span></p> + +<p><span class="smcap">Fig. 33.</span>—Diagram of simple Wireless Transmitting and Receiving Apparatus.</p> +</figcaption></figure> + +<p>If a stone is dropped into a pond, little waves are set in<span class="pagenum" id="Page_185">185</span> +motion, and these spread outwards in ever-widening rings. +Electric waves also are propagated outwards in widening +rings, but instead of travelling in one plane only, like the +water waves, they proceed in every plane; and when they +arrive at the receiving aerial they set up in it oscillations +of the same nature as those which produced the waves. +Let us suppose electric waves to reach the aerial wire of +<a href="#fig_33">Fig. 33<i>b</i></a>. The resistance of the coherer H is at once lowered +so that current from battery N flows and operates the relay +F, which closes the circuit of battery M. This battery +has a twofold task. It operates the sounder E, and it +energizes the electro-magnet of the de-coherer K, as shown +by the dotted lines. This de-coherer is simply an electric +bell without the gong, arranged so that the hammer strikes +the coherer tube; and its purpose is to tap the tube +automatically and much more rapidly than is possible by +hand. The sounder therefore gives a click, and the de-coherer +taps the tube, restoring the resistance of the +filings. The circuit of battery N is then broken, and the +relay therefore interrupts the circuit of battery M. If +waves continue to arrive, the circuits are again closed, +another click is given, and again the hammer taps the +tube. As long as waves are falling upon the aerial, the +alternate makings and breakings of the circuits follow one +another very rapidly and the sounder goes on working. +When the waves cease, the hammer of the de-coherer has +the last word, and the circuits of both batteries remain +broken. To confine the electric waves to their proper +sphere two coils of wire, LL, called choking coils, are +inserted as shown.</p> + +<p>In this simple apparatus we have all the really essential +features of a wireless installation for short distances. For +long distance work various modifications are necessary, +but the principle remains exactly the same. In land wireless<span class="pagenum" id="Page_186">186</span> +stations the single vertical aerial wire becomes an +elaborate arrangement of wires carried on huge masts and +towers. The distance over which signals can be transmitted +and received depends to a considerable extent upon +the height of the aerial, and consequently land stations +have the supporting masts or towers from one to several +hundred feet in height, according to the range over which +it is desired to work. As a rule the same aerial is used both +for transmitting and receiving, but some stations have a +separate aerial for each purpose. A good idea of the +appearance of commercial aerials for long distance working +may be obtained from the frontispiece, which shows the +Marconi station at Glace Bay, Nova Scotia, from which +wireless communication is held with the Marconi station at +Clifden, in Galway, Ireland.</p> + +<p>In the first wireless stations what is called a “plain +aerial” transmitter was used, and this was almost the same +as the transmitting apparatus in <a href="#fig_33">Fig. 33<i>a</i></a>, except, of course, +that it was on a larger scale. This arrangement had many +serious drawbacks, including that of a very limited range, +and it has been abandoned in favour of the “coupled” +transmitter, a sketch of which is shown in <a href="#fig_34">Fig. 34</a>. In this +transmitter there are two separate circuits, having the same +rate of oscillation. A is an induction coil, supplied with +current from the battery B, and C is a condenser. A +condenser is simply an apparatus for storing up charges of +electricity. It may take a variety of forms, but in every +case it must consist of two conducting layers separated by +a non-conducting layer, the latter being called the +“dielectric.” The Leyden jar is a condenser, with conducting +layers of tinfoil and a dielectric of glass, but the +condensers used for wireless purposes generally consist of +a number of parallel sheets of metal separated by glass or +mica, or in some cases by air only. The induction coil<span class="pagenum" id="Page_187">187</span> +charges up the condenser with high tension electricity, until +the pressure becomes so great that the electricity is +discharged in the form of a spark between the brass balls +of the spark gap D. The accumulated electric energy in +the condenser then surges violently backwards and forwards, +and by induction corresponding surgings are produced in +the aerial circuit, these latter surgings setting up electric +waves in the ether.</p> + +<figure id="fig_34" class="figcenter" style="max-width: 20em;"> + <img src="images/i_219.png" width="1573" height="1941" alt=" "> + <figcaption class="caption"><span class="smcap">Fig. 34.</span>—Wireless “Coupled” Transmitter. +</figcaption></figure> + +<p>For the sake of simplicity we have represented the +apparatus as using an induction coil, but in all stations of +any size the coil is replaced by a step-up transformer, and<span class="pagenum" id="Page_188">188</span> +the current is supplied either from an electric light power +station at some town near by, or from a power house specially +built for the purpose. Alternating current is generally used, +and if the current supplied is continuous, it is converted into +alternating current. This may be done by making the +continuous current drive an electric motor, which in turn +drives a dynamo generating alternating current. In any +case, the original current is too low in voltage to be used +directly, but in passing through the transformer it is raised +to the required high pressure. The transmitting key, +which is inserted between the dynamo and the transformer, +is specially constructed to prevent the operator from receiving +accidental shocks, and the spark gap is enclosed in a +sort of sound-proof box, to deaden the miniature thunders +of the discharge.</p> + +<p>During the time that signals are being transmitted, +sparks follow one another across the spark gap in rapid +succession, a thousand sparks per second being by no +means an uncommon rate. The violence of these rapid +discharges raises the brass balls of the gap to a great heat. +This has the effect of making the sparking spasmodic and +uncertain, with the result that the signals at the receiving +station are unsatisfactory. To get over this difficulty +Marconi introduced a rotary spark gap. This is a wheel +with projecting knobs or studs, mounted on the shaft of the +dynamo supplying the current, so that it rotates rapidly. +Two stationary knobs are fixed so that the wheel rotates +between them, and the sparks are produced between these +fixed knobs and those of the wheel, a double spark gap +thus being formed. Overheating is prevented by the +currents of air set up by the rapid movement of the wheel, +and the sparking is always regular.</p> + +<figure id="plate_XIIIa" class="figcenter" style="max-width: 26em;"> + <p class="caption">PLATE XIII.</p> + <img src="images/i_221.jpg" width="2076" height="1403" alt=" "> + <figcaption class="caption"> + +<p class="floatl"><i>Photo by</i></p> +<p class="floatr"><i>Daily Mirror</i>.</p> + +<p class="floatc">(<i>a</i>) MARCONI OPERATOR RECEIVING A MESSAGE.</p> +</figcaption></figure> + +<figure id="plate_XIIIb" class="figcenter" style="max-width: 27em;"> + <img src="images/i_221b.jpg" width="2081" height="1202" alt=" "> + <figcaption class="caption"> + +<p class="floatl"><i>By permission of</i></p> +<p class="floatr"><i>The Marconi Co. Ltd.</i></p> + +<p class="floatc">(<i>b</i>) MARCONI MAGNETIC DETECTOR.</p> +</figcaption></figure> + +<p>In the receiving apparatus already described a filings +coherer was used to detect the ether waves, and, by means +of a local battery, to translate them into audible signals with +a sounder, or printed signals with a Morse inker. This +coherer however is unsuitable for commercial working. +It is not sufficiently sensitive, and it can be used only for +comparatively short distances; while its action is so slow +that the maximum speed of signalling is not more than +about seventeen or eighteen words a minute. A number +of different detectors of much greater speed and sensitiveness +have been devised. The most reliable of these, +though not the most sensitive, is the Marconi magnetic detector, +<a href="#plate_XIIIb">Plate XIII.<i>b</i></a>. This consists of a moving band made +of several soft iron wires twisted together, and passing close +to the poles of two horse-shoe magnets. As the band +passes from the influence of one magnet to that of the other +its magnetism becomes reversed, but the change takes a +certain amount of time to complete owing to the fact that +the iron has some magnetic retaining power, so that it +resists slightly the efforts of one magnet to reverse the +effect of the other. The moving band passes through two +small coils of wire, one connected with the aerial, and the +other with a specially sensitive telephone receiver. When +the electric waves from the transmitting station fall upon +the aerial of the receiving station, small, rapidly oscillating +currents pass through the first coil, and these have the +effect of making the band reverse its magnetism instantly. +The sudden moving of the lines of magnetic force induces +a current in the second coil, and produces a click in the +telephone. As long as the waves continue, the clicks +follow one another rapidly, and they are broken up into the +long and short signals of the Morse code according to the +manipulation of the Morse key at the sending station. +Except for winding up at intervals the clockwork mechanism +which drives the moving band, this detector requires no +attention, and it is always ready for work.</p> + +<p><span class="pagenum" id="Page_190">190</span></p> + +<p>Another form of detector makes use of the peculiar +power possessed by certain crystals to rectify the oscillatory +currents received from the aerial, converting them into +uni-directional currents. At every discharge of the condenser +at the sending station a number of complete waves, +forming what is called a “train” of waves, is set in motion. +From each train of waves the crystal detector produces one +uni-directional pulsation of current, and this causes a click +in the telephone receiver. If these single pulsations follow +one another rapidly and regularly, a musical note is heard +in the receiver. Various combinations of crystals, and +crystals and metal points, are used, but all work in the +same way. Some combinations work without assistance, +but others require to have a small current passed through +them from a local battery. The crystals are held in small +cups of brass or copper, mounted so that they can be +adjusted by means of set-screws. Crystal detectors are +extremely sensitive, but they require very accurate adjustment, +and any vibration quickly throws them out of order.</p> + +<p>The “electrolytic” detector rectifies the oscillating +currents in a different manner. One form consists of a thin +platinum wire passing down into a vessel made of lead, +and containing a weak solution of sulphuric acid. The +two terminals of a battery are connected to the wire and +the vessel respectively. As long as no oscillations are +received from the aerial the current is unable to flow +between the wire and the vessel, but when the oscillations +reach the detector the current at once passes, and operates +the telephone receiver. The action of this detector is not +thoroughly understood, and the way in which the point of +the platinum wire prevents the passing of the current until +the oscillations arrive from the aerial is something of a +mystery.</p> + +<p>The last detector that need be described is the Fleming<span class="pagenum" id="Page_191">191</span> +valve receiver. This consists of an electric incandescent +lamp, with either carbon or tungsten filament, into which +is sealed a plate of platinum connected with a terminal outside +the lamp. The plate and the filament do not touch +one another, but when the lamp is lighted up a current can +be passed from the plate to the filament, but not from filament +to plate. This receiver acts in a similar way to the +crystal detector, making the oscillating currents into uni-directional +currents. It has proved a great success for +transatlantic wireless communication between the Marconi +stations at Clifden and Glace Bay, and is extensively used.</p> + +<p>The electric waves set in motion by the transmitting +apparatus of a wireless station spread outwards through +the ether in all directions, and so instead of reaching only +the aerial of the particular station with which it is desired +to communicate, they affect the aerials of all stations within +a certain range. So long as only one station is sending +messages this causes no trouble; but when, as is actually +the case, large numbers of stations are hard at work transmitting +different messages at the same time, it is evident +that unless something can be done to prevent it, each of +these messages will be received at the same moment by +every station within range, thus producing a hopeless confusion +of signals from which not a single message can be +read. Fortunately this chaos can be avoided by what is +called “tuning.”</p> + +<p>Wireless tuning consists in adjusting the aerial of the +receiving station so that it has the same natural rate of +oscillation as that of the transmitting station. A simple +experiment will make clearer the meaning of this. If we +strike a tuning-fork, so that it sounds its note, and while it +is sounding strongly place near it another fork of the same +pitch and one of a different pitch, we find that the fork of +similar pitch also begins to sound faintly, whereas the third<span class="pagenum" id="Page_192">192</span> +fork remains silent. The explanation is that the two forks +of similar pitch have the same natural rate of vibration, +while the other fork vibrates at a different rate. When +the first fork is struck, it vibrates at a certain rate, and sets +in motion air waves of a certain length. These waves +reach both the other forks, but their effect is different in +each case. On reaching the fork of similar pitch the first +wave sets it vibrating, but not sufficiently to give out a +sound. But following this wave come others, and as the +fork has the same rate of vibration as the fork which +produced the waves, each wave arrives just at the right +moment to add its impulse to that of the preceding wave, +so that the effect accumulates and the fork sounds. In the +case of the third fork of different pitch, the first wave sets +it also vibrating, but as this fork cannot vibrate at the same +rate as the one producing the waves, the latter arrive at +wrong intervals; and instead of adding together their +impulses they interfere with one another, each upsetting +the work of the one before it, and the fork does not sound. +The same thing may be illustrated with a pendulum. If +we give a pendulum a gentle push at intervals corresponding +to its natural rate of swing, the effects of all these +pushes are added together, and the pendulum is made to +swing vigorously. If, on the other hand, we give the pushes +at longer or shorter intervals, they will not correspond with +the pendulum’s rate of swing, so that while some pushes +will help the pendulum, others will hinder it, and the final +result will be that the pendulum is brought almost to a +standstill, instead of being made to swing strongly and +regularly. The same principle holds good with wireless +aerials. Any aerial will respond readily to all other aerials +having the same rate of oscillation, because the waves in +each case are of the same length; that is to say, they follow +one another at the same intervals. On the other hand, an<span class="pagenum" id="Page_193">193</span> +aerial will not respond readily to waves from another aerial +having a different rate of oscillation, because these do not +follow each other at intervals to suit it.</p> + +<p>If each station could receive signals only from stations +having aerials similar to its own, its usefulness would be +very limited, and so all stations are provided with means +of altering the rate of oscillation of their aerials. The +actual tuning apparatus by which this is accomplished need +not be described, as it is complicated, but what happens in +practice is this: The operator, wearing telephone receivers +fixed over his ears by means of a head band, sits at a +desk upon which are placed his various instruments. He +adjusts the tuning apparatus to a position in which +signals from stations of widely different wave-lengths are +received fairly well, and keeps a general look out over +passing signals. Presently he hears his own call-signal, +and knows that some station wishes to communicate with +him. Immediately he alters the adjustment of his tuner +until his aerial responds freely to the waves from this +station, but not to waves from other stations, and in this +way he is able to cut out signals from other stations and to +listen to the message without interruption.</p> + +<p>Unfortunately wireless tuning is yet far from perfect in +certain respects. For instance, if two stations are transmitting +at the same time on the same wave-length, it is +clearly impossible for a receiving operator to cut one out +by wave-tuning, and to listen to the other only. In such +a case, however, it generally happens that although the +wave-frequency is the same, the frequency of the wave +groups or trains is different, so that there is a difference +in the notes heard in the telephones; and a skilful operator +can distinguish between the two sufficiently well to read +whichever message is intended for him. The stations +which produce a clear, medium-pitched note are the easiest<span class="pagenum" id="Page_194">194</span> +to receive from, and in many cases it is possible to identify +a station at once by its characteristic note. Tuning is also +unable to prevent signals from a powerful station close at +hand from swamping to some extent signals from another +station at a great distance, the nearer station making the +receiving aerial respond to it as it were by brute force, +tuning or no tuning.</p> + +<p>Another source of trouble lies in interference by atmospheric +electricity. Thunderstorms, especially in the +tropics, interfere greatly with the reception of signals, +the lightning discharges giving rise to violent, irregular +groups of waves which produce loud noises in the telephones. +There are also silent electrical disturbances in the +atmosphere, and these too produce less strong but equally +weird effects. Atmospheric discharges are very irregular, +without any real wave-length, so that an operator cannot +cut them out by wave-tuning pure and simple in the way +just described, as they defy him by affecting equally all +adjustments. Fortunately, the irregularity of the atmospherics +produces correspondingly irregular sounds in the +telephones, quite unlike the clear steady note of a wireless +station; and unless the atmospherics are unusually strong +this note pierces through them, so that the signals can be +read. The effects of lightning discharges are too violent +to be got rid of satisfactorily, and practically all that can +be done is to reduce the loudness of the noises in the +telephones, so that the operator is not temporarily deafened. +During violent storms in the near neighbourhood of a +station it is usual to connect the aerial directly to earth, +so that in the event of its being struck by a flash the +electricity passes harmlessly away, instead of injuring the +instruments, and possibly also the operators. Marconi +stations are always fitted with lightning-arresters.</p> + +<p>The methods and apparatus we have described so far<span class="pagenum" id="Page_195">195</span> +are those of the Marconi system, and although in practice +additional complicated and delicate pieces of apparatus are +used, the description given represents the main features of +the system. Although Marconi was not the discoverer of +the principles of wireless telegraphy, he was the first to +produce a practical working system. In 1896 Marconi came +from Italy to England, bringing with him his apparatus, +and after a number of successful demonstrations of its +working, he succeeded in convincing even the most sceptical +experts that his system was thoroughly sound. Commencing +with a distance of about 100 yards, Marconi +rapidly increased the range of his experiments, and by +the end of 1897 he succeeded in transmitting signals +from Alum Bay, in the Isle of Wight, to a steamer 18 +miles away. In 1899 messages were exchanged between +British warships 85 miles apart, and the crowning achievement +was reached in 1901, when Marconi received readable +signals at St. John’s, Newfoundland, from Poldhu in Cornwall, +a distance of about 1800 miles. In 1907 the Marconi +stations at Clifden and Glace Bay were opened for public +service, and by the following year transatlantic wireless +communication was in full swing. The sending of wireless +signals across the Atlantic was a remarkable accomplishment, +but it did not represent by any means the limits +of the system, as was shown in 1910. In that year +Marconi sailed for Buenos Ayres, and wireless communication +with Clifden was maintained up to the almost incredible +distance of 4000 miles by day, and 6735 miles by night. +The Marconi system has had many formidable rivals, but +it still holds the proud position of the most successful commercial +wireless system in the world.</p> + +<p>We have not space to give a description of the other +commercial systems, but a few words on some of the chief +points in which they differ from the Marconi system may<span class="pagenum" id="Page_196">196</span> +be of interest. We have seen that an ordinary spark gap, +formed by two metal balls a short distance apart, becomes +overheated by the rapid succession of discharges, with the +result that the sparking is irregular. What actually +happens is that the violent discharge tears off and vaporizes +minute particles of the metal. This intensely heated +vapour forms a conducting path which the current is able +to cross, so that an arc is produced just in the same way +as in the arc lamp. This arc is liable to be formed by +each discharge, and it lasts long enough to prevent the +sparks from following one another promptly. In the +Marconi system this trouble is avoided by means of a +rotating spark gap, but in the German “Telefunken” +system, so named from Greek <em>tele</em>, far off, and German +<i lang="de">Funke</i>, a spark, a fixed compound spark gap is used +for the same purpose. This consists of a row of metal +discs about 1/100 inch apart, and the spark leaps these +tiny gaps one after the other. The discs are about +3 inches in diameter, and their effect is to conduct away +quickly the heat of the discharge. By this means the +formation of an arc is prevented, and the effect of each +discharge is over immediately, the sparks being said to be +“quenched.” The short discharges enable more energy to +be radiated from the aerial into the ether, and very high +rates of sparking are obtained, producing a high-pitched +musical note.</p> + +<p>The “Lepel” system also uses a quenched spark. +The gap consists of two metal discs clamped together and +separated only by a sheet of paper. The paper has a hole +through its centre, and through this hole the discharge +takes place, the discs being kept cool by water in constant +circulation. The discharge is much less noisy than in the +Marconi and Telefunken systems, and the musical note +produced is so sensitive that by varying the adjustments<span class="pagenum" id="Page_197">197</span> +simple tunes can be played, and these can be heard quite +distinctly in the telephone at the receiving stations.</p> + +<p>In the three systems already mentioned spark +discharges are used to set up oscillatory currents in the +aerial, which in turn set up waves in the ether. Each +discharge sets in motion a certain number of waves, +forming what is known as a train of waves. The discharges +follow one another very rapidly, but still there is a +minute interval between them, and consequently there is a +corresponding interval between the wave-trains. In the +“Goldschmidt” system the waves are not sent out in +groups of this kind, but in one long continuous stream. +The oscillatory currents which produce ether waves are +really alternating currents which flow backwards and +forwards at an enormous speed. The alternating current +produced at an ordinary power station is of no use for +wireless purposes, because its “frequency,” or rate of flow +backwards and forwards, is far too low. It has been +found possible however to construct special dynamos +capable of producing alternating current of the necessary +high frequency, and such dynamos are used in the +Goldschmidt system. The dynamos are connected directly +to the aerial, so that the oscillatory currents in the latter +are continuous, and the ether waves produced are continuous +also.</p> + +<p>The “Poulsen” system produces continuous waves in +an altogether different manner, by means of the electric +arc. The arc is formed between a fixed copper electrode +and a carbon electrode kept in constant rotation, and it is +enclosed in a kind of box filled with methylated spirit +vapour, hydrogen, or coal gas. A powerful electro-magnet +is placed close to the arc, so that the latter is surrounded +by a strong magnetic field. Connected with the terminals +of the arc is a circuit consisting of a condenser and a coil<span class="pagenum" id="Page_198">198</span> +of wire, and the arc sets up in this circuit oscillatory +currents which are communicated to the aerial. These +currents are continuous, and so also are the resulting +waves.</p> + +<p>The method of signalling employed in these two +continuous-wave systems is quite different from that used +in the Marconi and other spark systems. It is practically +impossible to signal by starting and stopping the alternating-current +dynamos or the arc at long or short intervals to +represent dashes or dots. In one case the sudden changes +from full load to zero would cause the dynamo to vary its +speed, and consequently the wave-length would be +irregular; besides which the dynamo would be injured by +the sudden strains. In the other case it would be +extremely difficult to persuade the arc to start promptly +each time. On this account the dynamo and the arc are +kept going continuously while a message is being transmitted, +and the signals are given by altering the wave-length. +In other words, the transmitting aerial is thrown +in and out of tune alternately at the required long or short +intervals, and the receiving aerial responds only during the +“in-tune” intervals.</p> + +<p>The various receiving detectors previously described +are arranged to work with dis-continuous waves, producing +a separate current impulse from each group or train of +waves. In continuous wave systems there are of course +no separate groups, and for this reason these detectors are +of no use, and a different arrangement is required. The +oscillatory currents set up in the aerial are allowed to +charge up a condenser, and this condenser is automatically +disconnected from the aerial and connected to the operator’s +telephones at regular intervals of about 1/1000 second. +Each time the condenser is connected to the telephones +it is discharged, and a click is produced. These clicks<span class="pagenum" id="Page_199">199</span> +continue only as long as the waves are striking the aerial, +and as the transmitting operator interrupts the waves at +long or short intervals the clicks are split up into groups of +corresponding length.</p> + +<p>Continuous waves have certain advantages over dis-continuous +waves, particularly in the matter of sharp +tuning, but these advantages are outweighed to a large +extent by weak points in the transmitting apparatus. The +dynamos used to produce the high-frequency currents in +the Goldschmidt system are very expensive to construct +and troublesome to keep in order; while in the Poulsen +system the arc is difficult to keep going for long periods, +and it is liable to fluctuations which greatly affect its +working power. Although all the commercial Marconi +installations make use of dis-continuous waves exclusively, +Mr. Marconi is still carrying out experiments with continuous +waves.</p> + +<p>There are many points in wireless telegraphy yet to be +explained satisfactorily. Our knowledge of the electric +ether waves is still limited, and we do not know for certain +how these waves travel from place to place, or exactly +what happens to them on their journeys. For this reason +we are unable to give a satisfactory explanation of the +curious fact that, generally speaking, it is easier to signal +over long distances at night than during the day. Still +more peculiar is the fact that it is easier to signal in a +north and south direction than in an east and west +direction. There are also remarkable variations in the +strength of the signals at certain times, particularly about +sunset and sunrise. Every station has a certain normal +range which does not vary much as a rule, but at odd +times astonishing “freak” distances are covered, stations +having for a short time ranges far beyond their usual limits. +These and other problems are being attacked by many<span class="pagenum" id="Page_200">200</span> +investigators, and no doubt before very long they will be +solved. Wireless telegraphy already has reached remarkable +perfection, but it is still a young science, and we may +confidently expect developments which will enable us to +send messages with all speed across vast gulfs of distance +at present unconquered.</p> + +<p>Wireless telephony, like wireless telegraphy, makes use +of electric waves set up in, and transmitted through the +ether. The apparatus is practically the same in each case, +except in one or two important points. In wireless +telegraphy either continuous or dis-continuous waves may +be used, and in the latter case the spark-frequency may be +as low as twenty-five per second. On the other hand, +wireless telephony requires waves which are either +continuous, or if dis-continuous, produced by a spark-frequency +of not less than 20,000 per second. In other +words, the frequency of the wave trains must be well above +the limits of audibility. Although dis-continuous waves of +a frequency of from 20,000 to 40,000 or more per second +can be used, it has been found more convenient to use +absolutely continuous waves for wireless telephony, and +these may be produced by the Marconi disc generator, by +the Goldschmidt alternator, or by the Poulsen arc, the last +named being largely employed.</p> + +<p>In wireless telegraphy the wave trains are split up by +a transmitting key so as to form groups of signals; but in +telephony the waves are not interrupted at all, but are +simply varied in intensity by means of the voice. All +telephony, wireless or otherwise, depends upon the production +of variations in the strength of a current of +electricity, these variations being produced by air vibrations +set up in speaking. In ordinary telephony with connecting +wires the current variations are produced by means of a +microphone in the transmitter, and in wireless telephony<span class="pagenum" id="Page_201">201</span> +the same principle is adopted. Here comes in the outstanding +difficulty in wireless transmission of speech. The +currents used in ordinary telephony are small, and the +microphone works with them quite satisfactorily; but in +wireless telephony very heavy currents have to be employed, +and so far no microphone has proved capable of +dealing effectively with these currents. Countless devices +to assist the microphone have been tried. It was found +that one of the causes of trouble was the overheating of +the carbon granules, which caused them to stick together, +so becoming insensitive. To remedy this the granules +have been cooled in various ways, by water, gas, or oil, but +although the results have been improved, still the microphones +worked far from perfectly. Improved results +have been obtained also by connecting a number of +microphones in parallel. The microphone difficulty is +holding back the development of wireless telephony, and +at present no satisfactory solution of the problem is in +sight.</p> + +<p>The transmitting and receiving aerials are the same as +in wireless telegraphy, and like them are tuned to the same +frequency. The receiving apparatus too is of the ordinary +wireless type, with telephones and electrolytic or other +detectors.</p> + +<p>Wireless telephony has been used with considerable +success in various German collieries, and at the Dinnington +Main Colliery, Yorkshire. Early last year Marconi succeeded +in establishing communication by wireless telephony +between Bournemouth and Chelmsford, which are about +100 miles apart; and about the same time a song sung +at Laeken, in Belgium, was heard clearly at the Eiffel +Tower, Paris, a distance of 225 miles. The German +Telefunken Company have communicated by wireless +telephony between Berlin and Vienna, 375 miles, and<span class="pagenum" id="Page_202">202</span> +speech has been transmitted from Rome to Tripoli, a total +distance of more than 600 miles. These distances are of +course comparatively small, but if the microphone trouble +can be overcome satisfactorily, transatlantic wireless +telephony appears to be quite possible.</p> +<hr class="chap x-ebookmaker-drop"> + +<div class="chapter"> +<p><span class="pagenum" id="Page_203">203</span></p> + +<h2 class="nobreak" id="toclink_203"><a id="chapter_XXI"></a>CHAPTER XXI<br> + +<span class="subhead">WIRELESS TELEGRAPHY—PRACTICAL APPLICATIONS</span></h2> +</div> + +<p class="in0"><span class="firstword">A fairly</span> good idea of the principles and apparatus of wireless +telegraphy should have been gained in reading <a href="#chapter_XX">Chapter +XX</a>., but so far little has been said about its practical use. +If we leave their power out of consideration, wireless +stations may be divided into two classes: fixed stations on +land, and moving stations, if the expression may be allowed, +on ships. For moving stations wireless telegraphy has +the field all to itself, but for communication between fixed +stations it comes into conflict with ordinary telegraphy by +wire or cable. As regards land messages over comparatively +short distances, say throughout Great Britain, wireless +telegraphy has no advantages over the older methods; and +it is extremely unlikely that it ever will be substituted +for the existing cable telegraphy. For long distances +overland wireless has the great advantage of having all its +apparatus concentrated at two points. A long land line +passing through wild country, and exposed to all kinds of +weather, requires constant labour to keep it in good repair, +and when a breakdown occurs at any point, the repairing +gang may be miles away, so that delay is caused. On the +other hand, whatever may go wrong at a wireless station, +no time is lost in effecting the necessary repairs, for everything +is on the spot.</p> + +<p>At present there is no great competition between wireless<span class="pagenum" id="Page_204">204</span> +and ordinary telegraphy for overland messages of any +kind, but the case is different when we come to communication +across seas and oceans. Already the cable companies have +been affected considerably, and there is little doubt that +they will feel the competition much more seriously before +long. The general public, always conservative in such +matters, have not yet grasped the fact that telegrams can +be handed in at any telegraph office in the British Isles, +and at most telegraph offices in the United States and +Canada, for wireless transmission across the Atlantic, via +the Marconi stations at Clifden and Glace Bay. The cost +is remarkably small, being eightpence a word for ordinary +messages.</p> + +<p>It is impossible to state with any accuracy how many +land wireless stations there are in the world, but the list +given in the <cite>Year-Book of Wireless Telegraphy</cite> for 1915 +enumerates about 700 stations. This list does not include +private or experimental stations, and also many stations +used exclusively for naval or military purposes are not +given. The information available about these 700 stations +is incomplete in many cases, but about 500 are controlled +by various departments of the governments of the different +states. Of the remainder, about 100 are controlled by the +Marconi Company, the rest being in the hands of various +wireless, commercial, or railway companies.</p> + +<p>Amongst the most important land stations are the +Clifden and Glace Bay transatlantic stations. They are +very similar in plan, and each has a separate aerial for +sending and for receiving. Contrary to the usual practice, +continuous current is used to charge the condensers. In +<a href="#chapter_IV">Chapter IV</a>. we saw how a current of high voltage could +be obtained by connecting a number of cells in series, and +at these stations the necessary high voltage is produced by +connecting a number of powerful dynamos in series, on the<span class="pagenum" id="Page_205">205</span> +same principle. Along with the dynamos a huge battery +of accumulators, consisting of about 6000 cells, is used as +a sort of reservoir of current. These stations have a +normal range of considerably over 3000 miles. Last year +a large transmitting station was completed at Cefndu, near +Carnarvon. This station, which is probably the most +powerful in existence, is intended to communicate directly +with New Jersey, United States, as an alternative to the +Clifden-Glace Bay route.</p> + +<p>Other powerful stations are Poldhu, in Cornwall, of +which we shall speak later; the French Eiffel Tower +station; the German station at Nauen, near Berlin, which +last year succeeded in exchanging messages with Windhoek, +German South-West Africa, a distance of nearly 6000 +miles; and the extremely powerful station at Coltano, Italy. +France has three stations in West Africa with a night +range of 1600 miles; and Italy one in Somaliland with a +normal range of about the same distance. The recently +opened Chinese stations at Canton, Foochow, and Woosung +have a range of 1300 miles by night, and 650 miles by day. +With the fall of Tsingtau, China, Germany lost a wireless +station capable of signalling over 1350 miles at night. +Japan has six stations with a night range of over 1000 +miles. Massawa, on the Red Sea, has a range of 1600 +miles, and New Zealand has two stations with ranges of +1200 miles by day, and 2500 miles by night. Australia +has a large number of stations with a normal range of +about 500 miles. In the United States, which has a very +large number of stations, Arlington, Virginia, covers 1000 +miles, and Sayville from 600 to 2300 miles. South +America has not many high-power stations, but Cerrito, in +Uruguay, has a range of about 1000 miles.</p> + +<p>Until a thoroughly practical system of long-distance +wireless telephony is developed, wireless telegraphy will<span class="pagenum" id="Page_206">206</span> +remain the only possible means of communication between +ships and shore, or between one ship and another, except +where the distance is so small that some method of semaphore +signalling can be used. In the days when wireless +was unknown, a navigator was thrown entirely upon his +own resources as soon as his vessel was out of sight of +land, for no information of any kind could reach him. +Even with a wireless installation on board, the captain of a +vessel still needs the same skill and watchfulness as of old, +but in the times of uncertainty and danger to which all +ships are liable, he often is able to obtain information which +may prevent disaster. In order to determine accurately +his position, a navigator requires to know the exact Greenwich +Mean Time, and he gets this time from his chronometers. +These are wonderfully reliable instruments, but +even they may err at times. To avoid the possibility of +mistakes from this cause, wireless time signals are sent out +at regular intervals by certain high-power stations, and as +long as a vessel is within range of one of these stations the +slightest variation in the chronometers may be detected +immediately. Amongst these stations are the Eiffel Tower, +giving time signals at 10 a.m. and at midnight; and Norddeich, +Germany, giving signals at noon and midnight. +These time signals have proved most useful also on land, +more particularly for astronomers and for explorers engaged +on surveying work.</p> + +<p>In addition to time signals, other valuable information +is conveyed by wireless to ships at sea. A ship encountering +ice, or a derelict, reports its discovery to other ships +and to the shore stations, and in this way vessels coming +along the same route are warned of the danger in time to +take the necessary precautions. Weather reports are issued +regularly from various shore stations in most parts of the +world. The completeness of the information given varies<span class="pagenum" id="Page_207">207</span> +a good deal with different stations, but in many cases it +includes a report of the existing state of the weather at a +number of different places, a forecast of the winds likely +to be encountered at sea, say at a distance of 100 miles +from land, and warnings of approaching storms, with +remarks on any special atmospheric conditions at the time +of sending. In Europe weather reports are issued daily +from the Admiralty station at Cleethorpes, the Eiffel Tower, +and Norddeich; and in the United States more than a +dozen powerful stations are engaged in this work.</p> + +<p>Another valuable use of wireless is in carrying on the +work of lighthouses and lightships during snowstorms or +dense fogs, which the light cannot penetrate. So far not +much has been done in this direction, but the French +Government have decided to establish wireless lighthouses +on the islands outside the port of Brest, and also at Havre. +Automatic transmitting apparatus will be used, sending +out signals every few seconds, and working for periods of +about thirty hours without attention.</p> + +<p>The improvement in the conditions of ocean travel +wrought by wireless telegraphy is very remarkable. The +days when a vessel, on passing out of sight of land, entered +upon a period of utter isolation, is gone for ever. Unless +it strays far from all recognized trade routes, a ship fitted +with a wireless installation is never isolated; and with the +rapidly increasing number of high-power stations both on +land and sea, it soon will be almost impossible for a vessel +to find a stretch of ocean beyond the reach of wave-borne +messages. The North Atlantic Ocean is specially remarkable +for perfection of wireless communication. For the +first 250 miles or so after leaving British shores, liners are +within reach of various coast stations, and beyond this +Poldhu takes up the work and maintains communication +up to about mid-Atlantic. On passing beyond the reach<span class="pagenum" id="Page_208">208</span> +of Poldhu, liners come within range of other Marconi +stations at Cape Cod, Massachusetts, and Cape Race, +Newfoundland, so that absolutely uninterrupted communication +is maintained throughout the voyage. On many +liners a small newspaper is published daily, in which are +given brief accounts of the most striking events of the +previous day, together with Stock Exchange quotations and +market prices. This press news is sent out during the +night from Poldhu and Cape Cod. During the whole +voyage messages may be transmitted from ship to shore, +or from shore to ship, with no more difficulty, as far as the +public are concerned, than in sending an ordinary inland +telegram.</p> + +<p>The transmitting ranges of ship installations vary +greatly, the range of the average ocean liner being about +250 miles. Small ships come as low as 50 miles, while a +few exceptional vessels have night ranges up to 1200 or +even 2500 miles. Although an outward-bound vessel is +almost always within range of some high-power shore +station, it is evident that it soon must reach a point beyond +which it is unable to communicate directly with the shore. +This difficulty is overcome by a system of relaying from +ship to ship. The vessel wishing to speak with the shore +hands on its message to some other vessel nearer to land +or with a longer range, and this ship sends forward the +message to a shore station if one is within its reach, and if +not to a third vessel, which completes the transmission.</p> + +<p>The necessity for wireless installations on all sea-going +vessels has been brought home to us in startling fashion on +several occasions during the last few years. Time after +time we have read thrilling accounts of ocean disasters in +which wireless has come to the rescue in the most wonderful +way. A magnificent liner, with its precious human +freight, steams majestically out of harbour, and ploughs its<span class="pagenum" id="Page_209">209</span> +way out into the waste of waters. In mid-ocean comes +disaster, swift and awful, and the lives of all on board are +in deadly peril. Agonized eyes sweep the horizon, but no +sail is in sight, and succour seems hopeless. But on the +deck of that vessel is a small, unpretentious cabin, and at a +desk in that cabin sits a young fellow with strange-looking +instruments before him. At the first tidings of disaster he +presses a key, and out across the waters speed electric +waves bearing the wireless cry for help, “S.O.S.,” incessantly +repeated. Far away, on another liner, is a +similar small cabin, and its occupant is busy with messages +of everyday matters. Suddenly, in the midst of his work, +comes the call from the stricken vessel, and instantly all +else is forgotten. No matter what the message in hand, it +must wait, for lives are in danger. Quickly the call is +answered, the position of the doomed ship received, and +the captain is informed. A few orders are hurriedly given, +the ship’s course is changed, and away she steams to the +rescue, urged on by the full power of her engines. In an +hour or two she arrives alongside, boats are lowered, and +passengers and crew are snatched from death and placed +in safety. This scene, with variations, has been enacted +many times, and never yet has wireless failed to play its +part. It is only too true that in some instances many +lives have been lost, but in these cases it is necessary to +remember that without wireless every soul on board might +have gone down. The total number of lives already saved +by wireless is estimated at about 5000, and of these some +3000 have been saved in the Atlantic.</p> + +<p>Ship aerials are carried from one mast to another, as +high up as possible. The transmitting and receiving +apparatus is much the same as in land stations, so that it +need not be described. In addition, most liners carry a +large induction coil and a suitable battery, so that distress<span class="pagenum" id="Page_210">210</span> +signals can be transmitted even when the ordinary +apparatus is rendered useless by the failure of the current +supply. Most of the wireless systems are represented +amongst ship installations, but the great majority of vessels +have either Marconi or Telefunken apparatus.</p> + +<p>Every wireless station, whether on ship or on shore, has +a separate call-signal, consisting of three letters. For +instance, Clifden is MFT, Poldhu MPD, Norddeich KAV, +s.s. <i>Lusitania</i> MFA, and H.M.S. <i>Dreadnought</i> BAU. +Glace Bay, GB, and the Eiffel Tower, FL, have two +letters only. In order to avoid confusion, different countries +have different combinations of letters assigned to them +exclusively, and these are allotted to the various ship +and shore stations. For example, Great Britain has all +combinations beginning with B, G, and M; France all +combinations beginning with F, and also the combinations +UAA to UMZ; while the United States is entitled to use +all combinations beginning with N and W, and the combinations +KIA to KZZ. There are also special signals to +indicate nationality, for use by ships, British being indicated +by - - — -, Japanese by — - — -, and so on.</p> + +<p>Wireless telegraphy apparently has a useful future in +railway work. In spite of the great perfection of present-day +railway signalling, no railway company is able to avoid +occasional accidents. Some of these accidents are due to +circumstances which no precautions can guard against +entirely, such, for instance, as the sudden breakage of some +portion of the mechanism of the train itself. In many cases, +however, the accident is caused by some oversight on the +part of the signalman or the engine-driver. Probably the +great majority of such accidents are not due to real carelessness +or inattention to duty, but to unaccountable freaks +of the brain, through which some little detail, never before +forgotten, is overlooked completely until too late. We all<span class="pagenum" id="Page_211">211</span> +are liable to these curious mental lapses, but happily in most +cases these do not lead to disaster of any kind. The ever-present +possibility of accidents brought about in this way is +recognized fully by railway authorities, and every effort is +made to devise mechanism which will safeguard a train in +case of failure of the human element. The great weakness +of the ordinary railway system is that there is no reliable +means of communicating with the driver of a train except +by the fixed signals, so that when a train has passed one +set of signals it is generally beyond the reach of a message +until it arrives at the next set. On the enterprising +Lackawanna Railroad, in the United States, an attempt has +been made to remove this defect by means of wireless +telegraphy, and the experiment has been remarkably +successful. Wireless communication between moving +passenger trains and certain stations along the route has +been established, and the system is being rapidly developed.</p> + +<p>The wireless equipment of the stations is of the usual +type, and does not call for comment, but the apparatus on +the trains is worth mention. The aerial, which must be +low on account of bridges and tunnels, consists of rectangles +of wire fixed at a height of 18 inches above the roof of +each car. These separate aerials are connected together +by a wire running to a small operating room containing a +set of Marconi apparatus, and situated at the end of one of +the cars. The earth connexion is made to the track rails, +and the current is taken from the dynamos used to supply +the train with electric light. With this equipment messages +have been transmitted and received while the train was +running at the rate of 70 miles an hour, and distances +up to 125 miles have been covered. During a severe +storm in the early part of last year the telegraph and +telephone lines along the railroad broke down, but +uninterrupted communication was maintained by wireless,<span class="pagenum" id="Page_212">212</span> +and the operations of the relief gangs and the snow-ploughs +were directed by this means. For emergency signalling +this system is likely to prove of enormous importance. If +signals are set wrongly, through some misunderstanding, +and a train which should have been held up is passed +forward into danger, it can be stopped by a wireless message +in time to prevent an accident. Again, if a train has a +breakdown, or if it sticks fast in a snow-drift, its plight and +its exact position can be signalled to the nearest station, so +that help may be sent without delay. The possibilities of +the system in fact are almost unlimited, and it seems not +unlikely that wireless telegraphy will revolutionize the +long-distance railway travelling of the future.</p> +<hr class="chap x-ebookmaker-drop"> + +<div class="chapter"> +<p><span class="pagenum" id="Page_213">213</span></p> + +<h2 class="nobreak" id="toclink_213"><a id="chapter_XXII"></a>CHAPTER XXII<br> + +<span class="subhead">ELECTROPLATING AND ELECTROTYPING</span></h2> +</div> + +<p class="in0"><span class="firstword">In</span> our chapter on the accumulator or storage cell we saw +that a current of electricity has the power of decomposing +certain liquids; that is to say, it is able to split them up +into their component parts. This power has given rise +to the important art of electroplating and electrotyping. +Electroplating is the process of depositing a coating of a +rarer metal, such as gold, silver, or nickel, upon the +surface of baser or commoner metals; and electrotyping is +the copying of casts, medals, types, and other similar +objects. The fact that metals could be deposited by the +decomposition of a solution by a current was known in the +early days of the voltaic cell, but no one seems to have +paid much attention to it. An Italian chemist published +in 1805 an account of his success in coating two silver +medals with gold, and some thirty years later Bessemer +transformed lead castings into fairly presentable ornaments +by coating them with copper, but commercial electroplating +may be said to have begun about 1840, when an +Englishman named Elkington took out a patent for the +process. Since then the processes of electroplating and +electrotyping have rapidly come more and more into use, +until to-day they are practised on a vast scale, giving employment +to thousands.</p> + +<p>Electroplating on a small scale is a very simple affair. +A solution of the metal which it is desired to deposit is<span class="pagenum" id="Page_214">214</span> +placed in a suitable vessel. Two metal rods are placed +across the top of this vessel, and from one of these is +suspended a plate of the same metal as that in the solution, +and from the other is hung the article to receive the +coating. The positive terminal of a voltaic battery is +connected to the rod supporting the plate, and the negative +terminal to the rod carrying the article to be plated. As +the current passes through the solution from the plate to the +article the solution is decomposed, and the article receives +a coating of metal. The solution through which the +current passes, and which is decomposed, is called the +<em>electrolyte</em>, and the terminal points at which the current +enters and leaves the solution are called <em>electrodes</em>. The +electrode by which the current enters the electrolyte is +called the <em>anode</em>, and the one by which it leaves is called +the <em>cathode</em>.</p> + +<p>If we wish to deposit a coating of copper on, say, an +old spoon which has been dismissed from household service, +a solution of sulphate of copper must be made up and +placed in a glass or stoneware jar. Two little rods of +brass, copper, or any other good conductor are placed +across the jar, one at each side, and by means of hooks of +wire a plate of copper is hung from one rod and the spoon +from the other. The positive terminal of a battery of +Daniell cells is then connected to the anode rod which +supports the copper plate, and the negative terminal to +the cathode rod carrying the spoon. The current now +commences its task of splitting up the copper-sulphate +solution into pure copper and sulphuric acid, and depositing +this copper upon the spoon. The latter is very quickly +covered with a sort of “blush” copper, and the coating +grows thicker and thicker as long as the current is kept +at work. If there were no copper plate forming the anode +the process would soon come to a standstill, on account of<span class="pagenum" id="Page_215">215</span> +the copper in the electrolyte becoming used up; but as it +is the sulphuric acid separated out of the electrolyte takes +copper from the plate and combines with it to form a +further supply of copper sulphate. In this way the strength +of the solution is kept up, and the copper anode becomes +smaller and smaller as the coating on the spoon increases +in thickness. It is not necessary that the anode should +consist of absolutely pure copper, because any impurities +will be precipitated to the bottom or mixed with the +solution, nothing but quite pure copper being deposited on +the spoon. At the same time if the copper anode is very +impure the electrolyte quickly becomes foul, and has to be +purified or replaced by new solution.</p> + +<figure id="fig_35" class="figcenter" style="max-width: 21em;"> + <img src="images/i_249.jpg" width="1625" height="861" alt=" "> + <figcaption class="caption"> + +<p class="floatl"><i>By permission of</i>]</p> +<p class="floatr">[<i>W. Canning & Co.</i></p> + +<p class="floatc"><span class="smcap">Fig. 35.</span>—Small Electroplating Outfit.</p> +</figcaption></figure> + +<p>To nickel-plate the spoon we should require a nickel +plate for the anode and a nickel solution; to silver-plate it, +a silver anode and solution, and so on. <a href="#fig_35">Fig. 35</a> shows at +simple but effective arrangement for amateur electroplating +in a small way.</p> + +<p>Electroplating on a commercial scale is of course a +much more elaborate process, but the principle remains +exactly the same. <a href="#fig_36">Fig. 36</a> shows the general arrangement +of a plating shop. It is obviously extremely important<span class="pagenum" id="Page_216">216</span> +that the deposit on a plated article should be durable, and +to ensure that the coating will adhere firmly the article +must be cleaned thoroughly before being plated. Cleanliness +in the ordinary domestic sense is not sufficient, for +the article must be chemically clean. Some idea of the +care required in this respect may be gained from the fact +that if the cleaned surface is touched with the hand before +being plated, the coating will strip off the parts that have +been touched. The surfaces are first cleaned mechanically, +and then chemically by immersion in solutions of acids or +alkalies, the cleaning process varying to some extent with +different metals. There is also a very interesting process +of cleaning by electricity. The article is placed in a vat +fitted with anode and cathode rods, just as in an ordinary +plating vat, and containing a solution of hydrate of potash +and cyanide of potassium. The anode consists of a carbon +plate, and the article is hung from the cathode rod. +Sufficient current is passed through the solution to cause +gas to be given off rapidly at the cathode, and as this gas +rises to the surface it carries with it the grease and dirt +from the article, in the form of a dirty scum. After a +short time the article becomes oxidized and discoloured, +and the current is then reversed, so that the article becomes +the anode, and the carbon plate the cathode. The +current now removes the oxide from the surface of the +article, which is left quite bright and chemically clean.</p> + +<figure id="fig_36" class="figcenter" style="max-width: 40em;"> + <img src="images/i_251.jpg" width="3127" height="1843" alt=" "> + <figcaption class="caption"> + +<p class="floatl"><i>By Permission of</i>]</p> +<p class="floatr">[<i>W. Canning & Co.</i></p> + +<p class="floatc"><span class="smcap">Fig. 36.</span>—General Arrangement of an Electroplating Shop.</p> +</figcaption></figure> + +<p>When thoroughly cleaned the articles are ready to be +placed in the plating vats. These vats are usually made +of wood lined with chemically pure lead, or of iron lined +with enamel or cement. Anode and cathode rods made +of brass are placed across the vats, and from these the +anodes of the various metals and the articles to be plated +are hung by hooks of nickel or brass. Any number of +rods may be used, according to the size of the vat, so long<span class="pagenum" id="Page_218">218</span> +as the articles have an anode on each side. If three rods +are used the articles are hung from the centre one, and the +anodes from the outside ones. If a number of small +articles are to be plated together they are often suspended +in perforated metal trays. Small articles are also plated +by placing them in a perforated barrel of wood, or wood +and celluloid, which revolves in the solution. While the +articles are being plated the revolving of the barrel makes +them rub one against the other, so that they are brightly +burnished. Dog chains, cycle chain links, button-hooks, +and harness fittings are amongst the articles plated by +means of the revolving barrel.</p> + +<p>The strength of current required for different kinds of +plating varies considerably, and if the work is to be of the +best quality it is very important that the current should be +exactly right for the particular process in hand. In order +to adjust it accurately variable resistances of German silver +wire are provided for each vat, the current having to pass +through the resistance before reaching the solution. The +volume and the pressure of the current are measured by +amperemeters and voltmeters attached to the resistance +boards. If the intensity of the current is too great the +articles are liable to be “burnt,” when the deposit is dark +coloured and often useless.</p> + +<p>When exceptionally irregular surfaces have to be plated +it is sometimes necessary to employ an anode of special +shape, in order to keep as uniform a distance as possible +between the electrodes. If this is not done, those parts of +the surface nearest the anode get more than their share of +the current, and so they receive a thicker deposit than the +parts farther away.</p> + +<p>An interesting process is that known as “parcel-plating,” +by which decorative coatings of different coloured metals +can be deposited on one article. For instance, if it is<span class="pagenum" id="Page_219">219</span> +desired to have gold flowers on a silver brooch, the parts +which are not to be gilded are painted over with a non-conducting +varnish. When this varnish is quite dry the +brooch is placed in the gilding vat and the current sent +through in the usual way. The gold is then deposited only +on the parts unprotected by varnish, and after the gilding +the varnish is easily removed by softening it in turpentine +and brushing with a bristle brush. More elaborate +combinations of different coloured metals can be made in +the same way.</p> + +<p>Sugar basins, cream jugs, ornamental bowls, cigarette +cases, and other articles are often gilded only on the inside. +The article is filled with gold solution and connected to the +cathode rod. A piece of gold wrapped in calico is attached +to the anode rod, suspended in the solution inside the +article, and moved about quickly until the deposit is of the +required thickness.</p> + +<p>The time occupied in plating is greatly shortened by +stirring or agitating the solutions. This sets up a good +circulation of the liquid, and a continual supply of fresh +solution is brought to the cathode. At the same time the +resistance to the current is decreased, and more current +may be used without fear of burning. <a href="#fig_37">Fig. 37</a> shows an +arrangement for this purpose. The solution is agitated by +compressed air, and at the same time the cathode rods are +moved backwards and forwards. Plating solutions are +also frequently heated in order to hasten the rate of +deposition.</p> + +<p>When the plating process is complete, the articles are +removed from the vat, thoroughly swilled in water, and +dried. They are then ready for finishing by polishing and +burnishing, or they may be given a sort of frosted surface. +During the finishing processes the appearance of the articles +changes considerably, the rather dead-looking surface<span class="pagenum" id="Page_220">220</span> +produced by the plating giving place to the bright lustre +of the particular metal.</p> + +<figure id="fig_37" class="figcenter" style="max-width: 27em;"> + <img src="images/i_254.jpg" width="2159" height="2366" alt=" "> + <figcaption class="caption"> + +<p class="floatl"><i>By Permission of</i>]</p> +<p class="floatr">[<i>W. Canning & Co.</i></p> + +<p class="floatc"><span class="smcap">Fig. 37.</span>—Method of agitating solution in Plating Vat.</p> +</figcaption></figure> + +<p>It sometimes happens that an article which has been +plated and polished shows little defects here and there in +the deposit. In such a case it is not necessary to re-plate +the whole article, for the defects can be made good by a<span class="pagenum" id="Page_221">221</span> +process of “doctoring.” A piece of the same metal as that +forming the deposit is placed between two pieces of wood, +and a wire fastened to one end of it. At the other end +several thicknesses of flannel are wrapped round and +securely tied. This strip, which forms a miniature anode, +is connected to the anode rod of the plating vat, and the +article is connected to the cathode rod. The flannel is +saturated with the plating solution, and the strip is rubbed +gently over the defective places until the deposit formed is +as thick as that on the rest of the article. If the work is +done carefully the “doctored” portions cannot be distinguished +from the rest of the surface.</p> + +<p>Electroplating may be employed to give ships’ plates +a coating of copper to prevent barnacles from sticking to +them. The work is done in sections by building up to the +side of the vessel a sort of vat of which the plate to be +coated forms one side. The plate is thus at the same time +the cathode and part of the vat.</p> + +<p>So far we have spoken only of electroplating objects +made of metal. If we tried to copperplate a plaster cast +by simply suspending it as we did our spoon, we should +get no result at all, because the plaster is a non-conductor. +But if we sprinkle plumbago over the cast so as to give it +a conducting surface, we can plate it quite well. Practically +all materials can be electroplated, but if they are non-conductors +they must be given a conducting surface in the +way just described or by some similar means. Even +flowers and insects may be plated, and by giving them first +a coating of copper and then a coating of gold, delicately +beautiful results are obtained.</p> + +<p>Electrotyping is practically the same as electroplating, +except that the coating is removed from the support on +which it is deposited. The process is largely used for +copying engraved plates for printing purposes. The plate<span class="pagenum" id="Page_222">222</span> +is first rubbed over with a very weak solution of beeswax +in turpentine, to prevent the deposit from adhering to it, +and it is then placed in a copperplating vat and given a +good thick coating. The coating is then stripped off, and +in this way a reversed copy of the plate is obtained. This +copy is then replaced in the vat, and a coating of copper +deposited upon it, and this coating, when stripped off, forms +an exact reproduction of the original, with every detail +faithfully preserved. An engraved plate may be copied +also by making from it a mould of plaster or composition. +The surface of this mould is then rendered conducting by +sprinkling over it a quantity of plumbago, which is well +brushed into all the recesses, and a coating of copper +deposited on it. As the mould was a reversed copy of the +original, the coating formed upon it is of course an exact +copy of the plate. If the copy has to be made very quickly +a preliminary deposit of copper is chemically formed on the +mould before it is placed in the vat. This is done by +pouring on to the mould a solution of sulphate of copper, +and sprinkling iron filings over the surface. The filings +are then brushed down on to the face of the mould with a +fine brush, and a chemical reaction takes place, resulting in +the precipitation of copper from the solution. After the +filings have been washed away, the mould is placed in the +vat, and the deposition of copper takes place very rapidly.</p> + +<p>Engraved copperplates are often nickel or steel-plated +to give their surface greater hardness, so that the printer +may obtain a larger number of sharp impressions from the +same plate. Stereotypes also are coated with nickel for a +similar reason.</p> + +<p>Before the dynamo came into general use all electroplating +and electrotyping was done with current supplied +by voltaic cells, and though the dynamo is now used exclusively +in large plating works, voltaic cells are still<span class="pagenum" id="Page_223">223</span> +employed for work on a very small scale. A cell which +quickly polarizes is quite useless for plating purposes, and +one giving a constant and ample supply of current is +required. The Daniell cell, which was described in +<a href="#chapter_IV">Chapter IV</a>., is used, and so also is the Bunsen cell, which +consists of a porous pot containing strong nitric acid and a +carbon rod, placed in an outer stoneware vessel containing +dilute sulphuric acid and a zinc plate. The drawback to +this cell is that it gives off very unpleasant fumes. The +dynamos used for plating work are specially constructed to +give a large amount of current at very low pressure. +Continuous current only can be used, for alternating current +would undo the work as fast as it was done, making the +article alternately a cathode and an anode.</p> +<hr class="chap x-ebookmaker-drop"> + +<div class="chapter"> +<p><span class="pagenum" id="Page_224">224</span></p> + +<h2 class="nobreak" id="toclink_224"><a id="chapter_XXIII"></a>CHAPTER XXIII<br> + +<span class="subhead">INDUSTRIAL ELECTROLYSIS</span></h2> +</div> + +<p class="in0"><span class="firstword">The</span> metal copper, as obtained from copper ore, contains +many impurities of various kinds. For most purposes +these impurities greatly affect the value of the copper, and +before the metal can be of much commercial use they must +be got rid of in some way. In the previous chapter, in +describing how to copperplate an old spoon, we saw that +the anode need not consist of pure copper, because in any +case nothing but the pure metal would be deposited upon +the spoon. This fact forms the basis of the important +industry of electrolytic copper refining. The process is +exactly the same as ordinary copperplating, except that +the cathode always consists of absolutely pure copper. +This is generally in the form of a sheet no thicker than +thin paper, but sometimes a number of suspended wires are +used instead. A solution of copper sulphate is used as +usual for the electrolyte, and the anode is a thick cast plate +of the impure copper. The result of passing a current +through the solution is that copper is taken from the anode +and carried to the cathode, the impurities falling to the +bottom of the vat and accumulating as a sort of slime. In +this way thick slabs of pure copper are obtained, ready to +be melted down and cast into ingots.</p> + +<p>The impurities in the raw copper vary according to the +ore from which it is obtained, and sometimes gold and +silver are found amongst them. When the copper is known<span class="pagenum" id="Page_225">225</span> +to contain these metals the deposit at the bottom of the +refining vats is carefully collected, and from it a considerable +quantity of gold and silver is recovered. It is +estimated that about half a million tons of copper are +refined every year. An immense amount of this pure +copper is used for electrical purposes, for making conducting +wires and cables, and innumerable parts of electric +appliances and machinery of all kinds; in fact it is calculated +that more than half of the copper produced all over the +world is used in this way.</p> + +<p>A similar method is employed to obtain the precious +metals in a pure state, from the substance known as +“bullion”; which consists usually of an intermingling of +gold, silver, and copper, with perhaps also lead. Just as in +copper refining, the raw material is used as the anode, and +a strip of pure gold or silver, according to which metal is +required, as the cathode. A silver solution is used if +silver is wanted, and a gold solution if gold is to be +deposited.</p> + +<p>The metal aluminium has come into general use with +surprising rapidity, and during the last twenty-five or +thirty years the amount of this metal produced annually +has increased from two or three tons to many thousands +of tons. Aluminium occurs naturally in large quantities, +in the form of alumina, or oxide of aluminium, but for a +long time experimenters despaired of ever obtaining the +pure metal cheaply on a commercial scale. The oxides of +most metals can be reduced, that is deprived of their oxygen, +by heating them with carbon; but aluminium oxide holds +on to its oxygen with extraordinary tenacity, and absolutely +refuses to be parted from it in this way. One process +after another was tried, without success, and cheap +aluminium seemed to be an impossibility until about 1887, +when two chemists, Hall, an American, and Héroult, a<span class="pagenum" id="Page_226">226</span> +Frenchman, discovered a satisfactory solution of the +problem. These chemists, who were then scarcely out of +their student days, worked quite independently of one +another, and it is a remarkable fact that their methods, +which are practically alike, were discovered at almost the +same time. The process is an interesting mixture of +electrolysis and electric heating. An iron crucible containing +a mixture of alumina, fluorspar, and cryolite is +heated. The two last-named substances are quickly fused, +and the alumina dissolves in the resulting fluid. When +the mixture has reached the fluid state, electrodes made of +carbon are dipped into it, and a current is passed through; +with the result that oxygen is given off at the anode, and +metallic aluminium is produced at the cathode, in molten +drops. This molten metal is heavier than the rest of the +fluid, and so it falls to the bottom. From here it is drawn +off at intervals, while fresh alumina is added as required, +so that the process goes on without interruption. After +the first fusing of the mixture no further outside heat is +required, for the heat produced by the passage of the +current is sufficient to keep the materials in a fluid state. +Vast quantities of aluminium are produced in this way at +Niagara Falls, and in Scotland and Switzerland.</p> + +<p>Most of us are familiar with the substance known as +caustic soda. The chemical name for this is sodium hydrate, +and its preparation by electrolysis is interesting. Common +salt is a chemical compound of the metal sodium and the +greenish coloured, evil smelling gas chlorine, its proper +name being sodium chloride. A solution of this in water +is placed in a vat or cell, and a current is sent through it. +The solution is then split up into chlorine, at the anode, +and sodium at the cathode. Sodium has a remarkably +strong liking for water, and as soon as it is set free from +the chlorine it combines with the water of the solution, and<span class="pagenum" id="Page_227">227</span> +a new solution of sodium hydrate is formed. The water +in this is then got rid of, and solid caustic soda remains.</p> + +<p>Amongst the many purposes for which caustic soda is +used is the preparation of oxygen and hydrogen. Water, +to which a little sulphuric acid has been added, is split up +by a current into oxygen and hydrogen, as we saw in +<a href="#chapter_V">Chapter V</a>. This method may be used for the preparation +of these two gases on a commercial scale, but more usually +a solution of caustic soda is used as the electrolyte. If the +oxygen and hydrogen are not to be used at the place where +they are produced, they are forced under tremendous +pressure into steel cylinders, and at a lantern lecture these +cylinders may be seen supplying the gas for the lime-light. +Although the cylinders are specially made and tested for +strength, they are covered with a sort of rope netting; so +that if by any chance one happened to burst, the shattered +fragments of metal would be caught by the netting, instead +of flying all over the room and possibly injuring a number +of people. A large quantity of hydrogen is prepared by +this process for filling balloons and military airships.</p> +<hr class="chap x-ebookmaker-drop"> + +<div class="chapter"> +<p><span class="pagenum" id="Page_228">228</span></p> + +<h2 class="nobreak" id="toclink_228"><a id="chapter_XXIV"></a>CHAPTER XXIV<br> + +<span class="subhead">THE RÖNTGEN RAYS</span></h2> +</div> + +<p class="in0"><span class="firstword">In</span> the chapter on electricity in the atmosphere we saw that +whereas air at ordinary pressure is a bad conductor, its +conducting power increases rapidly as the pressure is +lowered. Roughly speaking, if we wish to obtain a spark +across a gap of 1 inch in ordinary air, we must have an +electric pressure of about 50,000 volts. The discharge +which takes place under these conditions is very violent, +and it is called a “disruptive” discharge. If however the +air pressure is gradually lowered, the discharge loses its +violent character, and the brilliant spark is replaced by a +soft, luminous glow.</p> + +<p>The changes in the character of the discharge may be +studied by means of an apparatus known as the “electric +egg.” This consists of an egg-shaped bulb of glass, having +its base connected with an air-pump. Two brass rods +project into the bulb, one at each end; the lower rod being +a fixture, while the upper one is arranged to slide in and +out, so that the distance between the balls can be varied. +The outer ends of the rods are connected to an induction +coil or to a Wimshurst machine. If the distance between +the balls has to be, say, half an inch, to produce a spark +with the air at normal pressure, then on slightly reducing +the pressure by means of the air-pump it is found that a +spark will pass with the balls an inch or more apart. The +brilliance of an electric spark is due to the resistance of the<span class="pagenum" id="Page_229">229</span> +air, and as the pressure decreases the resistance becomes +smaller, so that the light produced is much less brilliant. +If the exhaustion is carried still further the discharge +becomes redder in colour, and spreads out wider and wider +until it loses all resemblance to a spark, and becomes a +luminous glow of a purple or violet colour. At first this +glow seems to fill the whole bulb, but at still higher vacua +it contracts into layers of definite shape, these layers being +alternately light and dark. Finally, when the pressure +becomes equal to about one-millionth of an atmosphere, a +luminous glow surrounds the cathode or negative rod, +beyond this is dark space almost filling the bulb, and the +walls of the bulb between the cathode and the anode +glow with phosphorescent light. This phosphorescence is +produced by rays coming from the cathode and passing +through the dark space, and these rays have been given the +name of “cathode rays.”</p> + +<p>Many interesting experiments with these rays may be +performed with tubes permanently exhausted to the proper +degree. The power of the rays to produce phosphorescence +is shown in a most striking way with a tube fixed in a +horizontal position upon a stand, and containing a light +cross made of aluminium, placed in the path of the rays. +This is hinged at the base, so that it can be stood up on end +or thrown down by jerking the tube. Some of the rays +streaming from the cathode are intercepted by the cross, +while others pass by it and reach the other end of the tube. +The result is that a black shadow of the cross is thrown on +the glass, sharply contrasted with those parts of the tube +reached by the rays, and which phosphoresce brilliantly. +After a little while this brilliance decreases, for the glass +becomes fatigued, and loses to a considerable extent its +power of phosphorescing. If now the cross is jerked down, +the rays reach the portions of the tube before protected by<span class="pagenum" id="Page_230">230</span> +the cross, and this glass, being quite fresh, phosphoresces +with full brilliance. The black cross now suddenly becomes +brilliantly illuminated, while the tired glass is dark in +comparison. If the tired glass is allowed to rest for a while +it partly recovers its phosphorescing powers, but it never +regains its first brilliance.</p> + +<p>An even more striking experiment may be made with a +horizontal tube containing a tiny wheel with vanes of mica, +something like a miniature water-wheel, mounted on glass +rails. When the discharges are sent through the tube, the +cathode rays strike against the vanes and cause the little +wheel to move forward in the direction of the anode. Other +experiments show that the cathode rays have great heating +power, and that they are deflected by a magnet held close +to the tube.</p> + +<p>For a long time the nature of these cathode rays was in +dispute. German physicists held that they were of the same +character as ordinary light, while English scientists, headed +by Sir William Crookes, maintained that they were streams +of extremely minute particles of matter in a peculiar fourth +state. That is to say, the matter was not liquid, or solid, or +gaseous in the ordinary sense, but was <em>ultra-gaseous</em>, and +Crookes gave it the name of <em>radiant matter</em>. Most of us +have been taught to look upon the atom as the smallest +possible division of matter, but recent researches have made +it clear that the atom itself is divisible. It is believed that +an atom is made up of very much more minute particles +called <em>electrons</em>, which are moving about or revolving all the +time with incredible rapidity. According to Sir Oliver +Lodge, if we imagine an atom of hydrogen to be as big as +an ordinary church, then the electrons which constitute it +will be represented by about 700 grains of sand, 350 being +positively electrified and 350 negatively electrified. It is not +yet definitely determined whether these electrons are minute<span class="pagenum" id="Page_231">231</span> +particles of matter charged with electricity, or whether they +are actually atoms of electricity. The majority of scientists +now believe that the cathode rays consist of a stream of +negative electrons repelled from the cathode at a speed of +124 miles per second, or not quite 1/1000 of the velocity of +light.</p> + +<p>In November 1895, Professor Röntgen, a German +physicist, announced his discovery of certain invisible rays +which were produced at the same time as the cathode rays, +and which could penetrate easily solids quite opaque to +ordinary light. He was experimenting with vacuum tubes, +and he found that certain rays emerged from the tube. These +were not cathode rays, because they were able to pass +through the glass, and were not deflected by a magnet. To +these strange rays he gave the name of the “<em>X</em>,” or unknown +rays, but they are very frequently referred to by the name +of their discoverer.</p> + +<p>It was soon found that the Röntgen rays affected an +ordinary photographic plate wrapped up in black paper so +as to exclude all ordinary light, and that they passed +through flesh much more easily than through bone. This +fact makes it possible to obtain what we may call “shadow-graphs” +of the bones through the flesh, and the value of +this to the medical profession was realized at once. The +rays also were found to cause certain chemical compounds +to become luminous. A cardboard screen covered with +one of these compounds is quite opaque to ordinary light, +but if it is examined when the Röntgen rays are falling +upon it, it is seen to be brightly illuminated, and if the +hand is held between the screen and the rays the bones +become clearly visible.</p> + +<figure id="fig_38" class="figcenter" style="max-width: 19em;"> + <img src="images/i_266.png" width="1518" height="1065" alt=" "> + <figcaption class="caption"><span class="smcap">Fig. 38.</span>—X-Ray Tube, showing paths of Cathode and X-Rays. +</figcaption></figure> + +<p>Röntgen rays are produced when the cathode rays fall +upon, and as it were bombard, an obstacle of some kind. +Almost any tube producing cathode rays will produce also<span class="pagenum" id="Page_232">232</span> +Röntgen rays, but special forms of tube are used when the +main object is to obtain these latter rays. <a href="#fig_38">Fig. 38</a> shows +a typical form of simple X-ray tube. This, like all other +tubes for X-ray work, is exhausted to a rather higher +vacuum than tubes intended for the production of cathode +rays only. The cathode C is made of aluminium, and is +shaped like a saucer, its curvature being arranged so that +the cathode rays are focused on to the anti-cathode A. +The focusing as a rule is not done very accurately, for +although sharper radiographs are obtained when the cathode +rays converge exactly to a point on the anti-cathode, the +heating effect at this point is so great that a hole is quickly +burned. The target, or surface of the anti-cathode, is +made of some metal having an extremely high melting-point, +such as platinum, iridium, or tungsten. It has a flat +surface inclined at an angle of about 45°, so that the rays +emanating from it proceed in the direction shown by the +dotted lines in the figure. The continuous lines show the +direction of the cathode rays. The anode is made of +aluminium, and it is shown at N. It is not necessary to<span class="pagenum" id="Page_233">233</span> +have a separate anode, and the anti-cathode may be used +as the anode. In the tube shown in <a href="#fig_38">Fig. 38</a> the anode +and the anti-cathode are joined by an insulated wire, so +that they both act as anodes. The tube is made of soda-glass, +as the X-rays do not pass at all readily through lead-glass.</p> + +<figure id="fig_39" class="figcenter" style="max-width: 18em;"> + <img src="images/i_267.png" width="1372" height="759" alt=" "> + <figcaption class="caption"> + +<p class="floatl"><i>By permission of</i>]</p> +<p class="floatr">[<i>C. H. F. Muller.</i></p> + +<p class="floatc"><span class="smcap">Fig. 39.</span>—Diagram of Mica Vacuum Regulator +for X-Ray Tubes.</p> +</figcaption></figure> + +<p>The penetrating power of the X-rays varies with the +vacuum of the tube, a low vacuum giving rays of small +penetration, and a high vacuum rays of great penetration. +Tubes are called hard or soft according to the degree of +the vacuum, a hard +tube having a high +vacuum and a soft +tube a low one. It +should be remembered +that the +terms high and +low, as applied to +the vacuum of X-ray +tubes, are only +relative, because +the vacuum must +be very high to admit of the production of X-rays at all. +The vacuum becomes higher as the tube is used, and after a +while it becomes so high that the tube is practically useless, +for the penetrating power of the rays is then so great that +sharp contrasts between different substances, such as flesh +and bone, cannot be obtained, and the resulting radiographs +are flat and poor. The vacuum of a hard tube may be +lowered temporarily by gently heating the tube, but this is +not a very convenient or satisfactory process, and tubes are +now made with special arrangements for lowering the +vacuum when necessary. There are several vacuum-regulating +devices, and <a href="#fig_39">Fig. 39</a> is a diagram of the<span class="pagenum" id="Page_234">234</span> +“Standard” mica regulator used in most of the well-known +“Muller” X-ray tubes. This consists of a small additional +bulb containing an electrode D carrying a series of mica +discs. A wire DF is attached to D by means of a hinged +cap. The vacuum is lowered while the discharges are +passing through the tube. The wire DF is moved towards +the cathode terminal B, and kept there for a few seconds. +Sparks pass between F and B, and the current is now +passing through the electrode D in the regulator chamber. +This causes the mica to become heated, so that it gives off +a small quantity of gas, which passes into the main tube +and so lowers the vacuum. The wire DF is then moved +well away from B, and after a few hours’ rest the tube, now +of normal hardness, is ready for further use.</p> + +<p>We have already referred to the heating of the anti-cathode +caused by the bombardment of the cathode rays. +Even if these rays are not focused very sharply, the anti-cathode +of an ordinary tube becomes dangerously hot if +the tube is run continuously for a fairly long period, and +for hospital and other medical work on an extensive scale +special tubes with water-cooled anti-cathodes are used. +These tubes have a small bulb blown in the anti-cathode +neck. This bulb is filled with water, which passes down a +tube to the back of the target of the anti-cathode. By this +arrangement the heat generated in the target is absorbed +by the water, so that the temperature of the target can +become only very slightly higher then 212° F., which +is the temperature of boiling water, and quite a safe +temperature for the anti-cathode. In some tubes the rise +in temperature is made slower by the use of broken bits of +ice in place of water. <a href="#fig_40">Fig. 40</a> shows a Muller water-cooled +tube, and <a href="#fig_41">Fig. 41</a> explains clearly the parts of an X-ray +tube and their names.</p> + +<figure id="fig_40" class="figcenter" style="max-width: 22em;"> + <img src="images/i_269.png" width="1732" height="905" alt=" "> + <figcaption class="caption"><span class="smcap">Fig. 40.</span>—Muller Water-cooled X-Ray Tube. +</figcaption></figure> + +<figure id="fig_41" class="figcenter" style="max-width: 26em;"> + <img src="images/i_269b.png" width="2031" height="2225" alt=" "> + <figcaption class="caption"> + +<p class="floatl"><i>By permission of</i>]</p> +<p class="floatr">[<i>C. H. F. Muller.</i></p> + +<p class="floatc"><span class="smcap">Fig. 41.</span>—Diagram showing parts of X-Ray Tube.</p> +</figcaption></figure> + +<p>An induction coil is generally used to supply the high-tension<span class="pagenum" id="Page_236">236</span><span class="pagenum" id="Page_235">235</span> +electricity required for the production of the Röntgen +rays. For amateur or experimental purposes a coil +giving continuous 4-inch or even 3-inch sparks will +do, but for medical work, in which it is necessary to take +radiographs with very short exposures, coils giving sparks +of 10, 12, or more inches in length are employed. An +electrical influence machine, such as the Wimshurst, may +be used instead of an induction coil. Very powerful +machines with several pairs of plates of large diameter, +and driven by an electric motor, are in regular use for +X-ray work in the United States, but in this country they +are used only to a very small extent. A Wimshurst +machine is particularly suitable for amateur work. If a +screen is to be used for viewing bones through the flesh a +fairly large machine is required, but for screen examination +of such objects as coins in a box, or spectacles in a case, +and for taking radiographs of these and other similar +objects, a machine giving a fairly rapid succession of sparks +as short as 2 inches can be used. Of course the exposure +required for taking radiographs with a machine as small as +this are very long, but as the objects are inanimate this +does not matter very much.</p> + +<p>For amateur X-ray work the arrangement of the +apparatus is simple. The tube is held in the required +position by means of a wooden clamp attached to a stand +in such a way that it is easily adjustable. Insulated wires +are led from the coil or from the Wimshurst machine to the +tube, the positive wire being connected to the anode, and +the negative wire to the cathode. With a small Wimshurst +machine light brass chains may be used instead of wires, +and these have the advantage of being easier to manipulate. +For medical purposes the arrangements are more complicated, +and generally a special room is set apart for X-ray +work.</p> + +<p><span class="pagenum" id="Page_237">237</span></p> + +<p>If the connexions have been made correctly, then on +starting the coil or the machine the tube lights up. The +bulb appears to be sharply divided into two parts, the +part in front of the anti-cathode glowing with a beautiful +greenish-yellow light, while the part behind the anti-cathode +is dark, except for lighter patches close to the +anode. The Röntgen rays are now being produced. The +illumination is not steady like that of an electric lamp, but +it consists of a series of flickers, which, with powerful +apparatus, follow one another so rapidly as to give the +impression of continuity. If the connexions are wrong, so +that the negative wire goes to the anode instead of to the +cathode, the bulb is not divided in this way, but has +patches of light almost all over. As soon as this appearance +is seen the apparatus must be stopped and the connexions +reversed, for the tube is quickly damaged by passing the +discharge through it in the wrong direction.</p> + +<p>Having produced the X-rays, we will suppose that it +is desired to examine the bones of the hand. For this +purpose a fluorescent screen is required. This consists of +a sheet of white cardboard coated usually with crystals of +barium platino-cyanide. In order to shut out all light but +that produced by the rays, the cardboard is placed at the +larger end of a box or bellows shaped like a pyramid. +This pyramid is brought close to the X-ray tube, with its +smaller end held close to the eyes, and the hand is placed +against the outer side of the cardboard sheet. The outline +of the hand is then seen as a light shadow, and the very +much blacker shadow of the bones is clearly visible. For +screen work it is necessary to darken the room almost +entirely, on account of the feebleness of the illumination of +the screen.</p> + +<p>If a radiograph of the bones of the hand is to be taken, +a very sensitive photographic plate is necessary. An<span class="pagenum" id="Page_238">238</span> +ordinary extra-rapid plate will do fairly well, but for the +best work plates made specially for the purpose are used. +The emulsion of an ordinary photographic plate is only +partially opaque to the X-rays, so that while some of the +rays are stopped by it, others pass straight through. The +silver bromide in the emulsion is affected only by those +rays which are stopped, so that the energy of the rays +which pass through the emulsion is wasted. If a plate is +coated with a very thick film, a larger proportion of the +rays can be stopped, and many X-ray plates differ from +photographic plates only in the thickness of the emulsion. +A thick film however is undesirable because it makes the +after processes of developing, fixing, and washing very +prolonged. In the “Wratten” X-ray plate the emulsion is +made highly opaque to the rays in a different and ingenious +manner. Salts of certain metals have the power of +stopping the X-rays, and in this plate a metallic salt of this +kind is contained in the emulsion. The film produced in +this way stops a far larger proportion of the rays than any +ordinary film, and consequently the plate is more sensitive +to the rays, so that shorter exposures can be given.</p> + +<p>X-ray plates are sold usually wrapped up separately in +light-tight envelopes of black paper, upon which the film +side of the plate is marked. If there is no such wrapping +the plate must be placed in a light-tight envelope, with +its film facing that side of the envelope which has no folds. +The ordinary photographic double envelopes, the inner one +of yellow paper and the outer one of black paper, are very +convenient for this purpose. The plate in its envelope is +then laid flat on the table, film side upwards, and the +X-ray tube is clamped in a horizontal position so that the +anti-cathode is over and pointing towards the plate. The +hand is laid flat on the envelope, and the coil or machine is +set working. The exposure required varies so much with<span class="pagenum" id="Page_239">239</span> +the size of the machine or coil, the distance between the +tube and the plate, the condition of the tube, and the nature +of the object, that it is impossible to give any definite +times, and these have to be found by experiment. The +hand requires a shorter exposure than any other part of the +body. If we call the correct exposure for the hand 1, then +the exposures for other parts of the body would be +approximately 3 for the foot and the elbow, 6 for the +shoulder, 8 for the thorax, 10 for the spine and the hip, +and about 12 for the head. The exposures for such objects +as coins in a box are much less than for the hand. After +exposure, the plate is developed, fixed, and washed just as +in ordinary photography. <a href="#plate_XIV">Plate XIV</a>. shows a Röntgen +ray photograph of a number of fountain pens, British and +foreign.</p> + +<p>Prolonged exposure to the X-rays gives rise to a +painful and serious disease known as X-ray dermatitis. +This danger was not realized by the early experimenters, +and many of them contracted the disease, with fatal results +in one or two cases. Operators now take ample precautions +to protect themselves from the rays. The tubes +are screened by substances opaque to the rays, so that +these emerge only where they are required, and +impenetrable gloves or hand-shields, aprons, and face-masks +made of rubber impregnated with lead-salts are +worn.</p> + +<p>X-ray work is a most fascinating pursuit, and it can be +recommended strongly to amateurs interested in electricity. +There is nothing particularly difficult about it, and complete +outfits can be obtained at extremely low prices, although it +is best to get the most powerful Wimshurst machine or +induction coil that can be afforded. As radiography is +most likely to be taken up by photographers, it may be +well to state here that any photographic plates or papers<span class="pagenum" id="Page_240">240</span> +left in their usual wrappings in the room in which X-rays +are being produced are almost certain to be spoiled, and +they should be placed in a tightly fitting metal box or be +taken into the next room. It is not necessary for the +amateur doing only occasional X-ray work with small +apparatus to take any of the precautions mentioned in the +previous paragraph, for there is not the slightest danger in +such work.</p> + +<figure id="plate_XIV" class="figcenter" style="max-width: 35em;"> + <p class="caption">PLATE XIV.</p> + <img src="images/i_275.jpg" width="3106" height="1886" alt=" "> + <figcaption class="caption"> + +<p class="floatl"><i>By permission of</i></p> +<p class="floatr"><i>Kodak Ltd.</i></p> + +<p class="floatc">RÖNTGEN RAY PHOTOGRAPH OF BRITISH AND FOREIGN FOUNTAIN PENS. TAKEN ON WRATTEN X-RAY PLATE.</p> +</figcaption></figure> + +<hr class="chap x-ebookmaker-drop"> + +<div class="chapter"> +<h2 class="nobreak" id="toclink_241"><a id="chapter_XXV"></a>CHAPTER XXV<br> + +<span class="subhead">ELECTRICITY IN MEDICINE</span></h2> +</div> + +<p class="in0"><span class="firstword">One</span> of the most remarkable things about electricity is the +immense number of different purposes for which it may be +used. We have already seen it driving trams and trains, +lighting and heating our houses, and carrying our messages +thousands of miles over land and sea, and now we come to its +use in medical work. In the minds of many people medical +electricity is associated with absolute quackery. Advertisements +of electric belts, rings, and other similar appliances +have appeared regularly for many years in our newspapers +and magazines, and constant exposures of the utter worthlessness +of almost all these appliances have produced the +impression that medical electricity is nothing but a bare-faced +fraud, while the disgusting exhibitions of so-called +electric healing which have been given on the music-hall +stage have greatly deepened this impression. This state +of things is very unfortunate, because electricity, in the +hands of competent medical men, is a healing agent of +wonderful potency. Still another source of prejudice +against electricity may be found in the fact that electric +healing is popularly associated with more or less violent +shocks. On this account nervously-sensitive people shrink +from the idea of any kind of electrical treatment. As a +matter of fact electric shocks have no healing value, but on +the contrary they are frequently harmful, and a very severe +shock to a sensitive person may cause permanent injury.<span class="pagenum" id="Page_242">242</span> +No shocks whatever are given in electric treatment by +medical men, and indeed in the majority of cases the treatment +is unaccompanied by unpleasant sensations of any kind.</p> + +<p>In the previous chapter we spoke of the use of the +Röntgen or X-rays in examining the various bones of the +body. By means of the fluorescent screen it is quite easy +to find and examine fractures and dislocations, and many +of the diseases of the bones and joints can be seen and +recognized. Metals are opaque to the X-rays, and so the +screen shows plainly such objects as needles or bullets +embedded in the flesh. Sometimes people, especially +young children, swallow coins and other small metal articles, +and here again the X-rays will show the exact position of +the intruder. A particularly valuable application of the +rays is in the discovering and locating of tiny fragments of +metal in the eye, for very often it is quite impossible to do +this by ordinary observation. Most of these fragments are +of steel or iron, and they are most easily removed by means +of an electro-magnet. If the fragment is very small a +powerful magnet is used, one capable of supporting 500 +or 600 lb.; but if it is fairly large a weaker magnet, +supporting perhaps 30 lb., must be employed, because +the forceful and rapid dragging out of a large body might +seriously damage the eye.</p> + +<p>If the chest is examined by the Röntgen rays the lungs +are seen as light spaces between the clearly marked ribs, +and any spot of congestion appears as a darker patch. In +this way the early stages of consumption may be revealed, +and in pneumonia and other similar complaints valuable +information regarding the condition of the lungs can be +obtained. It is possible also to follow to a considerable extent +the processes of digestion. X-rays easily pass through ordinary +food, but if bismuth oxychloride, which is quite harmless, +is mixed with the food, the mixture becomes opaque<span class="pagenum" id="Page_243">243</span> +to the rays, and so its course may be followed on the screen. +The normal movements of the food are well known, and an +abnormal halt is probably caused by an obstruction of some +kind, and thus the X-rays enable the physician to locate +the obstruction, and to form an opinion of its nature.</p> + +<p>In our chapter on wireless telegraphy we saw that the +discharge of a Leyden jar takes the form of a number of +rapid oscillations backwards and forwards. These oscillations +take place at a rate of more than half a million per +second, but by the use of an apparatus called a “high frequency +transformer” the rate is increased to more than a +million per second. Electricity in this state of rapid oscillation +is known as high frequency electricity, and high frequency +currents are very valuable for some kinds of medical +work. The application of these currents is quite painless, and +but for the strange-looking apparatus the patient probably +would not know that anything unusual was taking place. +To some extent the effect maybe said to be not unlike that +of a powerful tonic. Insomnia and other troubles due to +disordered nerves are quickly relieved, and even such +obstinate complaints as neuritis and crippling rheumatism +have been cured. The treatment is also of great value in +certain forms of heart trouble. By increasing the strength +of the high frequency currents the tissues actually may +be destroyed, and this power is utilized for exterminating +malignant growths, such as lupus or cancer.</p> + +<p>The heat produced by a current of electricity is made +use of in cauterizing. The burner is a loop of platinum +wire, shaped according to the purpose for which it is +intended, and it is used at a dull red heat. Very tiny +electric incandescent lamps, fitted in long holders of special +shape, are largely used for examining the throat and the +various cavities of the body.</p> + +<p>In the Finsen light treatment electric light is used for<span class="pagenum" id="Page_244">244</span> +a very different purpose. The spectrum of white light consists +of the colours red, orange, yellow, green, blue, indigo, +and violet. Just beyond the violet end of the spectrum are +the ultra-violet rays. Ultra-violet light consists of waves +of light which are so short as to be quite invisible to the +eye, and Dr. N. R. Finsen, a Danish physician, made the +discovery that this light is capable of destroying bacterial +germs. In the application of ultra-violet rays to medical +work, artificial light is used in preference to sunlight; for +though the latter contains ultra-violet light, a great deal of +it is absorbed in passing through the atmosphere. Besides +this, the sun sends out an immense amount of radiant heat, +and this has to be filtered out before the light can be used. +The usual source of light is the electric arc, and the arc is +much richer in ultra-violet rays if it is formed between +electrodes of iron, instead of the usual carbon rods. The +light, which, in addition to the ultra-violet rays, includes +the blue, indigo, and violet parts of the spectrum, is passed +along a tube something like that of a telescope, and is +focused by means of a double lens, consisting of two +separate plates of quartz. Glass cannot be used for the +lens, because it is opaque to the extreme ultra-violet rays. +A constant stream of water is passed between the two +plates forming the lens, and this filters out the heat rays, +which are not wanted. In some forms of Finsen lamp an +electric spark is used as the source of light, in place of the +arc.</p> + +<p>The most important application of the Finsen light is +in the cure of the terribly disfiguring disease called lupus. +This is a form of tuberculosis of the skin, and it is produced +by the same deadly microbe which, when it attacks +the lungs, causes consumption. In all but extreme cases +the Finsen light effects a remarkable cure. A number of +applications are necessary, each of half an hour or more;<span class="pagenum" id="Page_245">245</span> +and after a time the disease begins to disappear, leaving +soft, normal skin. The exact action of the light rays is a +disputed point. Finsen himself believed that the ultra-violet +rays attacked and exterminated the microbe, but a +later theory is that the rays stimulate the tissues to such +an extent that they are enabled to cure themselves. As +early as the year 1899 Finsen had employed his light +treatment in 350 cases of lupus, and out of this number +only five cases were unsuccessful.</p> + +<p>The ultra-violet rays are said to have a very beneficial +effect upon the teeth. Experiments carried out in Paris, +using a mercury vapour lamp as the source of light, show +that discoloured teeth are whitened and given a pearly +lustre by these rays, at the same time being sterilized so +that they do not easily decay. The Röntgen rays are +used for the treatment of lupus, and more particularly for +deeper growths, such as tumours and cancers, for which +the Finsen rays are useless, owing to their lack of penetrating +power. The action of these two kinds of rays appears +to be similar, but the X-rays are much the more active of +the two.</p> + +<p>Electricity is often applied to the body through water, +in the form of the hydro-electric bath, and such baths +are used in the treatment of different kinds of paralysis. +Electric currents are used too for conveying drugs into the +tissues of the body. This is done when it is desired to +concentrate the drug at some particular point, and it has +been found that chemicals can be forced into the tissues for +a considerable distance.</p> + +<p>Dr. Nagelschmidt, a great authority on medical electricity, +has suggested the use of electricity for weight reducing. +In the ordinary way superfluous flesh is got rid +of by a starvation diet coupled with exercise, but in many +cases excessively stout people are troubled with heart<span class="pagenum" id="Page_246">246</span> +disorders and asthma, so that it is almost impossible for +them to undergo the necessary muscular exertion. By the +application of electric currents, however, the beneficial +effects of the gentle exercise may be produced without any +exertion on the part of the patient, and an hour’s treatment +is said to result in a decrease in weight of from 200 to 800 +grammes, or roughly 7 to 27 ounces.</p> +<hr class="chap x-ebookmaker-drop"> + +<div class="chapter"> +<p><span class="pagenum" id="Page_247">247</span></p> + +<h2 class="nobreak" id="toclink_247"><a id="chapter_XXVI"></a>CHAPTER XXVI<br> + +<span class="subhead">OZONE</span></h2> +</div> + +<p class="in0"><span class="firstword">The</span> great difference between the atmospheric conditions +before and after a thunderstorm must have been noticed +by everybody. Before the storm the air feels lifeless. It +does not satisfy us as we draw it into our lungs, and however +deeply we breathe, we feel that something is lacking. +After the storm the air is delightful to inhale, and it refreshes +us with every breath. This remarkable transformation +is brought about to a very large extent by ozone +produced by the lightning discharges.</p> + +<p>As far back as 1785 it was noticed that oxygen became +changed in some way when an electric spark was passed +through it, and that it acquired a peculiar odour. No +particular attention was paid to the matter however until +about 1840, when Schönbein, a famous German chemist, and +the discoverer of gun-cotton and collodion, became interested +in it. He gave this strange smelling substance the name +of “ozone,” and he published the results of his experiments +with it in a treatise entitled, “On the Generation of +Ozone.” Schönbein showed that ozone could be produced +by various methods, chemical as well as electrical. For +instance, if a piece of phosphorus is suspended in a jar of +air containing also a little water, in such a manner that it +is partly in the water and partly out of it, the air acquires +the characteristic smell of ozone, and it is found to have +gained increased chemical energy, so that it is a more<span class="pagenum" id="Page_248">248</span> +powerful oxidizing agent. For a long time the exact +chemical nature of ozone could not be determined, mainly +because it was impossible to obtain the substance in +quantities sufficiently large for extensive experimental +research, but also on account of its extremely energetic +properties, which made it very troublesome to examine. +These difficulties were so great that investigators were in +doubt as to whether ozone was an element or a compound +of two or more elements; but finally it was proved that it +was simply oxygen in a condensed or concentrated state.</p> + +<p>Apparently ozone is formed by the contraction of +oxygen, so that from three volumes of oxygen two volumes +of ozone are produced. In other words, ozone has one and +a half times the density of oxygen. Ozone has far greater +oxidizing power than oxygen itself; in fact it is probably +the most powerful of all oxidizing agents, and herein lies +its great value. It acts as nature’s disinfectant or sterilizer, +and plays a very important part in keeping the air pure, +by destroying injurious organic matter. Bacteria apparently +have a most decided objection to dying; at any +rate they take an extraordinary amount of killing. Ozone +is more than a match for them however, and under its +influence they have a short life and probably not a merry +one.</p> + +<p>Ozone exists naturally in the atmosphere in the open +country, and more especially at the seaside. It is produced +by lightning discharges, by silent electrical discharges +in the atmosphere, by the evaporation of water, +particularly salt water, by the action of sunlight, and also +by the action of certain vegetable products upon the air. +The quantity of ozone in the air is always small, and even +pure country or sea air contains only one volume of ozone +in about 700,000 volumes of air. No ozone can be detected +in the air of large towns, or over unhealthy swamps or<span class="pagenum" id="Page_249">249</span> +marshes. The exhilarating effects of country and sea air, +and the depressing effects of town air, are due to a very +large extent to the presence or absence of ozone.</p> + +<p>A great proportion of our common ailments are caused +directly or indirectly by a sort of slow poisoning, produced +by the impure air in which we live and work. It is popularly +supposed that the tainting of the air of rooms in +which large numbers of people are crowded together is due +to an excessive amount of carbonic acid gas. This is a +mistake, for besides being tasteless and odourless, carbonic +acid gas is practically harmless, except in quantities far +greater than ever exist even in the worst ventilated rooms. +The real source of the tainted air is the great amount of +animal matter thrown off as waste products from the skin +and lungs, and this tainting is further intensified by the +absence of motion in the air. Even in an over-crowded +room the conditions are made much more bearable if the +air is kept in motion, and in a close room ladies obtain +relief by the use of their fans. What we require, therefore, +in order to maintain an agreeable atmosphere under +all conditions, is some means of keeping the air in gentle +motion, and at the same time destroying as much as possible +of the animal matter contained in it. Perhaps the +most interesting and at the same time the most scientific +method of doing this is by ozone ventilation.</p> + +<p>In the well-known “Ozonair” system of ventilation, +ozone is generated by high-tension current. Low-tension +current is taken from the public mains or from accumulators, +and raised to a very high voltage by passing it through a +step-up transformer. The secondary terminals of the +transformer are connected to a special form of condenser, +consisting of layers of fine metal gauze separated by an +insulating substance called “micanite.” The high tension +between the gauze layers produces a silent electrical discharge<span class="pagenum" id="Page_250">250</span> +or glow. A small fan worked by an electric motor +draws the air over the condenser plates, and so a certain +proportion of the oxygen is ozonized, and is driven out of +the other side of the apparatus into the room. The amount +of ozone generated and the amount of air drawn over the +condenser are regulated carefully, so that the ozonized air +contains rather less than one volume of ozone in one +million volumes of air, experiment having shown that this +is the most suitable strength for breathing. Ozone diluted +to this degree has a slight odour which is very refreshing, +and besides diminishing the number of organic germs in +the air, it neutralizes unpleasant smells, such as arise from +cooking or stale tobacco smoke. Ozone ventilation is now +employed successfully in many hotels, steamships, theatres +and other places of entertainment, municipal and public +buildings, and factories.</p> + +<figure id="fig_42" class="figcenter" style="max-width: 38em;"> + <img src="images/i_287.jpg" width="2999" height="2089" alt=" "> + <figcaption class="caption"> + +<p class="floatl"><i>By permission of</i>]</p> +<p class="floatr">[<i>Ozonair, Ltd.</i></p> + +<p class="floatc"><span class="smcap">Fig. 42.</span>—Diagram of Ozonizing Plant, Central London Tube Electric Railway.</p> +</figcaption></figure> + +<p>One of the most interesting examples of ozone ventilation +is that of the Central London tube electric railway. +The installation consists of a separate ozonizing plant at +every station, except Shepherd’s Bush, which is close to +the open end of the tunnel. <a href="#fig_42">Fig. 42</a> is a diagram of the +general arrangement of one of these plants, and it shows +how the air is purified, ozonized, and sent into the tunnel. +The generating plant is seen at the top left-hand corner of +the figure. Air is drawn in as shown by the arrows, and +by passing through the filter screen F it is freed from dirt +and smuts, and from most of the injurious gases which +always are present in town air. The filter screen is kept +moist by a continual flow of water from jets above it, the +waste water falling into the trough W. The ozone +generator is shown at O. Continuous current at about +500 volts, from the power station, is passed through a +rotary converter, which turns it into alternating current at +380 volts. This current goes to the transformer T, from<span class="pagenum" id="Page_252">252</span> +which it emerges at a pressure of 5000 volts, and is supplied +to the ozone generator. From the generator the strongly +ozonized air is taken by way of the ozone pipe P, to the +mixing chamber of the large ventilating fan M, where it is +mixed with the main air current and then blown down the +main air trunk. From this trunk it is distributed to various +conduits, and delivered at the air outlets marked A. +Altogether the various plants pump more than eighty +million cubic feet of ozonized air into the tunnels every +working day.</p> + +<p>In many industries pure air is very essential, especially +during certain processes. This is the case in brewing, in +cold storage, and in the manufacture and canning of food +products; and in these industries ozone is employed as an +air purifier, with excellent results. Other industries cannot +be carried on without the production of very unpleasant +fumes and smells, which are a nuisance to the workers and +often also to the people living round about; and here +again ozone is used to destroy and remove the offending +odours. It is employed also in the purification of sewage +and polluted water; in bleaching delicate fabrics; in drying +and seasoning timber; in maturing tobacco, wines and +spirits, and in many other processes too numerous to +mention.</p> +<hr class="chap x-ebookmaker-drop"> + +<div class="chapter"> +<p><span class="pagenum" id="Page_253">253</span></p> + +<h2 class="nobreak" id="toclink_253"><a id="chapter_XXVII"></a>CHAPTER XXVII<br> + +<span class="subhead">ELECTRIC IGNITION</span></h2> +</div> + +<p class="in0"><span class="firstword">The</span> petrol motor, which to-day is busily engaged all over +the world in driving thousands upon thousands of self-propelled +vehicles or automobiles, belongs to the important +class of internal-combustion engines. Combustion means +the operation of burning, and an internal-combustion engine +is one in which the motive power is produced by the combustion +of a highly explosive mixture of gases. In the +ordinary petrol motor this mixture consists of petrol and air, +and it is made by means of a device called a “carburetter.” +By suction, a quantity of petrol is forced through a jet with +a very fine nozzle, so that it is reduced to an extremely fine +spray. A certain proportion of air is allowed to enter, and +the mixture passes into the cylinder. Here it is compressed +by the rising piston so that it becomes more and more +heated, and at the right point it is ignited. Combustion +takes place with such rapidity that it takes the form of an +explosion, and the energy produced in this way drives +forward the piston, which turns the crank-shaft and so +communicates motion to the driving-wheels.</p> + +<p>The part played by electricity in this process is confined +to the ignition of the compressed charge of petrol and air. +This may be done in two ways; by means of an accumulator +and a small induction coil, or by means of a dynamo driven +by the engine. At one time the first method was employed +exclusively, but to-day it is used as a rule only for starting<span class="pagenum" id="Page_254">254</span> +the car engine, the second or magneto method being used +when the engine has started up.</p> + +<p>In accumulator ignition the low-tension current from +the accumulator passes through an induction coil, and is +thus transformed to high-tension current. This current +goes through a sparking plug, which is fixed in the head +of the cylinder. The sparking plug contains two metal +points separated by a tiny air gap of from about 1/30 to 1/50 +inch. This gap provides the only possible path for the +high-tension current, so that the latter leaps across it in +the form of a spark. The spark is arranged to take place +when the piston is at the top of its stroke, that is, when the +explosive mixture is at its maximum compression, and the +heat of the spark ignites the mixture, the resulting explosion +forcing down the piston with great power. In practice it +is found better as a rule to cause the spark to pass very +slightly before the piston reaches the extreme limit of its +stroke. The reason of this is that the process of igniting +and exploding the charge occupies an appreciable, though +of course exceedingly small amount of time. Immediately +on reaching the top of its stroke the piston begins to +descend again, and if the spark and the top of the stroke +coincide in time the explosion does not take place until the +piston has moved some little distance down the cylinder, +and so a certain amount of power is lost. By having the +spark a little in advance of the piston, the explosion occurs +at the instant when the piston begins to return, and so the +full force of the explosion is utilized.</p> + +<p>In magneto ignition the current is supplied by a small +dynamo. This generates alternating current, and it is +driven by the car engine. The current is at first at low +pressure, and it has to be transformed to high-tension +current in order to produce the spark. There are two +methods of effecting this transformation. One is by turning<span class="pagenum" id="Page_255">255</span> +the armature of the dynamo into a sort of induction coil, by +giving it two separate windings, primary and secondary; +so that the dynamo delivers high-tension current directly. +The other method is to send the low-tension current +through one or more transformer coils, just as in accumulator +ignition. Accumulators can give current only for a +certain limited period, and they are liable consequently to +run down at inconvenient times and places. They also +have the defect of undergoing a slight leakage of current +even when they are not in use. Magneto ignition has +neither of these drawbacks, and on account of its superior +reliability it has come into universal use.</p> + +<p>In the working of quarries and mines of various kinds, +and also in large engineering undertakings, blasting plays +a prominent part. Under all conditions blasting is a more +or less dangerous business, and it has been the cause of +very many serious accidents to the men engaged in carrying +it out. Many of these accidents are due to the carelessness +resulting from long familiarity with the work, but apart +from this the danger lies principally in uncertainty in +exploding the charge. Sometimes the explosion occurs +sooner than expected, so that the men have not time to get +away to a safe distance. Still more deadly is the delayed +explosion. After making the necessary arrangements the +men retire out of danger, and await the explosion. This +does not take place at the expected time, and after waiting +a little longer the men conclude that the ignition has failed, +and return to put matters right. Then the explosion takes +place, and the men are killed instantly or at least seriously +injured. Although it is impossible to avoid altogether +dangers of this nature, the risk can be reduced to the +minimum by igniting the explosives by electricity.</p> + +<p>Electrical shot firing may be carried out in different +ways, according to circumstances. The current is supplied<span class="pagenum" id="Page_256">256</span> +either by a dynamo or by a battery, and the firing is controlled +from a switchboard placed at a safe distance from the point +at which the charge is to be exploded, the connexions being +made by long insulated wires. The actual ignition is +effected by a hot spark, as in automobile ignition, or by an +electric detonator or fuse. Explosives such as dynamite +cannot be fired by simple ignition, but require to be +detonated. This is effected by a detonator consisting of a +small cup-shaped tube, made of ebonite or other similar +material. The wires conveying the current project into this +tube, and are connected by a short piece of very fine wire +having a high resistance. Round this wire is packed a +small quantity of gun-cotton, and beyond, in a sort of continuation +of the tube, is placed an extremely explosive +substance called “fulminate of mercury,” the whole arrangement +being surrounded by the dynamite to be fired. When +all is ready the man at the switchboard manipulates a +switch, and the current passes to the detonator and forces +its way through the resistance of the thin connecting wire. +This wire becomes sufficiently hot to ignite the gun-cotton, +and so explode the fulminate of mercury. The explosion +is so violent that the dynamite charge is detonated, and +the required blasting carried out. Gunpowder and similar +explosives do not need to be detonated, and so a simple +fuse is used. Electric fuses are much the same as detonators, +except that the tube contains gunpowder instead of +fulminate of mercury, this powder being ignited through an +electrically heated wire in the same way. These electrical +methods do away with the uncertainty of the slow-burning +fuses formerly employed, which never could be relied upon +with confidence.</p> + +<p>Enormous quantities of explosives are now used in +blasting on a large scale, where many tons of hard rock +have to be removed. One of the most striking blasting<span class="pagenum" id="Page_257">257</span> +feats was the blowing up of Flood Island, better known as +Hell Gate. This was a rocky islet, about 9 acres in +extent, situated in the East River, New York. It was a +continual menace to shipping, and after many fine vessels +had been wrecked upon it the authorities decided that it +should be removed. The rock was bored and drilled in all +directions, the work taking more than a year to complete; +and over 126 tons of explosives were filled into the borings. +The exploding was carried out by electricity, and the +mighty force generated shattered nearly 300,000 cubic +yards of solid rock.</p> +<hr class="chap x-ebookmaker-drop"> + +<div class="chapter"> +<p><span class="pagenum" id="Page_258">258</span></p> + +<h2 class="nobreak" id="toclink_258"><a id="chapter_XXVIII"></a>CHAPTER XXVIII<br> + +<span class="subhead">ELECTRO-CULTURE</span></h2> +</div> + +<p class="in0"><span class="firstword">About</span> thirty years ago a Swedish scientist, Professor +Lemström, travelled extensively in the Polar regions, and +he was greatly struck by the development of the Polar +vegetation. In spite of the lack of good soil, heat, and +light, he observed that this vegetation came to maturity +quicker than that of regions having much more favourable +climates, and that the colours of the flowers were remarkably +fresh and clear, and their perfumes exceptionally +strong. This was a surprising state of things, and +Lemström naturally sought a clue to the mystery. He +knew that peculiar electrical conditions prevailed in these +high latitudes, as was shown by the wonderful displays of +the Aurora Borealis, and he came to the conclusion that +the development of the vegetation was due to small currents +of electricity continually passing backwards and forwards +between the atmosphere and the Earth. On his return to +civilization Lemström at once began a series of experiments +to determine the effect of electricity upon the growth of +plants, and he succeeded in proving beyond all doubt that +plants grown under electrical influence flourished more +abundantly than those grown in the ordinary way. +Lemström’s experiments have been continued by other +investigators, and striking and conclusive results have been +obtained.</p> + +<p>The air surrounding the Earth is always charged to<span class="pagenum" id="Page_259">259</span> +some extent with electricity, which in fine weather is +usually positive, but changes to negative on the approach +of wet weather. This electricity is always leaking away to +the earth more or less rapidly, and on its way it passes +through the tissues of the vegetation. An exceedingly +slow but constant discharge therefore is probably taking +place in the tissues of all plants. Experiments appear to +indicate that the upper part of a growing plant is negative, +and the lower part positive, and at any rate it is certain that +the leaves of a plant give off negative electricity. In dull +weather this discharge is at its minimum, but under the +influence of bright sunshine it goes on with full vigour. It +is not known exactly how this discharge affects the plant, +but apparently it assists its development in some way, and +there is no doubt that when the discharge is at its maximum +the flow of sap is most vigorous. Possibly the electricity +helps the plant to assimilate its food, by making this more +readily soluble.</p> + +<p>This being so, a plant requires a regular daily supply of +uninterrupted sunshine in order to arrive at its highest +possible state of maturity. In our notoriously variable +climate there are many days with only short intermittent +periods of bright sunshine, and many other days without +any sunshine at all. Now if, on these dull days, we can +perform at least a part of the work of the sunshine, and +strengthen to some extent the minute currents passing +through the tissues of a plant, the development of this +plant should be accelerated, and this is found to be the +case. Under electrical influence plants not only arrive at +maturity quicker, but also in most cases their yield is +larger and of finer quality.</p> + +<p>Lemström used a large influence machine as the source +of electricity in his experiments in electro-culture. Such +machines are very suitable for experimental work on a<span class="pagenum" id="Page_260">260</span> +small scale, and much valuable work has been done with +them by Professor Priestly and others; but they have the +great drawback of being uncertain in working. They are +quite satisfactory so long as the atmosphere remains dry, +but in damp weather they are often very erratic, and may +require hours of patient labour to coax them to start. For +this reason an induction coil is more suitable for continuous +work on an extensive scale.</p> + +<p>The most satisfactory apparatus for electro-culture is +that used in the Lodge-Newman method, designed by Sir +Oliver Lodge and his son, working in conjunction with +Mr. Newman. This consists of a large induction coil +supplied with current from a dynamo driven by a small +engine, or from the public mains if available. This coil +is fitted with a spark gap, and the high-tension current goes +through four or five vacuum valve globes, the invention +of Sir Oliver Lodge, which permit the current to pass +through them in one direction only. This is necessary +because, as we saw in <a href="#chapter_VIII">Chapter VIII</a>., two opposite currents +are induced in the secondary winding of the coil, one at the +make and the other at the break of the primary circuit. +Although the condenser fitted in the base of the coil +suppresses to a great extent the current induced on making +the circuit, still the current from the coil is not quite +uni-directional, but it is made so by the vacuum rectifying +valves. These are arranged to pass only the positive +current, and this current is led to overhead wires out in the +field to be electrified. Lemström used wires at a height of +18 inches from the ground, but these were very much in +the way, and in the Lodge-Newman system the main wires +are carried on large porcelain insulators fixed at the top of +poles at a height of about 15 feet. This arrangement +allows carting and all other agricultural operations to be +carried on as usual. The poles are set round the field,<span class="pagenum" id="Page_261">261</span> +about one to the acre, and from these main wires finer +ones are carried across the field. These wires are placed +about 30 feet apart, so that the whole field is covered by a +network of wires. The electricity supplied to the wires is +at a pressure of about 100,000 volts, and this is constantly +being discharged into the air above the plants. It then +passes through the plants, and so reaches the earth. This +system may be applied also to plants growing in greenhouses, +but owing to the confined space, and to the amount +of metal about, in the shape of hot-water pipes and wires +for supporting plants such as vines and cucumbers, it is +difficult to make satisfactory arrangements to produce the +discharge.</p> + +<p>The results obtained with this apparatus at Evesham, +in Gloucestershire, by Mr. Newman, have been most +striking. With wheat, increases of from 20 per cent. to +nearly 40 per cent. have been obtained, and the electrified +wheat is of better quality than unelectrified wheat grown at +the same place, and, apart from electrification, under exactly +the same conditions. In some instances the electrified +wheat was as much as 8 inches higher than the +unelectrified wheat. Mr. Newman believes that by +electrification land yielding normally from 30 to 40 bushels +of wheat per acre can be made to yield 50 or even 60 +bushels per acre. With cucumbers under glass increases +of 17 per cent. have been obtained, and in the case of +strawberries, increases of 36 per cent. with old plants, and +80 per cent. with one-year-old plants. In almost every +case electrification has produced a marked increase in the +crop, and in the few cases where there has been a decrease +the crops were ready earlier than the normal. For instance, +in one experiment with broad beans a decrease of 15 per +cent. resulted, but the beans were ready for picking five +days earlier. In another case a decrease of 11½ per cent.<span class="pagenum" id="Page_262">262</span> +occurred with strawberries, but the fruit was ready for +picking some days before the unelectrified fruit, and also +was much sweeter. In some of the experiments resulting +in a decrease in the yield it is probable that the electrification +was overdone, so that the plants were over-stimulated. +It seems likely that the best results will be obtained only +by adjusting the intensity and the duration of the electrification +in accordance with the atmospheric conditions, and +also with the nature of the crop, for there is no doubt that +plants vary considerably in their electrical requirements. +A great deal more experiment is required however to +enable this to be done with anything like certainty.</p> + +<p>Unlike the farmer, the market gardener has to produce +one crop after another throughout the year. To make up +for the absence of sufficient sunshine he has to resort to +“forcing” in many cases, but unfortunately this process, +besides being costly, generally results in the production of +a crop of inferior quality. Evidently the work of the +market gardener would be greatly facilitated by some +artificial substitute for sunshine, to keep his plants growing +properly in dull weather. In 1880, Sir William Siemens, +knowing that the composition of the light of the electric +arc was closely similar to that of sunlight, commenced +experiments with an arc lamp in a large greenhouse. His +idea was to add to the effects of the solar light by using +the arc lamp throughout the night. His first efforts were +unsuccessful, and he discovered that this was due to the +use of the naked light, which apparently contained rays +too powerful for the plants. He then passed the light +through glass, which filtered out the more powerful rays, +and this arrangement was most successful, the plants +responding readily to the artificial light. More scientifically +planned experiments were carried out at the London +Royal Botanic Gardens in 1907, by Mr. B. H. Thwaite,<span class="pagenum" id="Page_263">263</span> +and these showed that by using the arc lamp for about five +hours every night, a great difference between the treated +plants and other similar plants grown normally could be +produced in less than a month. Other experiments made +in the United States with the arc lamp, and also with +ordinary electric incandescent lamps, gave similar results, +and it was noticed that the improvement was specially +marked with cress, lettuce, spinach, and other plants of this +nature.</p> + +<p>In 1910, Miss E. C. Dudgeon, of Dumfries, commenced +a series of experiments with the Cooper-Hewitt mercury +vapour lamp. Two greenhouses were employed, one of +which was fitted with this lamp. Seeds of various plants +were sown in small pots, one pot of each kind being placed +in each house. The temperature and other conditions +were kept as nearly alike as possible in both houses, and +in the experimental house the lamp was kept going for +about five hours every night. In every case the seeds in +the experimental house germinated several days before +those in the other house, and the resulting plants were +healthy and robust. Later experiments carried out by +Miss Dudgeon with plants were equally successful.</p> + +<p>From these experiments it appears that the electric arc, +and still more the mercury vapour lamp, are likely to prove +of great value to the market gardener. As compared with +the arc lamp, the mercury vapour lamp has the great +advantage of requiring scarcely any attention, and also it +uses less current. Unlike the products of ordinary forcing +by heat, the plants grown under the influence of the +mercury vapour light are quite sturdy, so that they can be +planted out with scarcely any “hardening off.” The crop +yields too are larger, and of better quality. The wonderful +effects produced by the Cooper-Hewitt lamp are +certainly not due to heat, for this lamp emits few heat rays.<span class="pagenum" id="Page_264">264</span> +The results may be due partly to longer hours worked by +the plants, but this does not explain the greater accumulation +of chlorophyll and stronger development of fibre.</p> + +<p>Most of us are familiar with the yarn about the poultry +keeper who fitted all his nests with trap-doors, so that when +a hen laid an egg, the trap-door opened under the weight +and allowed the egg to fall through into a box lined with +hay. The hen then looked round, and finding no egg, at +once set to work to lay another. This in turn dropped, +another egg was laid, and so on. It is slightly doubtful +whether the modern hen could be swindled in this bare-faced +manner, but it is certain that she can be deluded into +working overtime. The scheme is absurdly simple. +Electric lamps are fitted in the fowl-house, and at sunset +the light is switched on. The unsuspecting hens, who are +just thinking about retiring for the night, come to the conclusion +that the day is not yet over, and so they continue +to lay. This is not a yarn, but solid fact, and the increase +in the egg yield obtained in this way by different poultry +keepers ranges from 10 per cent. upwards. Indeed, one +poultry expert claims to have obtained an increase of about +40 per cent.</p> + +<p>The ease with which a uniform temperature can be +maintained by electric heating has been utilized in incubator +hatching of chickens. By means of a specially designed +electric radiator the incubator is kept at the right temperature +throughout the hatching period. When the chickens +emerge from the eggs they are transferred to another +contrivance called a “brooder,” which also is electrically +heated, the heat being decreased gradually day by day until +the chicks are sturdy enough to do without it. Even at +this stage however the chickens do not always escape +from the clutches of electricity. Some rearers have +adopted the electric light swindle for the youngsters,<span class="pagenum" id="Page_265">265</span> +switching on the light after the chickens have had a fair +amount of slumber, so that they start feeding again. In +this way the chickens are persuaded to consume more food +in the twenty-four hours, and the resulting gain in weight +is said to be considerable. More interesting than this +scheme is the method of rearing chickens under the +influence of an electric discharge from wires supplied with +high-tension current. Comparative tests show that electrified +chickens have a smaller mortality and a much greater +rate of growth than chickens brought up in the ordinary +way. It even is said that the electrified chickens have +more kindly dispositions than their unelectrified relatives!</p> + +<p>Possibly the high-tension discharge may turn out to be +as beneficial to animals as it has been proved to be for +plants, but so far there is little reliable evidence on this +point, owing to lack of experimenters. A test carried out +in the United States with a flock of sheep is worth +mention. The flock was divided into two parts, one-half +being placed in a field under ordinary conditions, and the +other in a field having a system of overhead discharge +wires, similar to those used in the Lodge-Newman system. +The final result was that the electrified sheep produced +more than twice as many lambs as the unelectrified sheep, +and also a much greater weight of wool. If further experiments +confirm this result, the British farmer will do well to +consider the advisability of electrifying his live-stock.</p> +<hr class="chap x-ebookmaker-drop"> + +<div class="chapter"> +<p><span class="pagenum" id="Page_266">266</span></p> + +<h2 class="nobreak" id="toclink_266"><a id="chapter_XXIX"></a>CHAPTER XXIX<br> + +<span class="subhead">SOME RECENT APPLICATIONS OF ELECTRICITY—AN ELECTRIC PIPE LOCATOR</span></h2> +</div> + +<p class="in0"><span class="firstword">One</span> of the great advantages of living in a town is the +abundant supply of gas and water. These necessary +substances are conveyed to us along underground pipes, +and a large town has miles upon miles of such pipes, +extending in all directions and forming a most complex +network. Gas and water companies keep a record of these +pipes, with the object of finding any pipe quickly when the +necessity arises; but in spite of such records pipes are +often lost, especially where the whole face of the neighbourhood +has changed since the pipes were laid. The finding +of a lost pipe by digging is a very troublesome process, and +even when the pipe is known to be close at hand, it is quite +surprising how many attempts are frequently necessary +before it can be located, and its course traced. As may be +imagined, this is an expensive business, and often it has been +found cheaper to lay a new length of pipe than to find the +old one. There is now an electrical method by which pipe +locating is made comparatively simple, and unless it is very +exceptionally deep down, a pipe never need be abandoned +on account of difficulty in tracing it.</p> + +<p>The mechanism of an electric pipe locator is not at all +complicated, consisting only of an induction coil with +battery, and a telephone receiver connected to a coil of a +large number of turns of thin copper wire. If a certain<span class="pagenum" id="Page_267">267</span> +section of a pipe is lost, and has to be located, operations +are commenced from some fitting known to be connected +with it, and from some other fitting which may or may not +be connected with the pipe, but which is believed to be so +connected. The induction coil is set working, and its +secondary terminals are connected one to each of these +fittings. If the second fitting is connected with the pipe, +then the whole length of the pipe between these two points +is traversed by the high-frequency current. The searcher, +wearing the head telephone receiver, with the coil hanging +down from it so as to be close to the ground, walks to and +fro over the ground beneath which the pipe must lie. +When he approaches the pipe the current passing through +the latter induces a similar current in the suspended coil, +and this produces a sort of buzzing or humming sound in +the telephone. The nearer he approaches to the pipe the +louder is the humming, and it reaches its maximum when +he is standing directly over the pipe. In this way the +whole course of the pipe can be traced without any digging, +even when the pipe is 15 or 20 feet down. The absence +of any sounds in the receiver indicates that the second +fitting is not on the required pipe line, and other fittings +have to be tried until one on this line is found.</p> + +<h3><span class="smcap">An Electric Iceberg Detector</span></h3> + +<p>Amongst the many dangers to which ships crossing the +Atlantic are exposed is that of collision with icebergs. +These are large masses of ice which have become detached +from the mighty ice-fields of the north, and which travel +slowly and majestically southwards, growing smaller and +smaller as they pass into warmer seas. Icebergs give no +warning of their coming, and in foggy weather, which is very +prevalent in the regions where they are encountered, they<span class="pagenum" id="Page_268">268</span> +are extremely difficult to see until they are at dangerously +close quarters.</p> + +<p>Attempts have been made to detect the proximity of +icebergs by noting the variations in the temperature of the +water. We naturally should expect the temperature of the +water to become lower as we approach a large berg, and +this is usually the case. On the other hand, it has been +found that in many instances the temperature near an +iceberg is quite as high as, and sometimes higher than the +average temperature of the ocean. For this reason the +temperature test, taken by itself, is not at all reliable. A +much more certain test is that of the salinity or saltness of +the water. Icebergs are formed from fresh water, and as +they gradually melt during their southward journey the +fresh water mixes with the sea water. Consequently the +water around an iceberg is less salt than the water of the +open ocean. The saltness of water may be determined by +taking its specific gravity, or by various chemical processes; +but while these tests are quite satisfactory when performed +under laboratory conditions, they cannot be carried out at sea +with any approach to accuracy. There is however an electrical +test which can be applied accurately and continuously. +The electrical conducting power of water varies greatly with +the proportion of salt present. If the conductivity of normal +Atlantic water be taken as 1000, then the conductivity of +Thames water is 8, and that of distilled water about 1/22. +The difference in conductivity between normal ocean water +and water in the vicinity of an iceberg is therefore very great.</p> + +<figure id="fig_43" class="figcenter" style="max-width: 37em;"> + <img src="images/i_305.jpg" width="2901" height="1623" alt=" "> + <figcaption class="caption"> + +<p class="floatl"><i>By permission of</i>]</p> +<p class="floatr">[<i>Dr. Myer Coplans.</i></p> + +<p class="floatc"><span class="smcap">Fig. 43.</span>—Diagram of Heat-compensated Salinometer.</p> +</figcaption></figure> + +<p>The apparatus for detecting differences in salinity by +measuring the conductivity of the water is called a “salinometer,” +and its most perfect form, known as the heat-compensated +conductivity salinometer, is due to Dr. Myer +Coplans. <a href="#fig_43">Fig. 43</a> shows a diagram of this interesting +piece of apparatus, which is most ingeniously devised. Two<span class="pagenum" id="Page_270">270</span> +insulated electrodes of copper, with platinum points, are +suspended in a <span class="sans bold">U</span>-tube through which the sea water passes +continuously, as indicated in the diagram. A steady current +is passed through the column of water between the two +platinum points, and the conductivity of this column is +measured continuously by very accurate instruments. +Variations in the conductivity, indicating corresponding +variations in the saltness of the water, are thus shown +immediately; but before these indications can be relied +upon the instrument must be compensated for temperature, +because the conductivity of the water increases with a rise, +and decreases with a fall in temperature. This compensation +is effected by the compound bars of brass and steel +shown in the vessel at the right of the figure. These bars +are connected with the wheel and disc from which the +electrodes are suspended. When the temperature of the +water rises, the bars contract, and exert a pull upon the +wheel and disc, so that the electrodes are raised slightly in +the <span class="sans bold">U</span>-tube. This increases the length of the column of +water between the platinum points, and so increases the +resistance, or, what amounts to the same thing, lowers the +conductivity, in exact proportion to the rise in temperature. +Similarly, a fall in temperature lowers the electrodes, and +decreases the resistance by shortening the column of water. +In this way the conductivity of the water remains constant +so far as temperature is concerned, and it varies only with +the saltness of the water. Under ordinary conditions a +considerable decrease in the salinity of the water indicates +the existence of ice in the near neighbourhood, but the +geographical position of the ship has to be taken into +account. Rivers such as the St. Lawrence pour vast +quantities of fresh water into the ocean, and the resulting +decrease in the saltness of the water within a considerable +radius of the mouth of the river must be allowed for.</p> + +<p><span class="pagenum" id="Page_271">271</span></p> + +<h3><span class="smcap">A “Flying Train”</span></h3> + +<p>Considerable interest was aroused last year by a model +of a railway working upon a very remarkable system. This +was the invention of Mr. Emile Bachelet, and the model +was brought to London from the United States. The main +principle upon which the system is based is interesting. +About 1884, Professor Elihu Thompson, a famous American +scientist, made the discovery that a plate of copper could be +attracted or repelled by an electro-magnet. The effects +took place at the moment when the magnetism was varied +by suddenly switching the current on or off; the copper +being repelled when the current was switched on, and +attracted when it was switched off. Copper is a non-magnetic +substance, and the attraction and repulsion are +not ordinary magnetic effects, but are due to currents +induced in the copper plate at the instant of producing or +destroying the magnetism. The plate is attracted or +repelled according to whether these induced currents flow in +the same direction as, or in the opposite direction to, the +current in the magnet coil. Brass and aluminium plates +act in the same way as the copper plate, and the effects are +produced equally well by exciting the magnet with alternating +current, which, by changing its direction, changes the +magnetism also. Of the two effects, the repulsion is +much the stronger, especially if the variations in the +magnetism take place very rapidly; and if a powerful +and rapidly alternating current is used, the plate is repelled +so strongly that it remains supported in mid-air above the +magnet.</p> + +<p>This repulsive effect is utilized in the Bachelet system +(<a href="#plate_XV">Plate XV</a>.). There are no rails in the ordinary sense, and +the track is made up of a continuous series of electro-magnets.<span class="pagenum" id="Page_272">272</span> +The car, which is shaped something like a cigar, +has a floor of aluminium, and contains an iron cylinder, +and it runs above the line of magnets. Along each side +of the track is a channel guide rail, and underneath the car +at each end are fixed two brushes with guide pieces, which +run in the guide rails. Above the car is a third guide rail, +and two brushes with guide pieces fixed on the top of the +car, one at each end, run in this overhead rail. These +guide rails keep the car in position, and also act as conductors +for the current. The repulsive action of the +electro-magnets upon the aluminium floor raises the car +clear of the track, and keeps it suspended; and while +remaining in this mid-air position it is driven, or rather +pulled forward, by powerful solenoids, which are supplied +with continuous current. We have referred previously to +the way in which a solenoid draws into it a core of iron. +When the car enters a solenoid, the latter exerts a pulling +influence upon the iron cylinder inside the car, and so the +car is given a forward movement. This is sufficient to +carry it along to the next solenoid, which gives it another +pull, and so the car is drawn forward from one solenoid +to another to the end of the line. The model referred +to has only a short track of about 30 feet, with one +solenoid at each end; but its working shows that the +pulling power of the solenoids is sufficient to propel the +car.</p> + +<figure id="plate_XV" class="figcenter" style="max-width: 40em;"> + <p class="caption">PLATE XV.</p> + <img src="images/i_309.jpg" width="3141" height="2019" alt=" "> + <figcaption class="caption"> + +<p class="floatl"><i>Photo by</i></p> +<p class="floatr"><i>Record Press.</i></p> + +<p class="floatc">BACHELET “FLYING TRAIN” AND ITS INVENTOR.</p> +</figcaption></figure> + +<p>To avoid the necessity of keeping the whole of the +electro-magnets energized all the time, these are arranged +in sections, which are energized separately. By means of +the lower set of brushes working in the track guides, each +of these sections has alternating current supplied to it as +the car approaches, and switched off from it when the car +has passed. The brushes working in the overhead guide +supply continuous current to each solenoid as the car enters +it, and switch off the current when the car has passed +through. The speed at which the model car travels is +quite extraordinary, and the inventor believes that in actual +practice speeds of more than 300 miles an hour are attainable +on his system.</p> +<hr class="chap x-ebookmaker-drop"> + +<div class="chapter"> +<p><span class="pagenum" id="Page_274">274</span></p> + +<h2 class="nobreak" id="toclink_274"><a id="chapter_XXX"></a>CHAPTER XXX<br> + +<span class="subhead">ELECTRICITY IN WAR</span></h2> +</div> + +<p class="in0"><span class="firstword">One</span> of the most striking features of modern naval warfare +is the absolute revolution in methods of communication +brought about by wireless telegraphy. To-day every +warship has its wireless installation. Our cruiser squadrons +and destroyer flotillas, ceaselessly patrolling the waters of +the North Sea, are always in touch with the Admiral of +the Fleet, and with the Admiralty at Whitehall. In the +Atlantic, and in the Pacific too, our cruisers, whether +engaged in hunting down the marauding cruisers of the +enemy or in searching for merchant ships laden with contraband, +have their comings and goings directed by wireless. +Even before the actual declaration of war between +Great Britain and Germany wireless telegraphy began its +work. At the conclusion of the great naval review of +July 1914, the Fleet left Portland to disperse as customary +for manœuvre leave, but a wireless message was dispatched +ordering the Fleet not to disperse. As no state of war +then existed, this was a precautionary measure, but subsequent +events quickly proved how urgently necessary it +had been to keep the Fleet in battle array. Immediately +war was declared Great Britain was able to put into the +North Sea a fleet which hopelessly outnumbered and outclassed +the German battle fleet.</p> + +<p>At the outset Germany had a number of cruisers in the +Atlantic and the Pacific Oceans. Owing to the vigilance<span class="pagenum" id="Page_275">275</span> +of our warships these vessels were unable to join the +German Home Fleet, and they immediately adopted the +rôle of commerce destroyers. In this work they made +extensive use of wireless telegraphy to ascertain the whereabouts +of British merchant ships, and for a short time they +played quite a merry game. Prominent among these +raiders was the <i>Emden</i>. It was really astonishing how +this cruiser obtained information regarding the sailings of +British ships. It is said that on one occasion she called up +by wireless a merchant ship, and inquired if the latter had +seen anything of a German cruiser. The unsuspecting +merchantman replied that there was no such thing as a +German warship in the vicinity. “Oh yes, there is,” +returned the <i>Emden</i>; “I’m it!” and shortly afterwards she +appeared on the horizon, to the great discomfiture of the +British skipper. An interesting account of the escape of a +British liner from another notorious raider, the <i>Karlsruhe</i>, +has been given in the <cite>Nautical Magazine</cite>. The writer +says:</p> + +<p>“I have just returned home after a voyage to South +America in one of the Pacific Steam Navigation Company’s +cargo boats. When we left Montevideo we heard that +France and Germany were at war, and that there was +every possibility of Great Britain sending an ultimatum to +Germany. We saw several steamers after leaving the +port, but could get no information, as few of them were +fitted with wireless and passed at some distance off. When +about 200 miles east of Rio, our wireless operator overheard +some conversation between the German cruiser +<i>Karlsruhe</i> and a German merchant ship at anchor in Rio. +It was clearly evident that the German merchant ship had +no special code, as the conversation was carried on in plain +German language, and our operator, who, by the way, was +master of several languages, was able to interpret these<span class="pagenum" id="Page_276">276</span> +messages without the slightest difficulty. It was then that +we learned that Great Britain was at war. The German +cruiser was inquiring from the German merchant ship what +British vessels were leaving Rio, and asking for any information +which might be of use. We also picked up some +news of German victories in Belgium, which were given +out by the German merchant ship. It was clearly evident +that the <i>Karlsruhe</i> had information about our ship, and +expected us to be in the position she anticipated, for she +sent out a signal to us in English, asking us for our latitude +and longitude. This our operator, under the instructions +of the captain, declined to give. The German operator +evidently got furious, as he called us an English ‘swine-hound,’ +and said, ‘This is a German warship, <i>Karlsruhe</i>; +we will you find.’ Undoubtedly he thought he was going +to strike terror to our hearts, but he made a mistake.</p> + +<p>“That night we steamed along without lights, and we +knew from the sound of the wireless signals that were +being flashed out from the German ship that we were +getting nearer and nearer to her. Fortunately for us, +about midnight a thick misty rain set in and we passed the +German steamer, and so escaped. Our operator said that +we could not have been more than 8 or 10 miles away +when we passed abeam. Undoubtedly our wireless on +this occasion saved us from the danger from which we +escaped.”</p> + +<p>Apparently little is known of the end of the <i>Karlsruhe</i>, +but the <i>Emden</i> met with the fate she richly deserved; and +fittingly enough, wireless telegraphy, which had enabled +her to carry out her marauding exploits, was the means of +bringing her to her doom. On 9th November 1914 the +<i>Emden</i> anchored off the Cocos-Keeling Islands, a group +of coral islets in the Indian Ocean, and landed a party of +three officers and forty men to cut the cable and destroy<span class="pagenum" id="Page_277">277</span> +the wireless station. Before the Germans could get to the +station, a wireless message was sent out stating the +presence of the enemy warship, and this call was received +by the Australian cruisers <i>Melbourne</i> and <i>Sydney</i>. These +vessels, which were then only some 50 miles away, were +engaged, along with a Japanese cruiser, in escorting transports. +The <i>Sydney</i> at once went off at full speed, caught +the <i>Emden</i>, and sent her to the bottom after a short but +sharp engagement. As the <i>Emden</i> fled at sight of the +Australian warship, the landing party had not time to get +aboard, and consequently were left behind. They seized +an old schooner, provisioned her, and set sail, but what +became of them is not known.</p> + +<p>In land warfare field telegraphs play a very important +part; indeed it is certain that without them the vast military +operations of the present war could not be carried on. +The General Headquarters of our army in France is in +telegraphic communication not only with neighbouring +French towns, but also with Paris and London. From +Headquarters also run wires to every point of the firing-line, +so that the Headquarters Staff, and through them the +War Office in London, know exactly what is taking place +along the whole front. The following extract from a letter +from an officer, published by <cite>The Times</cite>, gives a remarkably +good idea of the work of the signal companies of the +Royal Engineers.</p> + +<p>“As the tide of battle turns this way and the other, and +headquarters are constantly moving, some means have to be +provided to keep in constant touch with General Headquarters +during the movement. This emergency is met +by cable detachments. Each detachment consists of two +cable waggons, which usually work in conjunction with one +another, one section laying the line whilst the other remains +behind to reel up when the line is finished with. A<span class="pagenum" id="Page_278">278</span> +division is ordered to move quickly to a more tactical +position. The end of the cable is connected with the +permanent line, which communicates to Army Headquarters, +and the cable detachment moves off at the trot; across +country, along roads, through villages, and past columns of +troops, the white and blue badge of the signal service +clears the way. Behind the waggon rides a horseman, who +deftly lays the cable in the ditches and hedges out of danger +from heavy transport and the feet of tramping infantry, +with the aid of a crookstick. Other horsemen are in the +rear tying back and making the line safe. On the box of +the waggon sits a telegraphist, who is constantly in touch +with headquarters as the cable runs swiftly out. An +orderly dashes up with an important message; the waggon +is stopped, the message dispatched, and on they go again.”</p> + +<p>Wireless telegraphy too has its part to play in land +war, and for field purposes it has certain advantages over +telegraphy with wires. Ordinary telegraphic communication +is liable to be interrupted by the cutting of the wire by +the enemy, or, in spite of every care in laying, by the +breaking of the wire by passing cavalry or artillery. No +such trouble can occur with wireless telegraphy, and if it +becomes necessary to move a wireless station with great +rapidity, as for instance on an unexpected advance of the +enemy, it is an advantage to have no wire to bother about. +The Marconi portable wireless sets for military purposes +are marvels of compactness and lightness, combined with +simplicity. They are of two kinds, pack-saddle sets and +cart sets. The former weigh about 360 lb., this being +divided amongst four horses. They can be set up in ten +minutes by five or six men, and require only two men to +work them. Their guaranteed range is 40 miles, but +they are capable of transmitting twice this distance or even +more under favourable conditions. The cart sets can be<span class="pagenum" id="Page_279">279</span> +set up in twenty minutes by seven or eight men, and they +have a guaranteed range of from 150 to 200 miles.</p> + +<p>It is obviously very important that wireless military +messages should not be intercepted and read by the enemy, +and the method of avoiding danger of this kind adopted +with the Marconi field stations is ingenious and effective. +The transmitter and the receiver are arranged to work on +three different fixed wave-lengths, the change from one to +another being effected quickly by the movement of a three-position +switch. By this means the transmitting operator +sends three or four words on one wave-length, then changes +to another, transmits a few words on this, changes the wave-length +again, and so on. Each change is accompanied by +the sending of a code letter which informs the receiving +operator to which wave-length the transmitter is passing. +The receiving operator adjusts his switch accordingly, and +so he hears the whole message without interruption, the +change from one wave-length to another taking only a small +fraction of a second. An enemy operator might manage +to adjust his wave-length so as to hear two or three words, +but the sudden change of wave-length would throw him out +of tune, and by the time he had found the new wave-length +this would have changed again. Thus he would hear at +most only a few disconnected words at intervals, and he +would not be able to make head or tail of the message. +To provide against the possibility of the three wave-lengths +being measured and prepared for, these fixed lengths themselves +can be changed, if necessary, many times a day, so +that the enemy operators would never know beforehand +which three were to be used.</p> + +<p>Wireless telegraphy was systematically employed in +land warfare for the first time in the Balkan War, during +which it proved most useful both to the Allies and to the +Turks. One of the most interesting features of the war<span class="pagenum" id="Page_280">280</span> +was the way in which wireless communication was kept up +between the beleaguered city of Adrianople and the +Turkish capital. Some time before war broke out the +Turkish Government sent a portable Marconi wireless set +to Adrianople, and this was set up at a little distance from +the city. When war was declared the apparatus was +brought inside the city walls and erected upon a small hill. +Then came the siege. For 153 days Shukri Pasha kept +the Turkish flag flying, but the stubborn defence was +broken down in the end through hunger and disease. All +through these weary days the little wireless set did its duty +unfalteringly, and by its aid regular communication was +maintained with the Government station at Ok Meidan, +just outside Constantinople, 130 miles away. Altogether +about half a million words were transmitted from Adrianople +to the Turkish capital.</p> + +<figure id="plate_XVIa" class="figcenter" style="max-width: 26em;"> + <p class="caption">PLATE XVI.</p> + <img src="images/i_319.jpg" width="2068" height="1406" alt=" "> + <figcaption class="caption"> + +<p>(<i>a</i>) CAVALRY PORTABLE WIRELESS CART SET.</p> +</figcaption></figure> + +<figure id="plate_XVIb" class="figcenter" style="max-width: 26em;"> + <img src="images/i_319b.jpg" width="2053" height="1471" alt=" "> + <figcaption class="caption"> + +<p class="floatl"><i>By permission of</i></p> +<p class="floatr"><i>Marconi Co. Ltd.</i></p> + +<p class="floatc">(<i>b</i>) AEROPLANE FITTED WITH WIRELESS TELEGRAPHY.</p> +</figcaption></figure> + +<p>The rapid development of aviation during the past few +years has drawn attention to the necessity for some means +of communication between the land and airships and +aeroplanes in flight. At first sight it might appear that +wireless telegraphy could be used for this purpose without +any trouble, but experience has shown that there are +certain difficulties in the way, especially with regard to +aeroplanes. The chief difficulty with aeroplanes lies in the +aerial. This must take the form either of a long trailing +wire or of fixed wires running between the planes and the +tail. A trailing wire is open to the objection that it is +liable to get mixed up with the propeller, besides which it +appears likely to hamper to some slight extent the movements +of a small and light machine. A fixed aerial between +planes and tail avoids these difficulties, but on the other +hand its wave-length is bound to be inconveniently small. +The heavy and powerful British military aeroplanes +apparently use a trailing wire of moderate length, carried +in a special manner so as to clear the propeller, but few +details are available at present. A further trouble with +aeroplanes lies in the tremendous noise made by the +engine, which frequently makes it quite impossible to hear +incoming signals; and the only way of getting over this +difficulty appears to be for the operator to wear some sort +of sound-proof head-gear. Signals have been transmitted +from an aeroplane in flight up to distances of 40 or 50 miles +quite successfully, but the reception of signals by aeroplanes +is not so satisfactory, except for comparatively short distances. +Although few particulars have been published +regarding the work of the British aeroplanes in France, it +seems evident that wireless telegraphy is in regular use. +In addition to their value as scouts, our aeroplanes appear +to be extremely useful for the direction of heavy artillery +fire, using wireless to tell the gunners where each shell falls, +until the exact range is obtained. In the case of airships +the problem of wireless communication is much simpler. +A trailing wire presents no difficulties, and on account of +their great size much more powerful sets of apparatus can +be carried. The huge German Zeppelin airships have a +long freely-floating aerial consisting of a wire which can be +wound in or let out as required, its full length being about +750 feet. The total weight of the apparatus is nearly +300 lb., and the transmitting range is said to be from +about 120 to 200 miles.</p> + +<p>Electricity is used in the navy for a great variety of +purposes besides telegraphy. Our battleships are lighted +by electricity, which is generated at a standard pressure of +220 volts. This current is transformed down for the +searchlights, and also for the intricate systems of telephone, +alarm, and firing circuits. The magazines containing the +deadly cordite are maintained at a constant temperature of +70° F. by special refrigerating machinery driven by electricity,<span class="pagenum" id="Page_282">282</span> +and the numerous fans for ventilating the different parts of +the ship are also electrically driven. Electric power is used +for capstans, coaling winches, sounding machines, lifts, +pumps, whether for drainage, fire extinction, or raising +fresh water from the tanks, and for the mechanism for +operating boats and torpedo nets. The mechanism for +manipulating the great guns and their ammunition is +hydraulic. Electricity was tried for this purpose on the +battle cruiser <i>Invincible</i>, but was abandoned in favour of +hydraulic power. But though electricity is apparently out of +favour in this department, it takes an extremely important +share in the work of controlling and firing the guns; its duties +being such as could not be carried out by hydraulic power.</p> + +<p>The guns are controlled and fired from what is known +as the fire-control room, which is situated in the interior of +the ship, quite away from the guns themselves. The +range-finder, from his perch up in the gigantic mast, +watches an enemy warship as she looms on the horizon, +and when she comes within range he estimates her distance +by means of instruments of wonderful precision. He then +telephones to the fire-control room, giving this distance, +and also the enemy’s speed and course. The officer in +charge of the fire-control room calculates the elevation of +the gun required for this distance, and decides upon the +instant at which the gun must be fired. A telephoned +order goes to the gun-turret, and the guns are brought to +bear upon the enemy, laid at the required elevation, and +sighted. At the correct instant the fire-control officer +switches on an electric current to the gun, which fires a +small quantity of highly explosive material, and this in +turn fires the main charge of cordite. The effect of the +shell is watched intently from the fire-control top, up above +the range-finder, and if, as is very likely, this first shell +falls short of, or overshoots the mark, an estimate of the<span class="pagenum" id="Page_283">283</span> +amount of error is communicated to the fire-control room. +Due corrections are then made, the gun is laid at a slightly +different elevation, and this time the shell finds its mark +with unerring accuracy.</p> + +<p>The range of movement, horizontal and vertical, of +modern naval guns is so great that it is possible for two +guns to be in such relative positions that the firing of one +would damage the other. To guard against a disaster of +this kind fixed stops are used, supplemented by ingenious +automatic alarms. The alarm begins to sound as soon as +any gun passes into a position in which it could damage +another gun, and it goes on sounding until the latter gun +is moved out of the danger line.</p> + +<p>Since the outbreak of war the subject of submarine +mines has been brought to our notice in very forcible +fashion. Contrary to the general impression, the explosive +submarine mine is not a recent introduction. It is difficult +to say exactly when mines were first brought into use, but +at any rate we know that they were employed by Russia +during the Crimean War, apparently with little success. +The first really successful use of mines occurred in the +American Civil War, when the Confederates sank a number +of vessels by means of them. This practical demonstration +of their possibilities did not pass unnoticed by European +nations, and in the Franco-German War we find that mines +were used for harbour defence by both belligerents. It is +doubtful whether either nation derived much benefit from +its mines, and indeed as the war progressed Germany +found that the principal result of her mining operations was +to render her harbours difficult and dangerous to her own +shipping. Much greater success attended the use of mines +in the Russo-Japanese War, but all previous records shrink +into insignificance when compared with the destruction +wrought by mines in the present great conflict.</p> + +<p><span class="pagenum" id="Page_284">284</span></p> + +<p>Submarine mines may be divided into two classes; +those for harbour defence, and those for use in the open +sea. Harbour defence mines are almost invariably electrically +controlled; that is, they are connected with the shore +by means of a cable, and fired by an electric impulse sent +along that cable. In one system of control the moment of +firing is determined entirely by observers on shore, who, +aided by special optical instruments, are able to tell exactly +when a vessel is above any particular mine. The actual +firing is carried out by depressing a key which completes +an electric circuit, thus sending a current along the cable +to actuate the exploding mechanism inside the mine. A +hostile ship therefore would be blown up on arriving at the +critical position, while a friendly vessel would be allowed +to pass on in safety. In this system of control there is no +contact between the vessel and the mine, the latter being +well submerged or resting on the sea floor, so that the +harbour is not obstructed in any way. This is a great +advantage, but against it must be set possible failure of the +defence at a critical moment owing to thick weather, which +of course interferes seriously with the careful observation +of the mine field necessary for accurate timing of the explosions. +This difficulty may be surmounted by a contact +system of firing. In this case the mines are placed so near +the surface as to make contact with vessels passing over +them. The observers on shore are informed of the contact +by means of an electric impulse automatically transmitted +along the cable, so that they are independent of continuous +visual observation of the mined area. As in the previous +system, the observers give the actual firing impulse. The +drawback to this method is the necessity for special pilotage +arrangements for friendly ships in order to avoid unnecessary +striking of the mines, which are liable to have their +mechanism deranged by constant blows. If the harbour or<span class="pagenum" id="Page_285">285</span> +channel can be closed entirely to friendly shipping, the +observers may be dispensed with, their place being taken +by automatic electric apparatus which fires at once any mine +struck by a vessel.</p> + +<p>Shore-controlled mines are excellent for harbour +defence, and a carefully distributed mine-field, backed by +heavy fort guns, presents to hostile vessels a barrier which +may be regarded as almost impenetrable. A strong fleet +might conceivably force its way through, but in so doing it +would sustain tremendous losses; and as these losses would +be quite out of proportion to any probable gains, such an +attempt is not likely to be made except as a last resort.</p> + +<p>For use in the open sea a different type of mine is +required. This must be quite self-contained and automatic +in action, exploding when struck by a passing vessel. The +exploding mechanism may take different forms. The blow +given by a ship may be made to withdraw a pin, thus +releasing a sort of plunger, which, actuated by a powerful +spring, detonates the charge. A similar result is obtained +by the use of a suspended weight, in place of plunger and +spring. Still another form of mine is fired electrically by +means of a battery, the circuit of which is closed automatically +by the percussion. Deep-sea mines may be anchored +or floating free. Free mines are particularly dangerous on +account of the impossibility of knowing where they may be +at any given moment. They are liable to drift for considerable +distances, and to pass into neutral seas; and to +safeguard neutral shipping international rules require them +to have some sort of clockwork mechanism which renders +them harmless after a period of one hour. It is quite +certain that some, at least, of the German free mines have +no such mechanism, so that neutral shipping is greatly +endangered.</p> + +<p>Submarine mines are known as <em>ground</em> mines, or<span class="pagenum" id="Page_286">286</span> +<em>buoyant</em> mines, according to whether they rest on the sea +bottom or float below the surface. Ground mines are +generally made in the form of a cylinder, buoyant mines +being usually spherical. The cases are made of steel, and +buoyancy is given when required by enclosing air spaces. +Open-sea mines are laid by special vessels, mostly old +cruisers. The stern of these ships is partly cut away, and +the mines are run along rails to the stern, and so overboard. +The explosive employed is generally gun-cotton, fired by +a detonator, charges up to 500 lb. or more being used, +according to the depth of submersion and the horizontal +distance at which the mine is desired to be effective. +Ground mines can be used only in shallow water, and even +then they require a heavier charge than mines floating near +the surface. Mines must not be laid too close together, as +the explosion of one might damage others. The distance +apart at which they are placed depends upon the amount +of charge, 500-lb. mines requiring to be about 300 feet apart +for safety.</p> +<hr class="chap x-ebookmaker-drop"> + +<div class="chapter"> +<p><span class="pagenum" id="Page_287">287</span></p> + +<h2 class="nobreak" id="toclink_287"><a id="chapter_XXXI"></a>CHAPTER XXXI<br> + +<span class="subhead">WHAT IS ELECTRICITY?</span></h2> +</div> + +<p class="in0"><span class="firstword">The</span> question which heads this, our final chapter, is one +which must occur to every one who takes even the most +casual interest in matters scientific, and it would be very +satisfactory if we could bring this volume to a conclusion +by providing a full and complete answer. Unfortunately +this is impossible. In years to come the tireless labours of +scientific investigators may lead to a solution of the problem; +but, as Professor Fleming puts it: “The question—What +is electricity?—no more admits of a complete and final +answer to-day than does the question—What is life?”</p> + +<p>From the earliest days of electrical science theories of +electricity have been put forward. The gradual extension +and development of these theories, and the constant substitution +of one idea for another as experimental data +increased, provide a fascinating subject for study. To +cover this ground however, even in outline, would necessitate +many chapters, and so it will be better to consider +only the theory which, with certain reservations in some +cases, is held by the scientific world of to-day. This is +known as the <em>electron</em> theory of electricity.</p> + +<p>We have referred already, in <a href="#chapter_XXIV">Chapter XXIV</a>., to atoms +and electrons. All matter is believed to be constituted +of minute particles called “atoms.” These atoms are so +extremely small that they are quite invisible, being far +beyond the range of the most powerful microscope; and +their diameter has been estimated at somewhere about one<span class="pagenum" id="Page_288">288</span> +millionth of a millimetre. Up to a few years ago the atom +was believed to be quite indivisible, but it has been proved +beyond doubt that this is not the case. An atom may be +said to consist of two parts, one much larger than the +other. The smaller part is negatively electrified, and is +the same in all atoms; while the larger part is positively +electrified, and varies according to the nature of the atom. +The small negatively electrified portion of the atom consists of +particles called “electrons,” and these electrons are believed +to be indivisible units or atoms of negative electricity. To +quote Professor Fleming: “An atom of matter in its neutral +condition has been assumed to consist of an outer shell or +envelope of negative electrons associated with some core or +matrix which has an opposite electrical quality, such that if +an electron is withdrawn from the atom the latter is left +positively electrified.”</p> + +<p>The electrons in an atom are not fixed, but move with +great velocity, in definite orbits. They repel one another, +and are constantly endeavouring to fly away from the atom, +but they are held in by the attraction of the positive core. +So long as nothing occurs to upset the constitution of the +atom, a state of equilibrium is maintained and the atom is +electrically neutral; but immediately the atom is broken up +by the action of an external force of some kind, one or +more electrons break their bonds and fly away to join some +other atom. An atom which has lost some of its electrons +is no longer neutral, but is electro-positive; and similarly, +an atom which has gained additional electrons is electro-negative. +Electrons, or atoms of negative electricity, can +be isolated from atoms of matter, as in the case of the +stream of electrons proceeding from the cathode of a vacuum +tube. So far, however, it has been found impossible to +isolate corresponding atoms of positive electricity.</p> + +<p>From these facts it appears that we must regard a<span class="pagenum" id="Page_289">289</span> +positively charged body as possessing a deficiency of +electrons, and a negatively charged body as possessing an +excess of electrons. In <a href="#chapter_I">Chapter I</a>. we spoke of the +electrification of sealing-wax or glass rods by friction, and +we saw that according to the nature of the substance used +as the rubber, the rods were either positively or negatively +electrified. Apparently, when we rub a glass rod with a +piece of silk, the surface atoms of each substance are +disturbed, and a certain number of electrons leave the glass +atoms, and join the silk atoms. The surface atoms of the +glass, previously neutral, are now electro-positive through +the loss of electrons; and the surface atoms of the silk, +also previously neutral, are now electro-negative through +the additional electrons received from the glass atoms. +As the result we find the glass to be positively, and silk to +be negatively electrified. On the other hand, if we rub the +glass with fur, a similar atomic disturbance and consequent +migration of electrons takes place, but this time the glass +receives electrons instead of parting with them. In this case +the glass becomes negatively, and the fur positively electrified. +The question now arises, why is the movement of the electrons +away from the glass in the first instance, and toward it in +the second? To understand this we may make use of a +simple analogy. If we place in contact two bodies, one hot +and the other cold, the hot body gives up some of its heat +to the cold body; but if we place in contact with the hot +body another body which is still hotter, then the hot body +receives heat instead of parting with it. In a somewhat +similar manner an atom is able to give some of its electrons +to another atom which, in comparison with it, is deficient in +electrons; and at the same time it is able to receive electrons +from another atom which, compared with it, has an +excess of electrons. Thus we may assume that the glass +atoms have an excess of electrons as compared with<span class="pagenum" id="Page_290">290</span> +silk atoms, and a deficiency in electrons as compared with +fur atoms.</p> + +<p>A current of electricity is believed to be nothing more +or less than a stream of electrons, set in motion by the +application of an electro-motive force. We have seen that +some substances are good conductors of electricity, while +others are bad conductors or non-conductors. In order to +produce an electric current, that is a current of electrons, it +is evidently necessary that the electrons should be free to +move. In good conductors, which are mostly metals, it is +believed that the electrons are able to move from atom to +atom without much hindrance, while in a non-conductor +their movements are hampered to such an extent that inter-atomic +exchange of electrons is almost impossible. Speaking on +this point, Professor Fleming says: “There may be (in +a good conductor) a constant decomposition and recomposition +of atoms taking place, and any given electron so to +speak flits about, now forming part of one atom and now of +another, and anon enjoying a free existence. It resembles +a person visiting from house to house, forming a unit in +different households, and, in between, being a solitary +person in the street. In non-conductors, on the other hand, +the electrons are much restricted in their movements, and +can be displaced a little way but are pulled back again +when released.”</p> + +<p>Let us try to see now how an electric current is set up +in a simple voltaic cell, consisting of a zinc plate and a +copper plate immersed in dilute acid. First we must +understand the meaning of the word <em>ion</em>. If we place a +small quantity of salt in a vessel containing water, the salt +dissolves, and the water becomes salt, not only at the +bottom where the salt was placed, but throughout the +whole vessel. This means that the particles of salt must be +able to move through the water. Salt is a chemical<span class="pagenum" id="Page_291">291</span> +compound of sodium and chlorine, and its molecules +consist of atoms of both these substances. It is supposed +that each salt molecule breaks up into two parts, one part +being a sodium atom, and the other a chlorine atom; and +further, that the sodium atom loses an electron, while the +chlorine atom gains one. These atoms have the power of +travelling about through the solution, and they are called +<em>ions</em>, which means “wanderers.” An ordinary atom is unable +to wander about in this way, but it gains travelling power +as soon as it is converted into an ion, by losing electrons if +it be an atom of a metal, and by gaining electrons if it be +an atom of a non-metal.</p> + +<p>Returning to the voltaic cell, we may imagine that the +atoms of the zinc which are immersed in the acid are trying +to turn themselves into ions, so that they can travel through +the solution. In order to do this each atom parts with two +electrons, and these electrons try to attach themselves to +the next atom. This atom however already has two +electrons, and so in order to accept the newcomers it must +pass on its own two. In this way electrons are passed on +from atom to atom of the zinc, then along the connecting +wire, and so to the copper plate. The atoms of zinc which +have lost their electrons thus become ions, with power of +movement. They leave the zinc plate immediately, and so +the plate wastes away or dissolves. So we get a constant +stream of electrons travelling along the wire connecting the +two plates, and this constitutes an electric current.</p> + +<p>The electron theory gives us also a clear conception of +magnetism. An electric current flowing along a wire +produces magnetic effects; that is, it sets up a field of +magnetic force. Such a current is a stream of electrons, +and therefore we conclude that a magnetic field is produced +by electrons in motion. This being so, we are led to +suppose that there must be a stream of electrons in a steel<span class="pagenum" id="Page_292">292</span> +magnet, and this stream must be constant because the +magnetic field of such a magnet is permanent. The +electron stream in a permanent magnet however is not +quite the same as the electron stream in a wire conveying a +current. We have stated that the electrons constituting an +atom move in definite orbits, so that we may picture them +travelling round the core of the atom as the planets travel +round the Sun. This movement is continuous in every +atom of every substance. Apparently we have here the +necessary conditions for the production of a magnetic field, +that is, a constant stream of electrons; but one important +thing is still lacking. In an unmagnetized piece of steel the +atoms are not arranged symmetrically, so that the orbits of +their electrons lie some in one plane and some in another. +Consequently, although the electron stream of each atom +undoubtedly produces an infinitesimally small magnetic +field, no magnetic effect that we can detect is produced, +because the different streams are not working in unison and +adding together their forces. In fact they are upsetting +and neutralizing each other’s efforts. By stroking the piece +of steel with a magnet, or by surrounding it by a coil of +wire conveying a current, the atoms are turned so that their +electron orbits all lie in the same plane. The electron +streams now all work in unison, their magnetic effects are +added together, and we get a strong magnetic field as the +result of their combined efforts. Any piece of steel or iron +may be regarded as a potential magnet, requiring only a +rearrangement of its atoms in order to become an active +magnet. In <a href="#chapter_VI">Chapter VI</a>. it was stated that other substances +besides iron and steel show magnetic effects, and this is what +we should expect, as the electron movement is common to +all atoms. None of these substances is equal to iron +and steel in magnetic power, but why this is so is not +understood.</p> + +<p><span class="pagenum" id="Page_293">293</span></p> + +<p>This brings us to the production of an electric current by +the dynamo. Here we have a coil of wire moving across a +magnetic field, alternately passing into this field and out of +it. A magnetic field is produced, as we have just seen, by +the steady movement of electrons, and we may picture it +as being a region of the ether disturbed or strained by the +effect of the moving electrons. When the coil of wire +passes into the magnetic field, the electrons of its atoms are +influenced powerfully and set in motion in one direction, so +producing a current in the coil. As the coil passes away +from the field, its electrons receive a second impetus, which +checks their movement and starts them travelling in the +opposite direction, and another current is produced. The +coil moves continuously and regularly, passing into and out +of the magnetic field without interruption; and so we get a +current which reverses its direction at regular intervals, that +is, an alternating current. This current may be made continuous +if desired, as explained in <a href="#chapter_IX">Chapter IX</a>.</p> + +<p>Such, stated briefly and in outline, is the electron theory +of electricity. It opens up possibilities of the most fascinating +nature; it gives us a wonderfully clear conception of +what might be called the inner mechanism of electricity; and +it even introduces us to the very atoms of electricity. +Beyond this, at present, it cannot take us, and the actual +nature of electricity itself remains an enigma.</p> +<hr class="chap x-ebookmaker-drop"> + +<div class="chapter"> +<p><span class="pagenum" id="Page_295">295</span></p> + +<h2 class="nobreak" id="toclink_295">INDEX</h2> + +<div class="index"> +<ul class="index"> +<li class="ifrst">Accumulators, <a href="#Page_38">38</a>, <a href="#Page_90">90</a>.</li> + +<li class="indx">Alarms, electric, <a href="#Page_120">120</a>.</li> + +<li class="indx">Alternating currents, <a href="#Page_70">71</a>, <a href="#Page_75">75</a>.</li> + +<li class="indx">Amber, discovery of, <a href="#Page_2">2</a>.</li> + +<li class="indx">Ampère, <a href="#Page_33">33</a>.</li> + +<li class="indx">Arc lamp, <a href="#Page_93">93</a>.</li> + +<li class="indx">Armature, <a href="#Page_68">68</a>.</li> + +<li class="indx">Atlantic cable, <a href="#Page_145">145</a>.</li> + +<li class="indx">Atom, <a href="#Page_287">287</a>.</li> + +<li class="indx">Aurora borealis, <a href="#Page_25">25</a>.</li> + +<li class="indx">Automatic telephone exchange, <a href="#Page_164">165</a>.</li> + +<li class="indx">Aviation and “wireless,” <a href="#Page_280">280</a>.</li> + +<li class="ifrst">Bachelet “flying” train, <a href="#Page_271">271</a>.</li> + +<li class="indx">Bastian heater, the, <a href="#Page_110">110</a>.</li> + +<li class="indx">Battery, voltaic, <a href="#Page_33">33</a>.</li> + +<li class="indx">Bell telephone, the, <a href="#Page_156">156</a>.</li> + +<li class="indx">Bells and alarms, electric, <a href="#Page_116">116</a>.</li> + +<li class="indx">Blasting, <a href="#Page_256">256</a>.</li> + +<li class="indx">Bunsen cell, <a href="#Page_223">223</a>.</li> + +<li class="ifrst">Cable-laying, <a href="#Page_150">150</a>.</li> + +<li class="indx">Cables, telegraph, <a href="#Page_144">144</a>.</li> + +<li class="indx">Cell, voltaic, <a href="#Page_29">29</a>.</li> + +<li class="indx">Clocks, electric, <a href="#Page_124">124</a>.</li> + +<li class="indx">Coherer, the, <a href="#Page_183">183</a>.</li> + +<li class="indx">Commutator, <a href="#Page_70">70</a>.</li> + +<li class="indx">Compass, magnetic, <a href="#Page_52">52</a>.</li> + +<li class="indx">Condenser, <a href="#Page_63">63</a>.</li> + +<li class="indx">Conductors, <a href="#Page_6">6</a>.</li> + +<li class="indx">Conduit system, <a href="#Page_83">83</a>.</li> + +<li class="indx">Convectors, <a href="#Page_109">109</a>.</li> + +<li class="indx">Cookers, electric, <a href="#Page_110">110</a>.</li> + +<li class="indx">Creed telegraph, <a href="#Page_137">137</a>.</li> + +<li class="indx">Crookes, Sir W., <a href="#Page_230">230</a>.</li> + +<li class="indx">Current, electric, <a href="#Page_30">30</a>.</li> + +<li class="ifrst">Daniell cell, <a href="#Page_30">31</a>, <a href="#Page_223">223</a>.</li> + +<li class="indx">Davy, Sir Humphry, <a href="#Page_93">93</a>.</li> + +<li class="indx">Detector, in wireless telegraphy, <a href="#Page_188">188</a>, <a href="#Page_198">198</a>.</li> + +<li class="indx">Diamond-making, <a href="#Page_113">113</a>.</li> + +<li class="indx">Duplex telegraphy, <a href="#Page_139">139</a>.</li> + +<li class="indx">Dussaud cold light, <a href="#Page_106">106</a>.</li> + +<li class="indx">Dynamo, <a href="#Page_66">66</a>.</li> + +<li class="ifrst">Edison, Thomas A., <a href="#Page_103">103</a>.</li> + +<li class="indx">Electric cookers, <a href="#Page_110">110</a>.</li> + +<li class="indx">Electric heating, <a href="#Page_109">109</a>.</li> + +<li class="indx">Electric motor, <a href="#Page_66">66</a>.</li> + +<li class="indx">Electric lighting, <a href="#Page_70">70</a>, <a href="#Page_75">75</a>, <a href="#Page_93">93</a>.</li> + +<li class="indx">Electricity, early discoveries, <a href="#Page_1">1</a>;</li> +<li class="isub1">nature of, <a href="#Page_287">287</a>.</li> + +<li class="indx">Electro-culture, <a href="#Page_258">258</a>.</li> + +<li class="indx">Electrolysis, <a href="#Page_224">224</a>.</li> + +<li class="indx">Electro-magnets, <a href="#Page_58">58</a>.</li> + +<li class="indx">Electron, <a href="#Page_287">287</a>.</li> + +<li class="indx">Electroplating, <a href="#Page_213">213</a>.</li> + +<li class="indx">Electrophorus, the, <a href="#Page_11">11</a>.</li> + +<li class="indx">Electrotyping, <a href="#Page_213">213</a>.</li> + +<li class="ifrst">Faraday, <a href="#Page_66">66</a>.</li> + +<li class="indx">Finsen light treatment, <a href="#Page_243">243</a>.</li> + +<li class="indx">Franklin, Benjamin, <a href="#Page_19">19</a>.</li> + +<li class="indx">Frictional electricity, <a href="#Page_2">2</a>.</li> + +<li class="indx">Furnace, electric, <a href="#Page_111">111</a>.</li> + +<li class="ifrst">Galvani, <a href="#Page_27">27</a>.</li> + +<li class="indx">Galvanometer, <a href="#Page_59">59</a>.</li> + +<li class="indx">Glass, <a href="#Page_4">4</a>.</li> + +<li class="indx">Goldschmidt system, <a href="#Page_197">197</a>.</li> + +<li class="indx"><i>Great Eastern</i>, the, <a href="#Page_148">148</a>.</li> + +<li class="ifrst">Half-watt lamp, <a href="#Page_105">105</a>.</li> + +<li class="indx">Heating by electricity, <a href="#Page_109">109</a>.</li> + +<li class="indx">Hughes printing telegraph, <a href="#Page_136">136</a>.</li> + +<li class="ifrst">Iceberg detector, <a href="#Page_267">267</a>.</li> + +<li class="indx">Ignition, electric, <a href="#Page_253">253</a>.</li> + +<li class="indx">Incandescent lamps, <a href="#Page_103">103</a>.</li> + +<li class="indx">Induction, <a href="#Page_9">9</a>.</li> + +<li class="indx">Induction coil, <a href="#Page_61">61</a>.</li> + +<li class="indx">Ion, <a href="#Page_291">291</a>.</li> + +<li class="ifrst">Kelvin, Lord, <a href="#Page_152">152</a>.</li> + +<li class="indx">Korn’s photo-telegraph, <a href="#Page_174">174</a>.</li> + +<li class="ifrst">Lamps, electric, <a href="#Page_93">93</a>.</li> + +<li class="indx">Leclanché cell, <a href="#Page_32">32</a>, <a href="#Page_116">116</a>.</li> + +<li class="indx">Lemström’s experiments in electro-culture, <a href="#Page_258">258</a>.</li> + +<li class="indx">Lepel system, <a href="#Page_196">196</a>.</li> + +<li class="indx">Leyden jar, <a href="#Page_15">15</a>, <a href="#Page_181">181</a>.</li> + +<li class="indx">Light, <a href="#Page_23">23</a>.</li> + +<li class="indx"><span class="pagenum" id="Page_296">296</span>Lighting, electric, <a href="#Page_75">75</a>, <a href="#Page_93">93</a>.</li> + +<li class="indx">Lightning, <a href="#Page_1">1</a>, <a href="#Page_19">19</a>, <a href="#Page_23">23</a>.</li> + +<li class="indx">Lightning conductors, <a href="#Page_25">25</a>.</li> + +<li class="indx">Lindsay, wireless experiments, <a href="#Page_180">180</a>.</li> + +<li class="indx">Lodge, Sir Oliver, <a href="#Page_260">260</a>.</li> + +<li class="ifrst">Machines for producing static electricity, <a href="#Page_9">9</a>.</li> + +<li class="indx">Magnetic poles, <a href="#Page_50">50</a>.</li> + +<li class="indx">Magnetism, <a href="#Page_44">44</a>, <a href="#Page_56">56</a>, <a href="#Page_291">291</a>.</li> + +<li class="indx">Marconi, <a href="#Page_186">186</a>, <a href="#Page_195">195</a>.</li> + +<li class="indx">Medicine, electricity in, <a href="#Page_240">241</a>.</li> + +<li class="indx">Mercury-vapour lamp, <a href="#Page_99">99</a>.</li> + +<li class="indx">Microphone, <a href="#Page_159">159</a>.</li> + +<li class="indx">Mines, submarine, <a href="#Page_283">283</a>.</li> + +<li class="indx">Mines, telephones in, <a href="#Page_169">169</a>.</li> + +<li class="indx">Mono-railway, <a href="#Page_89">89</a>.</li> + +<li class="indx">Morse, telegraph, <a href="#Page_130">130</a>;</li> +<li class="isub1">experiments in wireless telegraphy, <a href="#Page_180">180</a>.</li> + +<li class="indx">Motor, electric, <a href="#Page_66">66</a>.</li> + +<li class="indx">Motor-car, electric, <a href="#Page_91">91</a>.</li> + +<li class="ifrst">Navy, use of wireless, <a href="#Page_274">274</a>;</li> +<li class="isub1">of electricity, <a href="#Page_282">282</a>.</li> + +<li class="indx">Negative electricity, <a href="#Page_5">5</a>.</li> + +<li class="indx">Neon lamps, <a href="#Page_102">102</a>.</li> + +<li class="indx">Non-conductors, <a href="#Page_6">6</a>.</li> + +<li class="ifrst">Ohm, <a href="#Page_33">33</a>.</li> + +<li class="indx">Oil radiator, <a href="#Page_110">110</a>.</li> + +<li class="indx">Ozone, <a href="#Page_23">23</a>, <a href="#Page_247">247</a>.</li> + +<li class="indx">Ozone ventilation, <a href="#Page_249">249</a>.</li> + +<li class="ifrst">Petrol, motor, ignition in, <a href="#Page_253">253</a>.</li> + +<li class="indx">Photographophone, the, <a href="#Page_173">173</a>.</li> + +<li class="indx">Pile, voltaic, <a href="#Page_28">28</a>.</li> + +<li class="indx">Pipe locator, <a href="#Page_266">266</a>.</li> + +<li class="indx">Plant culture, electric, <a href="#Page_258">258</a>.</li> + +<li class="indx">Polarization, <a href="#Page_30">31</a>.</li> + +<li class="indx">Pollak-Virag telegraph, <a href="#Page_137">137</a>.</li> + +<li class="indx">Positive electricity, <a href="#Page_5">5</a>.</li> + +<li class="indx">Poulsen, Waldemar, <a href="#Page_171">171</a>, <a href="#Page_197">197</a>.</li> + +<li class="indx">Poultry, electro-culture of, <a href="#Page_264">264</a>.</li> + +<li class="indx">Power stations, <a href="#Page_75">75</a>.</li> + +<li class="indx">Preece, wireless experiments, <a href="#Page_180">180</a>.</li> + +<li class="indx">Primary and secondary coils, <a href="#Page_62">62</a>.</li> + +<li class="ifrst">Radiator, <a href="#Page_109">109</a>.</li> + +<li class="indx">Railways, electric, <a href="#Page_86">87</a>;</li> +<li class="isub1">use of wireless, <a href="#Page_211">211</a>.</li> + +<li class="indx">Resistance, <a href="#Page_33">33</a>.</li> + +<li class="indx">Röntgen rays, <a href="#Page_228">228</a>, <a href="#Page_242">242</a>.</li> + +<li class="ifrst">Searchlights, <a href="#Page_98">98</a>.</li> + +<li class="indx">Ships, use of wireless, <a href="#Page_206">206</a>.</li> + +<li class="indx">Siphon recorder, the, <a href="#Page_252">252</a>.</li> + +<li class="indx">Sparking plug, <a href="#Page_154">154</a>.</li> + +<li class="indx">Static electricity, <a href="#Page_7">7</a>.</li> + +<li class="indx">Stations, wireless, <a href="#Page_204">204</a>.</li> + +<li class="indx">Steinheil telegraph, <a href="#Page_130">130</a>.</li> + +<li class="indx">Submarine telegraphy, <a href="#Page_144">144</a>.</li> + +<li class="indx">Submarine telephony, <a href="#Page_169">169</a>.</li> + +<li class="indx">Surface contact system, <a href="#Page_83">83</a>.</li> + +<li class="ifrst">Telefunken system, <a href="#Page_196">196</a>.</li> + +<li class="indx">Telegraph, the, <a href="#Page_128">128</a>, <a href="#Page_144">144</a>, <a href="#Page_171">171</a>, <a href="#Page_179">179</a>, <a href="#Page_203">203</a>.</li> + +<li class="indx">Telegraphone, <a href="#Page_171">171</a>.</li> + +<li class="indx">Telephone, the, <a href="#Page_154">154</a>, <a href="#Page_171">171</a>, <a href="#Page_179">179</a>, <a href="#Page_201">201</a>.</li> + +<li class="indx">Telephone exchange, <a href="#Page_160">160</a>.</li> + +<li class="indx">Thermopile, <a href="#Page_36">36</a>.</li> + +<li class="indx">Thermostat, <a href="#Page_120">121</a>.</li> + +<li class="indx">Thunderstorms, <a href="#Page_22">22</a>, <a href="#Page_194">194</a>.</li> + +<li class="indx">Trains, electric, <a href="#Page_86">87</a>;</li> +<li class="isub1">the Bachelet, <a href="#Page_271">271</a>.</li> + +<li class="indx">Tramways, electric, <a href="#Page_78">78</a>, <a href="#Page_83">83</a>.</li> + +<li class="indx">Trolley system, <a href="#Page_83">83</a>.</li> + +<li class="indx">Tubes for X-rays, <a href="#Page_233">233</a>.</li> + +<li class="indx">Tuning in wireless telegraphy, <a href="#Page_191">191</a>.</li> + +<li class="indx">Tungsten lamps, <a href="#Page_104">104</a>.</li> + +<li class="ifrst">Volt, <a href="#Page_33">33</a>.</li> + +<li class="indx">Voltaic electricity, <a href="#Page_28">28</a>, <a href="#Page_129">129</a>, <a href="#Page_290">290</a>.</li> + +<li class="ifrst">War, electricity in, <a href="#Page_274">274</a>;</li> +<li class="isub1">telegraph in, <a href="#Page_277">277</a>.</li> + +<li class="indx">Water, electrolysis of, <a href="#Page_38">38</a>.</li> + +<li class="indx">Water-power, <a href="#Page_80">81</a>.</li> + +<li class="indx">Waves, electric, <a href="#Page_181">181</a>, <a href="#Page_191">191</a>, <a href="#Page_199">199</a>.</li> + +<li class="indx">Welding, electric, <a href="#Page_114">114</a>.</li> + +<li class="indx">Welsbach lamp, <a href="#Page_103">103</a>.</li> + +<li class="indx">Wheatstone and Cooke telegraphs, <a href="#Page_130">130</a>.</li> + +<li class="indx">Wimshurst machine, <a href="#Page_12">12</a>.</li> + +<li class="indx">Wireless telegraphy and telephony, <a href="#Page_179">179</a>, <a href="#Page_203">203</a>, <a href="#Page_270">270</a>, <a href="#Page_280">280</a>.</li> + +<li class="indx">Wires, telegraph, <a href="#Page_141">141</a>.</li> + +<li class="ifrst">X-rays, <a href="#Page_231">231</a>, <a href="#Page_242">242</a>.</li> +</ul> +</div></div> + +<p class="p4 center wspace smaller"> +<span class="smcap">Morrison & Gibb Limited, Edinburgh</span><br> +5/15 <span class="inend">2½</span> +</p> + +<div class="chapter transnote"> +<h2 class="nobreak" id="Transcribers_Notes">Transcriber’s Notes</h2> + +<p>Punctuation, hyphenation, and spelling were made +consistent when a predominant preference was found +in the original book; otherwise they were not changed.</p> + +<p>Simple typographical errors were corrected; unbalanced +quotation marks were remedied when the change was +obvious, and otherwise left unbalanced.</p> + +<p>Illustrations in this eBook have been positioned +between paragraphs and outside quotations. In versions +of this eBook that support hyperlinks, the page +references in the List of Plates lead to the +corresponding illustrations. (There is no list +of the other illustrations.)</p> + +<p id="plate_VIII"><span class="bold">Plate VIII.</span>, “Typical Electric Locomotives,” listed as being on +<a href="#Page_90">page 90</a>, was not in the original book and therefore not in this ebook.</p> + +<p>The index was not checked for proper alphabetization +or correct page references.</p> +</div> +<div style='text-align:center'>*** END OF THE PROJECT GUTENBERG EBOOK ELECTRICITY ***</div> +</body> +</html> |