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diff --git a/38526-h/38526-h.htm b/38526-h/38526-h.htm new file mode 100644 index 0000000..34d0728 --- /dev/null +++ b/38526-h/38526-h.htm @@ -0,0 +1,5841 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" + "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> +<!-- $Id: header.txt 236 2009-12-07 18:57:00Z vlsimpson $ --> + +<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> + <head> + <meta http-equiv="Content-Type" content="text/html;charset=iso-8859-1" /> + <meta http-equiv="Content-Style-Type" content="text/css" /> + <title> + The Project Gutenberg eBook of Hertzian Wave Wireless Telegraphy, by Dr. J. A. FLEMING, F.R.S. + </title> + <style type="text/css"> + +body { + margin-left: 10%; + margin-right: 10%; +} + +h1 { + text-align: center; + font-size:200%; + font-weight:normal; + clear: both; +} + + +h2 { + text-align: center; + font-size:175%; + font-weight:normal; + clear: both; +} + +h3 { + text-align: center; + font-size:100%; + font-weight:normal; + clear: both; +} + +h4 { + text-align: center; + font-size:80%; + font-weight:normal; + clear: both; +} + +h5 { + text-align: center; + font-size:60%; + font-weight:normal; + clear: both; +} + +p { + margin-top: .75em; + text-align: justify; + margin-bottom: .75em; +} + +hr { + width: 33%; + margin-top: 2em; + margin-bottom: 2em; + margin-left: auto; + margin-right: auto; + clear: both; +} + +.pagenum { + position: absolute; + left: 2%; + font-size: smaller; + text-align: right; +} + + +.bbox { + border: solid 2px; + padding-top: 5px; + padding-bottom: 5px; + padding-left: 5px; + padding-right: 5px; +} + + +.smcap {font-variant: small-caps;} + + +.caption { + font-weight: normal; + font-size: smaller; +} + +/* Images */ +.eq { + margin: auto; + text-align: center; +} + + +.figcenter { + margin: auto; + text-align: center; +} + +.figleft { + float: left; + clear: left; + margin-left: 0; + margin-bottom: 1em; + margin-top: 1em; + margin-right: 1em; + padding: 0; + text-align: center; +} + +.figright { + float: right; + clear: right; + margin-left: 1em; + margin-bottom: + 1em; + margin-top: 1em; + margin-right: 0; + padding: 0; + text-align: center; +} + +/* Footnotes */ +.footnotes { + border: solid 1px; + border-color: gray; +} + +.footnote_title { + margin-left: 10%; +} + +.footnote {margin-left: 10%; margin-right: 10%; font-size: 0.9em;} + +.footnote .label {position: absolute; right: 84%; text-align: right;} + +.fnanchor { + vertical-align: super; + font-size: .8em; + text-decoration: + none; +} + +/* Poetry */ +.poem { + margin-left:10%; + margin-right:10%; + text-align: left; +} + +.poem br {display: none;} + +.poem .stanza {margin: 1em 0em 1em 0em;} + +.poem span.i0 { + display: block; + margin-left: 0em; + padding-left: 3em; + text-indent: -3em; +} + +.tnlink { + text-decoration:none; + border-bottom: thin dotted black; + color:black; +} + +p.cap:first-letter { + float: left; + clear: left; + margin: 0.05em 0 0 0; + padding: 0 0 0.1em 0; + line-height: 1.2em; + font-size: 350%; +} + +.text_l { + font-size:125%; + font-weight:normal; +} + + + + + </style> + </head> +<body> + + +<pre> + +The Project Gutenberg EBook of Hertzian Wave Wireless Telegraphy, by +John Ambrose Fleming + +This eBook is for the use of anyone anywhere at no cost and with +almost no restrictions whatsoever. You may copy it, give it away or +re-use it under the terms of the Project Gutenberg License included +with this eBook or online at www.gutenberg.org + + +Title: Hertzian Wave Wireless Telegraphy + +Author: John Ambrose Fleming + +Release Date: January 8, 2012 [EBook #38526] + +Language: English + +Character set encoding: ISO-8859-1 + +*** START OF THIS PROJECT GUTENBERG EBOOK HERTZIAN WAVE WIRELESS TELEGRAPHY *** + + + + +Produced by Robert Cicconetti, Tim Madden and the Online +Distributed Proofreading Team at https://www.pgdp.net (This +file was produced from images generously made available +by The Internet Archive/American Libraries.) + + + + + + +</pre> + + +<div class="bbox"> + <p><b>Transcriber's Notes:</b></p> + + <p>All apparent printer's errors and variations in spelling have been + retained, there are also some inconsistencies in the hyphenation of + words. All these have been detailed at the end of the text.</p> + + <p>The middle dot has been used in the original text, both as a + multiplication symbol and as a decimal point. These have been kept but + the middle dot as a multiplication symbol in formulae is surrounded + by single spaces.</p> + +</div> + + + + +<hr style="width: 65%;" /> + +<p> </p> +<p> </p> +<p> </p> +<p> </p> + +<h2>HERTZIAN WAVE WIRELESS TELEGRAPHY.</h2> + +<p> </p> +<p> </p> +<p> </p> + +<h3><span class="smcap">By Dr. J. A. FLEMING, F.R.S.</span></h3> + +<p> </p> +<p> </p> +<p> </p> + +<h3>[From the <span class="smcap">Popular Science Monthly</span>, June-December, 1903.]</h3> + +<p> </p> +<p> </p> +<p> </p> +<p> </p> + +<hr style="width: 65%;" /> + +<p><span class="pagenum"><a name="Page_1" id="Page_1">[Pg 1]</a></span></p> +<h4>[From the "Popular Science Monthly," June, 1903.]</h4> + +<p> </p> +<p> </p> + +<h1>HERTZIAN WAVE WIRELESS TELEGRAPHY.<a name="FNanchor_1_1" id="FNanchor_1_1"></a><a href="#Footnote_1_1" class="fnanchor">[1]</a></h1> + +<hr style="width: 15%" /> + +<h4><span class="smcap">By Dr. J. A. FLEMING, F.R.S.,</span></h4> +<h5><span class="smcap">PROFESSOR OF ELECTRICAL ENGINEERING, UNIVERSITY COLLEGE, LONDON.</span></h5> + +<hr style="width: 15%;" /> + + +<p class="cap">T<span class="text_l">HE</span> immense public interest which has been aroused of late years in +the subject of telegraphy without connecting wires has undoubtedly +been stimulated by the achievements of Mr. Marconi in effecting +communication over great distances by means of Hertzian waves. The +periodicals and daily journals, which are the chief avenues through +which information reaches the public, whilst eager to describe in a +sensational manner these wonderful applications of electrical +principles, have done little to convey an intelligible explanation of +them. Hence it appeared probable that a service would be rendered by +an endeavour to present an account of the present condition of +electric wave telegraphy in a manner acceptable to those unversed in +the advanced technicalities of the subject, but acquainted at least +with the elements of electrical science. It is the purpose of these +articles to attempt this task. We shall, however, limit the discussion +to an account of the scientific principles underlying the operation of +this particular form of wireless telegraphy, omitting, as far as +possible, references to mere questions of priority and development.</p> + +<p>The practical problem of electric wave wireless telegraphy, which has +been variously called Hertzian wave telegraphy, Marconi telegraphy, or +spark telegraphy (<i>Funkentelegraphie</i>), is that of the production of +an effect called an electric wave or train of electric waves, which +can be sent out from one place, controlled, detected at another place, +and interpreted into an alphabetic code. Up to the present time, the +chief part of that intercommunication has been effected by means of +the Morse code, in which a group of long and short signs form the +letter or symbol. Some attempts have been made with more or less, +success to work printing telegraphs and even writing or <span class="pagenum"><a name="Page_2" id="Page_2">[Pg 2]</a></span>drawing +telegraphs by Hertzian waves, but have not passed beyond the +experimental stage, whilst wireless telephony by this means is still a +dream of the future.</p> + +<p>We shall, in the first place, consider the transmitting arrangements +and, incidentally, the nature of the effect or wave transmitted; in +the second place, the receiving appliances; and, finally, discuss the +problem of the isolation or secrecy of the intelligence conveyed +between any two places.</p> + +<p>The transmitter used in Hertzian wave telegraphy consists essentially +of a device for producing electric waves of a type which will travel +over the surface of the land or sea without speedy dissipation, and +the important element in this arrangement is the <i>radiator</i>, by which +these waves are sent out. It will be an advantage to begin by +explaining the electrical action of the radiator, and then proceed to +discuss the details of the transmitting appliances.</p> + +<p>It will probably assist the reader to arrive most easily at a general +idea of the functions of the various portions of the transmitting +arrangements, and in particular of the radiator, if we take as our +starting point an analogy which exists between electric wave +generation for telegraphic purposes and air wave generation for sound +signal purposes. Most persons have visited some of the large +lighthouses which exist around our coasts and have there seen a steam +or air <i>siren</i>, as used for the production of sound signals during +fogs. If they have examined this appliance, they will know that it +consists, in the first place, of a long metal tube, generally with a +trumpet-shaped mouthpiece. At the bottom of this tube there is a fixed +plate with holes in it, against which revolves another similarly +perforated plate. These two plates separate a back chamber or wind +chest from the tube, and the wind chest communicates with a reservoir +of compressed air or a high-pressure steam boiler. In the +communication pipe there is a valve which can be suddenly opened for a +longer or shorter time. When the movable plate revolves, the +coincidence or non-coincidence of the holes in the two plates opens or +shuts the air passage way very rapidly. Hence when the blast of air or +steam is turned on, the flow is cut up by the revolving plates into a +series of puffs which inflict blows upon the stationary air in the +siren tube. If these blows come at the rate, say, of a hundred a +second, they give rise to aerial oscillations in the tube, which +impress the ear as a deep, musical note or roar; and this continuous +sound can be cut up by closing the valve intermittently into long and +short periods, and so caused to signal a letter according to the Morse +code, denoting the name of the lighthouse. In this case the object is +to produce: first, aerial vibrations in the tube, giving rise to a +train of powerful air waves; secondly, to intermit this wave-train so +as to produce an intelligible signal; and thirdly, to transmit this +wave as far as possible through space.</p> + +<p>The production of a sound or air wave can only be achieved by +administering a very sudden blow to the general mass of the air in the +tube. This impulse must be sufficient to call into operation the +inertia and elastic qualities of the air. It is found, moreover, that +the amplitude of the resulting wave, or the loudness of the sound, is +increased<span class="pagenum"><a name="Page_3" id="Page_3">[Pg 3]</a></span> by suitably proportioning the length of the siren pipe and +the frequency of the air puffs; whilst the distance at which it is +heard depends also in some degree upon the form of the mouthpiece.</p> + +<p>Inside the siren tube, when it is in operation, the air molecules are +in rapid vibratory motion in the direction of the length of the tube. +If we could at any one instant examine the distribution and changes of +air pressure in the tube, we should find that at some places there are +large, and at others small, variations in air pressure. These latter +places are called the <i>nodes</i> of pressure. At the pressure nodes, +however, we should find large variations in the velocity of the air +particles, and these points are called the <i>antinodes</i> of velocity. In +those places at which the pressure variation is greatest, the velocity +changes are least, and <i>vice versa</i>. Outside the tube, as a result of +these air motions in it, we have a hemispherical air wave produced, +which travels out from the mouthpiece as a centre; and if we could +examine the distribution of air pressure and velocity through all +external space, we should find a distribution which is periodic in +space as well as time, constituting the familiar phenomenon of an air +wave.</p> + +<p>Turning then to consider the production of an electric, instead of an +air wave, we notice in the first place that the medium with which we +are concerned is the <i>ether</i> filling all space. This ether permits the +production of physical changes in it which are analogous to, but not +identical in nature with, the pressures and movements which constitute +a sound wave. The Hertzian radiator is an appliance for acting on the +ether as the siren acts on the air. It produces a wave in it, and it +can be shown that all the parts of the above described siren apparatus +have their electrical equivalents in the transmitter employed in +Hertzian wave wireless telegraphy.</p> + +<p>To understand the nature of an electric wave we must consider, in the +first place, some properties of the ether. In this medium we can at +any place produce a state called <i>electric displacement</i> or <i>ether +strain</i> as we can produce compression or rarefaction in air; and, just +as the latter changes are said to be created by mechanical force, so +the former is said to be due to <i>electric force</i>. We can not define +more clearly the nature of this ether strain or displacement until we +know much more about the structure of the ether than we do at present. +We can picture to ourselves the operation of compressing air as an +approximation of the air molecules, but the difficulty of +comprehending the nature of an electric wave arises from the fact that +we cannot yet definitely resolve the notion of electric strain into +any simpler or more familiar ideas.</p> + +<p>We have to be content, therefore, to disguise our present ignorance by +the use of some descriptive term, such as <i>electric strain</i>, +<i>electrostatic strain</i> or <i>ether strain</i>, to describe the directed +condition of the space around a body in a state of electrification +which is produced by electric force. This electric strain is certainly +not of the nature of a compression in the ether, but much more akin to +a twist or rotational strain in a solid body.</p> + +<p>For our present purpose it is not so necessary to postulate any +particular theory of the ether as it is to possess some consistent +hypothesis, in terms of which we can describe the phenomena which +will<span class="pagenum"><a name="Page_4" id="Page_4">[Pg 4]</a></span> concern us. These effects are, as we shall see, partly states of +electrification on the surface or distributions of electric current in +wires or rods, and partly conditions in the space outside them, which +we are led to recognise as distributions of electric strain and of an +associated effect called <i>magnetic flux</i>.</p> + +<p>We find such a theory at hand at the present time in the electronic +theory of electricity, which has now been sufficiently developed and +popularised to make it useful as, a descriptive hypothesis.<a name="FNanchor_2_2" id="FNanchor_2_2"></a><a href="#Footnote_2_2" class="fnanchor">[2]</a> This +theory has the great recommendation that it offers a means of +abolishing the perplexing dualism of ether and ponderable matter, and +gives a definite and, in a sense, objective meaning to the word +electricity. In this physical speculation, the chief subject of +contemplation is the electron, or ultimate particle of negative +electricity, which, when associated in greater or less number with a +matrix of some description, forms the atom of ponderable matter. To +avoid further hypothesis, this matrix may be called the <i>co-electron</i>; +and we shall adopt the view that a single chemical atom is a union of +a <i>co-electron</i> with a surrounding envelope or group of electrons, one +or more of the latter being detachable. We need not stop to speculate +on the structure of the atomic core or co-electron, whether it is +composed of positive and negative electrons or of something entirely +different. The single electron is the indivisible unit or atomic +element of so-called negative electricity, and the neutral chemical +atom deprived of one electron is the unit of positive electricity. On +this hypothesis, the chemical atom is to be regarded as a microcosm, a +sort of a solar system in miniature, the component electrons being +capable of vibration relatively to the atomic centre of mass. +Furthermore, from this point of view it is the electron which is the +effective cause of radiation. It alone has a grip on the ether whereby +it is able to establish wave motion in the latter.</p> + +<p>Dr. Larmor has developed in considerable detail an hypothesis of the +nature of an electron which makes it the centre or convergence-point +of lines of a self-locked ether strain of a torsional type. The notion +of an atom merely as a "centre of force" was one familiar to Faraday +and much supported by Boscovich and others. The fatal objection to the +validity of this notion as originally stated was that it offers no +possibility of explaining the inertia of matter. On the electronic +hypothesis, the source of all inertia is the inertia of the ether, and +until we are able to dissect this last quality into anything simpler +than the time-element involved in the production of an ether strain or +displacement, we must accept it as an ultimate fact, not more +elucidated because we speak of it as the inductance of the electron.</p> + +<p>We postulate, therefore, the following ideas: We have to think of the +ether as a homogeneous medium in which a strain of some kind, most +probably of a rotational type, is possible. This strain appears only +under the influence of an appropriate stress called the electric +force, and disappears when the force is removed. Hence to create this +strain necessitates the expenditure of energy. An electron is a +<span class="pagenum"><a name="Page_5" id="Page_5">[Pg 5]</a></span> +centre or convergence-point of lines of permanent ether strain of such +nature that it cannot release itself. To obtain some idea of the +nature of such a structure, let us imagine a flat steel band formed +into a ring by welding the ends together. There is then no torsional +strain. If, however, we suppose the band cut in one place, one end +then given half a turn and the cut ends again welded, we shall have on +the band a self-locked twist, which can be displaced on the band, but +which can not release itself or be released except by cutting the +ring. Hence we see that to make an electron in an ether possessing +torsional elasticity would require creative energy, and when made, the +electron cannot destroy itself except by occupying simultaneously the +same place as an electron of opposite type. Every electron extends, +therefore, as Faraday said of the atom, throughout the universe, and +the properties that we find in the electron are only there because +they are first in the universal medium, the ether. Every line of ether +or electric strain must, therefore, be a self-closed line, or else it +must terminate on an electron and a co-electron.</p> + +<p>So far we have only considered the electron at rest. If, however, it +moves, it can be mathematically demonstrated that it must give rise to +a second form of ether strain which is related to the electric strain +as a twist is related to a thrust or a vortex ring to a squirt in +liquid or a rotation to a linear progression. The ether strain which +results from the lateral movement of lines of electric strain is +called the <i>magnetic flux</i>, and it can be mathematically shown that +the movement of an electron, <a name="tnd_5" id="tnd_5"></a><a href="#tn_5" class="tnlink" title="possible printer's error, a for at">consisting when a rest</a> of a radial +convergence of lines of electric strain, must be accompanied by the +production of self-closed lines of magnetic flux, distributed in +concentric circles or rings round it, the planes of these circles +being perpendicular to the direction of motion of the electron.</p> + +<p>This electronic hypothesis, therefore, affords a basis on which we can +build up a theory affording an explanation of the nature of the +intimate connection known to exist between ether, matter and +electricity. The electron is the connecting link between them all, for +it is in itself a centre of convergent ether strain; isolated, it +presents itself as electricity of the negative or resinous kind; and, +in combination with co-electrons and other electrons, it forms the +atoms of ponderable matter. At rest the electron or the co-electron +constitutes an electric charge, and when in motion it is an electric +current. A steady flux or drift of electrons in one direction and +co-electrons in the opposite direction is a continuous electric +current, whilst their mere oscillation about a mean position is an +alternating current. Furthermore, the vibration of an electron, if +sufficiently rapid, enables it to establish what are called electric +waves in the ether, but which are really detached and self-closed +lines of ether strain distributed in a periodic manner through space.</p> + +<p>We have, therefore, to start with, three conceptions concerning the +electron, viz.: Its condition when at rest; its state when in uniform +motion; and its operations when in vibration or rapid oscillation. In +the first case, by our fundamental supposition, it consists of lines +of ether strain of a type called the electric strain, radiating +uniformly in <span class="pagenum"><a name="Page_6" id="Page_6">[Pg 6]</a></span>all directions. When in uniform motion, it can be shown +that these lines of electric strain tend to group themselves in a +plane perpendicular to the line of motion drawn through the electron, +and their lateral motion generates another class of strain called the +magnetic strain, disposed in concentric circles described round the +electron and lying in this equatorial plane.</p> + +<p>The proof of the above propositions cannot be given verbally, but +requires the aid of mathematical analysis of an advanced kind. The +reader must be referred for the complete demonstration to the writings +of Professor J. J. Thomson<a name="FNanchor_3_3" id="FNanchor_3_3"></a><a href="#Footnote_3_3" class="fnanchor">[3]</a> and Mr. Oliver Heaviside.<a name="FNanchor_4_4" id="FNanchor_4_4"></a><a href="#Footnote_4_4" class="fnanchor">[4]</a></p> + +<p>In the third case, when the electron vibrates, we have a state in +which self-closed lines of electric strain and magnetic flux are +thrown off and move away through the <a name="tnd_6" id="tnd_6"></a><a href="#tn_6" class="tnlink" title=" printer's error, comma rather than full stop at end of sentence">ether constituting electric +radiation,</a> The manner in which this happens was first described by +Hertz in a Paper on "Electric Oscillations treated according to the +Method of Maxwell."<a name="FNanchor_5_5" id="FNanchor_5_5"></a><a href="#Footnote_5_5" class="fnanchor">[5]</a> As this phenomenon lies at the very root of +Hertzian wave wireless telegraphy, we must spend a moment or two in +its careful examination.</p> + +<div class="figright" style="width: 292px;"> +<img src="images/fig01.png" width="292" height="204" alt="FIG. 1.--LINES OF ELECTRIC STRAIN BETWEEN A POSITIVE +AND NEGATIVE ELECTRON AT REST." title="" /> +<span class="caption smcap">Fig. 1.—Lines of Electric Strain between a Positive +and Negative Electron at Rest.</span> +</div> + +<p>Let us imagine two metal rods placed in line and constituting what is +called a linear oscillator. Let these rods have adjacent ends +separated by a very small air space, and let one rod be charged with +positive and the other with negative electricity. On the electronic +theory this is explained by stating that there is an accumulation of +electrons in one and of co-electrons in the other. These charges +create a distribution of electric strain throughout their +neighbourhood, which follows approximately the same law of +distribution as the lines of magnetic force of a bar magnet, and may +be roughly represented as in Fig. 1. Suppose then that the air gap is +destroyed, these charges move towards each other and disappear by +uniting, the lines of electric strain then collapse, and as they +shrink in give rise to circular lines of magnetic flux embracing the +rods. This external distribution of magnetism constitutes an electric +current in the rods produced by the movement of the two opposite +electric charges. At this stage it may be explained that the electrons +or atoms of electricity can in some cases make their way freely +between the atoms of ponderable matter. The former are incomparably +smaller than the latter, and in those cases in which this electronic +movement can take place easily, we call the material a good conductor.</p> + +<p>Suppose then the electric charges reappear in reversed positions and +go through an oscillatory motion. The result in the external <span class="pagenum"><a name="Page_7" id="Page_7">[Pg 7]</a></span>space +would be the alternate production of lines of electric strain and +magnetic flux, the direction of these lines being reversed each half +cycle. Inside the rods we have a movement of electrons and +co-electrons to and fro, electric charges at the ends of the rods +alternating with electric currents in the rods, the charges being at a +maximum when the current is zero, and the current at a maximum when +the charges have for the moment disappeared. Outside the rods we have +a corresponding set of charges, lines of electric strain stretching +from end to end of the rod, alternating with rings of magnetic flux +embracing the rod. So far we have supposed the oscillation to be +relatively a slow one.</p> + +<div class="figcenter" style="width: 551px;"> +<img src="images/fig02.png" width="551" height="336" alt="FIG. 2.--SUCCESSIVE STAGES IN THE DEFORMATION OF A LINE +OF STRAIN BETWEEN POSITIVE AND NEGATIVE ELECTRONS IN RAPID +OSCILLATION, SHOWING CLOSED LOOP OF ELECTRIC STRAIN THROWN OFF." title="" /> +<span class="caption smcap">Fig. 2.—Successive Stages in the Deformation of a Line +of Strain between Positive and Negative Electrons in Rapid +Oscillation, showing Closed Loop of Electric Strain thrown off.</span> +</div> + +<p>Imagine next that the to and fro movement of the electrons or charges +is sufficiently rapid to bring into play the inertia quality of the +medium. We then have a different state of affairs. The lines of strain +in the external medium cannot contract or collapse quickly enough to +keep up with the course of events, or movements of the electrons in +the rods, and hence their regular contraction and absorption is +changed into a process of a different kind. As the electrons and +co-electrons, <i>i.e.</i>, the electric charges, vibrate to and fro, the +lines of electric strain connecting them are nipped in and thrown off +as completely independent and closed lines of electric strain, and at +each successive alternation, groups or batches of these loops of +strain are detached from the rod, and, so to speak, take on an +independent existence. The whole process of the formation of these +self-closed lines of electric strain is best understood by examining a +series of diagrams which roughly represent the various stages of the +process. In Fig. 2 we have a diagram (<i>a</i>) the curved line in which +delineates approximately the form of one line of electric strain round +a linear oscillator, with spark gap in the centre, one half being +charged positively and the other negatively. Let us then suppose that +the insulation of the spark gap is destroyed, so that the opposite +electric charges rush together and oscillate to and fro. The strain +lines at each oscillation <span class="pagenum"><a name="Page_8" id="Page_8">[Pg 8]</a></span>are then crossed or decussate, and the +result, as shown in Fig. 2, <i>d</i>, is that a portion of the energy of +the field is thrown off in the form of self-closed lines of strain +(see Fig. 2, <i>e</i>). At each oscillation of the charges the direction of +the lines of strain springing from end to end of the radiator is +reversed. It is a general property of lines of strain whether electric +or magnetic, that there is a tension along the line and a pressure at +right angles. In other words, these lines of electric strain are like +elastic threads, they tend to contract in the direction of their +length and press sideways on each other when in the same direction. +Hence it is not difficult to see that as each batch of self-closed +lines of strain is thrown off, the direction of the strain round each +loop is alternately in one direction and in the other. Hence these +loops of electric strain press each other out, and each one that is +formed squeezes the already formed loops further and further from the +radiator. The loops, therefore, march away into space (see Fig. 2, +<i>f</i>). If we imagine ourselves standing at a little distance at a point +on the equatorial line and able to see these loops of strain as they +pass, we should recognise a procession of loops, consisting of +alternately directed strain lines marching past. This movement through +the ether of self-closed lines of electric strain constitutes what is +called electric radiation.</p> + +<p>Hence along a line drawn perpendicular to the radiator through its +centre, there is a distribution of electric strain normal to that +line, which is periodic in space and in time. Moreover, in addition to +these lines of electric strain, there are at right angles to them +another set of self-closed lines of magnetic flux. Alternated between +the instants when the electric charges at the ends of the radiator are +at their maximum, we have instants when the radiator rod is the seat +of an electric current, and hence the field round it is filled with +circular lines of magnetic flux coaxial with the radiator. As the +current alternates in direction each half period, these rings of +magnetic flux alternate in direction as regards the flux, and hence we +must complete our mental picture of the space round the radiator rods +when the charges are oscillating by supposing it filled with +concentric rings of magnetic flux which are periodically reversed in +direction, and have their maximum values at those instants and places +where the lines of electric strain have their zero values. +Accordingly, along the equatorial line we have two sets of strains in +the ether, distributed periodically in space and in time. First, the +lines of electric strain in the plane of the radiator, and, secondly, +the lines of magnetic flux at right angles to these. At any one point +in space these two changes, the strain and the flux, succeed each +other periodically, being, however, at right angles in direction. At +any one moment these two effects are distributed periodically or +cyclically through space, and these changes in time and space +constitute an <i>electric wave</i> or electromagnetic wave.</p> + +<p>We may then summarise the above statements by saying that the most +recent hypothesis as to the nature of electrical action and of +electricity itself is briefly comprised in the following statements: +The universally diffused medium called the ether has had created in +it <span class="pagenum"><a name="Page_9" id="Page_9">[Pg 9]</a></span>certain centres of strain or radiating points from which proceed +lines of strain, and these centres of force are called electrons. +Electrons must, therefore, be of two kinds, positive and negative, +according to the direction of the strain radiating from the centre. +These electrons in their free condition constitute what we call +electricity, and the electrons themselves are the atoms of electricity +which, in one sense, is, therefore, as much material as that which we +call ordinary gross or ponderable matter.</p> + +<p>Collocations of these electrons constitute the atoms of gross matter, +and we must consider that the individuality of any atom is not +determined merely by the identity of the electrons composing it, but +by the permanence of their arrangement or form. In any mass of +material substance there is probably a continual exchange of electrons +from one atom to another, and hence at any one given moment, whilst a +number of the electrons are an association forming material atoms, +there will be a further number of isolated but intermingled electrons, +which are called the free electrons. In substances which we call good +conductors, we must imagine that the free electrons have the power of +moving freely through or between the material atoms, and this movement +of the electrons constitutes a current of electricity; whilst a +superfluity of electrons of either type in any one mass of matter +constitutes what we call a charge of electricity. Hence an electrical +oscillation, which is merely a very rapid alternating current taking +place in a conductor, is on this hypothesis assumed to consist in a +rapid movement to and fro of the free electrons. We may picture to +ourselves, therefore, a rod of metal in which electrical oscillations +are taking place, as similar to an organ-pipe or siren tube in which +movements of the air particles are taking place to and fro, the free +electrons corresponding with the air particles.</p> + +<div class="figright" style="width: 200px;"> +<img src="images/fig03.png" width="200" height="262" alt="FIG. 3.--SIMPLE MARCONI RADIATOR. B, battery; I, +induction coil; K, signalling key; S, spark gap; A, aerial wire; E, +earth plate." title="" /> +<span class="caption smcap">Fig. 3.—Simple Marconi Radiator.</span><span class="caption"> B, battery; I, +induction coil; K, signalling key; S, spark gap; A, aerial wire; E, +earth plate.</span> +</div> + +<p>Owing to the nature of the structure of an electron, it follows, +however, that every movement of an electron is accompanied by changes +in the distribution of the electric strain or ether strain taking +place throughout all surrounding space, and, as already explained, +certain very rapid movements of the electrons have the effect of +detaching closed lines of strain in the ether which move off through +space, forming, when cyclically distributed, an electric wave.</p> + +<p>We may next proceed to apply these principles to the explanation of +the action of the simplest form of Hertzian wave telegraphic radiator, +viz., the Marconi aerial wire. In its original form this consists of a +long vertical insulated wire, A, the lower end of which is attached to +one of the spark balls S of an induction coil, I, the other spark ball +being connected to earth E, and the two spark balls being placed a few +millimetres apart (see Fig. 3). When the coil is set in action +oscillatory or Hertzian sparks pass between the balls, electric +<span class="pagenum"><a name="Page_10" id="Page_10">[Pg 10]</a></span> +oscillations are set up in the wire and electric waves are radiated +from it. Deferring for the moment a more detailed examination of the +operations of the coil and at the spark gap, we may here say that the +action which takes place in the aerial wire is as follows: The wire is +first charged to a high potential, let us suppose, with negative +electricity. We may imagine this process to consist in forcing +additional electrons into it, the induction coil acting as an electron +pump. Up to a certain pressure the spark gap is a perfect insulator, +but at a critical pressure, which for spark gap lengths of four or +five millimetres and balls about one centimetre in diameter +approximates to <a name="tnd_10" id="tnd_10"></a><a href="#tn_10" class="tnlink" title="printer's error, millmetre for millimetre">three thousand volts per millmetre,</a> the insulation of +the air gives way, and the charge in the wire rushes into the earth. +In consequence, however, of the inertia of the medium or of the +electrons, the charge, so to speak, overshoots the mark, and the wire +is then left with a charge of opposite sign. This again in turn +rebounds, and so the wire is discharged by a series of electrical +oscillations, consisting of alternations of static charge and electric +discharge. We may fasten our attention either on the events taking +place in the vertical wire or in the medium outside, but the two sets +of phenomena are inseparably connected and go on together. When the +aerial wire is statically charged, we may describe it by saying that +there is an accumulation of electrons or co-electrons in it. Outside +the wire there is, however, a distribution of electric strain the +strain lines proceeding from the wire to the earth (see Fig. 4).</p> + +<div class="figleft" style="width: 287px;"> +<img src="images/fig04.png" width="287" height="259" alt="FIG. 4.--LINES OF ELECTRIC STRAIN (DOTTED LINES) +EXTENDING BETWEEN A MARCONI AERIAL, A, AND THE EARTH _ee_ BEFORE +DISCHARGE." title="" /> +<span class="caption smcap">Fig. 4.—Lines of Electric Strain (Dotted Lines) +extending between a Marconi Aerial, A, and the Earth</span> <span class="caption"><i>ee</i></span> <span class="caption smcap">before +Discharge.</span> +</div> + +<p>The wire has <i>capacity</i> with respect to the earth, and it acts like +the inner coating of a Leyden jar, of which the dielectric is the air +and ether around it, and the outer coating is the earth's surface. +When the discharge takes place, we may consider that electrons rush +out of the wire and then rush back again into it. At the moment when +the electrons rush out of or into the aerial wire, we say there is an +electric current flowing into or out of the wire, and this electron +movement, therefore, creates the magnetic flux which is distributed in +concentric circles round the wire. This current, and, therefore, +motion of electrons, can be proved to exist by its heating effect upon +a fine wire inserted in series with the aerial, and in the case of +large aerials it may have a mean value of many amperes and a maximum +value of hundreds of amperes. Inside the aerial wire we have, +therefore, alternations of electric potential or charge and electric +current, or we may call it electron-pressure and electron-movement.</p> + +<p>There is, therefore, an oscillation of electrons in the aerial wire, +just as in the case of an organ-pipe there is an oscillation of air +molecules in the pipe. Outside the aerial we have variations and +distributions of electric strain and magnetic flux. The resemblance +between the closed organ-pipe and the simple Marconi aerial is, in +<span class="pagenum"><a name="Page_11" id="Page_11">[Pg 11]</a></span> +fact, very complete. In the case of the closed organ-pipe, we have a +longitudinal oscillation of air molecules in the pipe. At the open end +or mouthpiece, where we have air moving in and out, the air movement +is alternating and considerable, but there is little or no variation +of air pressure. At the upper or closed end of the pipe we have great +variation of air pressure, but little or no air movement (see Fig. 5).</p> + +<p>Compare this now with the electrical phenomena of the aerial. At the +spark ball or lower end we have little or no variation of potential or +electron pressure, but we have electrons rushing into and out of the +aerial at each half oscillation, forming the electric discharge or +current. At the upper or insulated end we have little or no current, +but great variations of potential or electron pressure. Supposing we +could examine the wire inch by inch, all the way up from the spark +balls at the bottom to the top, we should find at each stage of our +journey that the range of variation and maximum value of the current +in the wire became less and those of the potential became greater. At +the bottom we have nearly zero potential or no electric pressure, but +large current, and at the top end, no current, but great variation of +potential.</p> + +<div class="figleft" style="width: 107px;"> +<img src="images/fig05.png" width="107" height="275" alt="FIG. 5.--AMPLITUDE OF PRESSURE VARIATION IN A CLOSED +ORGAN PIPE, INDICATED BY THE ORDINATES OF THE DOTTED LINE _xy_." title="" /> +<span class="caption smcap">Fig. 5.—Amplitude of Pressure Variation in a Closed +Organ Pipe, indicated by the Ordinates of the Dotted Line</span> <span class="caption"><i>xy</i></span><span class="caption smcap">.</span> +</div> + +<p>We can represent the amplitude of the current and potential values +along the aerial by the ordinates of a dotted line so drawn that its +distance from the aerial represents the potential oscillation or +current oscillation at that point (see Fig. 6).</p> + +<p>This distribution of potential and current along the wire does not +necessarily imply that any one electron moves far from its normal +position. The actual movement of any particular air molecule in the +case of a sound wave is probably very small, and reckoned in +millionths of an inch. So also we must suppose that any one electron +may have a small individual amplitude of movement, but the +displacement is transferred from one to another. Conduction in a solid +may be effected by the movement of free electrons intermingled with +the chemical atoms, but any one electron may be continually passing +from a condition of freedom to one of combination.</p> + +<div class="figright" style="width: 195px;"> +<img src="images/fig06.png" width="195" height="278" alt="FIG. 6.--(_a_) DISTRIBUTION OF ELECTRIC PRESSURE IN A +MARCONI AERIAL, A, (_b_) DISTRIBUTION OF ELECTRIC CURRENT IN A MARCONI +AERIAL, AS SHOWN BY THE ORDINATES OF THE DOTTED LINE _xy_." title="" /> +<span class="caption smcap">Fig. 6.—(</span><span class="caption"><i>a</i></span><span class="caption smcap">) Distribution of Electric Pressure in a +Marconi Aerial, A, (</span><span class="caption"><i>b</i></span><span class="caption smcap">) Distribution of Electric Current in a Marconi +Aerial, as shown by the Ordinates of the Dotted Line</span> <span class="caption"><i>xy</i></span><span class="caption smcap">.</span> +</div> + +<p>So much for the events inside the wire, but now outside the wire its +electric charge is represented by lines of electric strain springing +from the aerial to the earth. It must be remembered that every line of +strain terminates on an electron or a co-electron. Hence, when the +discharge or spark takes place between the spark balls, the rapid +movement of the electrons in the wire is accompanied by a +redistribution and movement of the lines <span class="pagenum"><a name="Page_12" id="Page_12">[Pg 12]</a></span>of strain outside. As the +negative charge flows out of the aerial the ends of the strain lines +abutting on to it run down the wire and are transferred to the earth, +and at the next instant this semi-loop of electric or ether strain, +with its ends on the earth, is pushed out sideways from the wire by +the growth of a new set of lines of ether strain in an opposite +direction. The process is best understood by consulting a series of +diagrams which represent the distribution and approximate form of a +few of the strain lines at successive instants (see Fig. 7). In +between the lines of formation of the successive strain lines between +the aerial and the earth, corresponding to the successive alternate +electric charges of the aerial with opposite sign, there are a set of +concentric rings of magnetic flux formed round it which are +alternately in opposite directions, and these expand out, keeping step +with the progress of the detached strain loops and having their planes +at right angles to the latter. As the semi-loops of electric strain +march outwards with their feet on the ground, these strain lines must +always be supposed to terminate on electrons, but not continually on +the same electrons. Since the earth is a conductor, we must suppose +that there is a continual migration of the electrons forming the atoms +of the earth, and that when one electron enters an atom, another +leaves it. Hence, corresponding to the electric wave in the space +above, there are electrical changes in the ground beneath. This view +is confirmed by the well-known fact that the achievement of Hertzian +wave telegraphy is much dependent on the nature of the surface over +which it is conducted, and can be carried on more easily over good +conducting material, like sea water, than over badly conducting dry +land.</p> + +<div class="figcenter" style="width: 426px;"> +<img src="images/fig07.png" width="426" height="462" alt="FIG. 7.--SUCCESSIVE STAGES IN THE PRODUCTION OF A +SEMI-LOOP OF ELECTRIC STRAIN BY A MARCONI AERIAL RADIATOR." title="" /> +<span class="caption smcap">Fig. 7.—Successive Stages in the Production of a +Semi-loop of Electric Strain by a Marconi Aerial Radiator.</span> +</div> + +<p>The matter may be viewed, however, from another standpoint. Good +conductors are opaque to Hertzian waves; in other words, are +<span class="pagenum"><a name="Page_13" id="Page_13">[Pg 13]</a></span> +non-absorptive. The energy of the electric wave is not so rapidly +absorbed when it glides over a sea surface as when it is passing over +a surface which is an indifferent conductor, like dry land. In fact, +it is possible by the improvement of the signals to detect a heavy +fall of rain in the space between two stations separated only by dry +land. It is, however, clear that on the electronic theory the +progression of the lines of electric strain can only take place if the +surface over which they move is a fairly good conductor, unless these +lines of strain form completely closed loops. Hence we may sum up by +saying that <a name="tnd_13a" id="tnd_13a"></a><a href="#tn_13a" class="tnlink" title="possible printer's error, set for sets">there are three set of phenomena</a> to which we must pay +attention in formulating any complete theory of the aerial. The first +is the operation taking place in the vertical wire, which is described +by saying that electrical oscillations or vibratory movements of +electrons are taking place in it, and, on our adopted theory, it may +be said to consist in a longitudinal vibration of electrons of such a +nature that we may appropriately call the aerial an ether organ-pipe. +Then in the next place, we have the distribution and movement of the +lines of electric strain and magnetic flux in the space outside the +wire, constituting the electric wave; and lastly, there are the +electrical changes in the conductor over which the wave travels, which +is the earth or water surrounding the aerial. In subsequently dealing +with the details of transmitting arrangements, attention will be +directed to the necessity for what telegraphists call a "good earth" +in connection with Hertzian wave telegraphy. This only means that +there must be a perfectly free egress and ingress for the electrons +leaving or entering the aerial, so that nothing hinders their access +to the conducting surface over which the wave travels. There must be +nothing to stop or throttle the rush of electrons into or out of the +aerial wire, or else the lines of strain cannot be <a name="tnd_13b" id="tnd_13b"></a><a href="#tn_13b" class="tnlink" title="printer's error, duplicate word">detached and and +travel away.</a></p> + +<p>We may next consider more particularly the energy which is available +for radiation and which is radiated. In the original form of simple +Marconi aerial, the aerial itself when insulated forms one coating or +surface of a condenser, the dielectric being the air and ether around +it, and the other conductor being the earth. The electric energy +stored up in it just before discharge takes place is numerically equal +to the product of the capacity of the aerial and half the square of +the potential to which it is charged.</p> + +<p>If we call C the capacity of the aerial in microfarads, and V the +potential in volts to which it is raised before discharge, then the +energy storage in joules E is given by the equation,</p> + +<div class="eq"> +<img class="tex" alt="E = \frac{CV^{2}}{2 \cdot 10^{6}}." src="images/eq_1.png"/> +</div> + +<p>Since one joule is nearly equal to three-quarters of a foot-pound, the +energy storage in foot-pounds F is roughly given by the rule +<span class="eq"><img class="tex" alt=" F =\frac{3}{8} CV^{2}/10^{6}" src="images/eq_2.png"/></span>. For spark lengths of the order of five to fifteen +millimetres, the disruptive voltage in air of ordinary pressure is at +the rate of 3,000 volts per millimetre. Hence, if S stands for the +spark length in millimetres, and C for the aerial capacity in +microfarads, it is easy to see that the energy storage in foot-pound +is</p> + +<div class="eq"> +<img class="tex" alt="F = \frac{27CS^{2}}{8}." src="images/eq_3.png"/> +</div> + +<p><span class="pagenum"><a name="Page_14" id="Page_14">[Pg 14]</a></span></p> +<p>If the aerial consists of a stranded wire formed of 7/22 and has a +length of 150 feet, and is insulated and held vertically with its +lower end near the earth, it would have a capacity of about one three +ten-thousandths of a microfarad or 0·0003 mfd.<a name="FNanchor_6_6" id="FNanchor_6_6"></a><a href="#Footnote_6_6" class="fnanchor">[6]</a> Hence, if it is used +as a Marconi aerial and operated with a spark gap of one centimetre in +length, the energy stored up in the wire before each discharge would +be only one-tenth (0·1) of a foot-pound.</p> + +<p>By no means can all of this energy be radiated as Hertzian waves; part +of it is dissipated as heat and light in the spark, and yet such an +aerial can, with a sensitive receiver such as that devised by Mr. +Marconi, make itself felt for a hundred miles over sea in every +direction. This fact gives us an idea of the extremely small energy +which, when properly imparted to the ether, can effect wireless +telegraphy over immense distances. Of course, the minimum telegraphic +signal, say the Morse dot, may involve a good many, perhaps +half-a-dozen, discharges of the wire, but even then the amount of +energy concerned in affecting the receiver at the distant place is +exceedingly small.</p> + +<p>The problem, therefore, of long-distance telegraphy by Hertzian waves +is largely, though not entirely, a matter of associating sufficient +energy with the aerial wire or radiator. There are obviously two +things which may be done; first, we may increase the capacity of the +aerial, and secondly, we may increase the charging voltage or, in +other words, lengthen the spark gap. There is, however, a well-defined +limit to this last achievement. If we lengthen the spark gap too much, +its resistance becomes too great and the spark ceases to be +oscillatory. We can make a discharge, but we obtain no radiation. When +using an induction coil, about a centimetre, or at most a centimetre +and a half, is the limiting length of oscillatory sparks; in other +words, our available potential difference is restricted to 30,000 or +40,000 volts. By other appliances we can, however, obtain oscillatory +sparks having a voltage of 100,000 or 200,000 volts, and so obtain +what Hertz called "active sparks" five or six centimetres in length.</p> + +<p>Turning then to the question of capacity, we may enquire in the next +place how the capacity of an aerial wire can be increased. This has +generally been done by putting up two or more aerial wires in +contiguity and joining them together, and so making arrangements +called in the admitted slang of the subject "multiple aerials." The +measurement of the capacity of insulated wires can be easily carried +out by means of an appliance devised by the author and Mr. W. C. +Clinton, consisting of a rotating commutator which alternately charges +the insulated wire at a source of known electromotive force and then +discharges it through a galvanometer. If this galvanometer is +subsequently standardised, so that the ampere value of its deflection +is known, we can determine easily the capacity C of the aerial or +insulated conductor, reckoned in microfarads, when it is charged to a +potential of V volts, and discharged <i>n</i> times a second through a +<span class="pagenum"><a name="Page_15" id="Page_15">[Pg 15]</a></span> +galvanometer. The series of discharges are equivalent to a current, of +which the value in amperes A is given by the equation</p> + +<div class="eq"> +<img class="tex" alt="A = \frac{nVC}{10^{6}}," src="images/eq_4.png"/> +</div> + +<p>and hence, if the value of the current resulting is known, we have the +capacity of the aerial or conductor expressed in microfarads, given by +the formula</p> + +<div class="eq"> +<img class="tex" alt="C = \frac{A10^{6}}{nV}." src="images/eq_5.png"/> +</div> + +<p>A series of experiments made on this plan have revealed the fact that +if a number of vertical insulated wires are hung up in the air and +rather near together, the electrical capacity of the whole of the +wires in parallel is not nearly equal to the sum of their individual +capacities. If a number of parallel insulated wires are separated by a +distance equal to about 3 per cent. of their length, the capacity of +the whole lot together varies roughly as the square root of their +number. Thus, if we call the capacity of one vertical wire in free +space unity, then the capacity of four wires placed rather near +together will only be about twice that of one wire, and that of +twenty-five wires will only be about five times one wire.</p> + +<div class="figright" style="width: 480px;"> +<img src="images/fig08.png" width="480" height="268" alt="FIG. 8.--VARIOUS FORMS OF AERIAL RADIATOR. _a_, single +wire; _b_, multiple wire; _c_, fan shape; _d_, cylindrical; _g_, +Conical." title="" /> +<span class="caption smcap">Fig. 8.—Various Forms of Aerial Radiator.</span> <span class="caption"><i>a</i>, single +wire; <i>b</i>, multiple wire; <i>c</i>, fan shape; <i>d</i>, cylindrical; <i>g</i>, +Conical.</span> +</div> + +<p>This approximate rule has been confirmed by experiments made with long +wires one hundred or two hundred feet in length in the open air. Hence +it points to the fact that the ordinary plan of endeavouring to obtain +a large capacity by putting several wires in parallel and not very far +apart is very uneconomical in material. The diagrams in Fig. 8 show +the various methods which have been employed by Mr. Marconi and others +in the construction of such multiple wire aerials. If, for instance, +we put four insulated stranded 7/22 wires each 100 feet long, about +six feet apart, all being held in a vertical position, the capacity of +the four together is not much more than twice that of a single wire. +In the same manner, if we arrange 150 similar wires, each 100 feet +long, in the form of a conical aerial, the wires being distributed at +the top round a circle 100 feet in diameter, the whole group will not +have much more than twelve times the capacity of one single wire, +although it weighs 150 times as much.</p> + +<p>The author has designed an aerial in which the wires, all of equal +length, are arranged sufficiently far apart not to reduce each other's +capacity.</p> + +<p><span class="pagenum"><a name="Page_16" id="Page_16">[Pg 16]</a></span></p> +<p>As a rough guide in practice, it may be borne in mind that a wire +about one tenth of an inch in diameter and one hundred feet long, held +vertical and insulated, with its bottom end about six feet from the +ground, has a capacity of 0·0002 of a microfarad, if no other earthed +vertical conductors are very near it. The moral of all this is that +the amount of electric energy which can be stored up in a simple +Marconi aerial is very limited, and is not much more than one-tenth of +a joule or one-fourteenth of a foot-pound, per hundred feet of 7/22 +wire. The astonishing thing is that with so little storage of energy +it should be possible to transmit intelligence to a distance of a +hundred miles without connecting wires.</p> + +<p>One consequence, however, of the small amount of energy which can be +accumulated in a simple Marconi aerial is that this energy is almost +entirely radiated in one oscillation or wave. Hence, strictly +speaking, a simple aerial of this type does not create a train of +waves in the ether, but probably at most a single impulse or two.</p> + +<div class="figleft" style="width: 247px;"> +<img src="images/fig09.png" width="247" height="251" alt="FIG. 9.--MARCONI-BRAUN SYSTEM OF INDUCING ELECTROMOTIVE +FORCE IN AN AERIAL, A. B, battery; K, key; I, induction coil; S, spark +gap; C, Leyden jar; E, earth plate; _ps_, oscillation transformer." title="" /> +<span class="caption smcap">Fig. 9.—Marconi-Braun System of inducing Electromotive +Force in an Aerial,</span><span class="caption"> A. B, battery; K, key; I, induction coil; S, spark +gap; C, Leyden jar; E, earth plate; <i>ps</i>, oscillation transformer.</span> +</div> + +<p>We shall later on consider some consequences which follow from this +fact. Meanwhile, it may be explained that there are methods by which +not only a much larger amount of energy can be accumulated in +connection with an aerial, but more sustained oscillations created +than by the original Marconi method. One of these methods originated +with Professor Braun, of Strasburg, and a modification was first +described by Mr. Marconi in a lecture before the Society of Arts of +London.<a name="FNanchor_7_7" id="FNanchor_7_7"></a><a href="#Footnote_7_7" class="fnanchor">[7]</a> In this method the charge in the aerial is not created by +the direct application to it of the secondary electromotive force of +an induction coil, but by means of an induced electromotive force +created in the aerial by an oscillation transformer. The method due to +Professor Braun is as follows: A condenser or Leyden jar has one +terminal, say, its inside, connected to one spark ball of an induction +coil. The other spark ball is connected to the outside of the Leyden +jar or condenser through the primary coil of a transformer of a +particular kind, called an oscillation transformer (see Fig. 9). The +spark balls are brought within a few millimetres of each other. When +the coil is set in operation, the jar is charged and discharged +through the spark gap, and electrical oscillations are set up in the +circuit consisting of the dielectric of the jar, the primary coil of +the oscillation transformer and the spark gap. The secondary circuit +of this oscillation transformer is connected in between the earth and +the insulated aerial wire; hence, when the oscillations take place in +the primary circuit, they induce other oscillations in the aerial +circuit. But the arrangement is not very effective unless, <span class="pagenum"><a name="Page_17" id="Page_17">[Pg 17]</a></span>as is +shown by Mr. Marconi, the two circuits of the oscillation transformer +are tuned together.</p> + +<p>We shall return presently to the consideration of this form of +transmitter; meanwhile we may notice that by means of such an +arrangement it is possible to create in the aerial a far greater +charging electromotive force than would be the case if the aerial were +connected directly to one terminal of the secondary circuit of the +induction coil, the other terminal being to earth, and the two +terminals connected as usual by spark balls. By the inductive +arrangement it is possible to create in an aerial electromotive forces +which are equivalent to a spark of a foot in length, and when the +length of the aerial is also properly proportioned the potential along +it will increase all the way up, until at the top or insulated end of +the aerial it may reach an amount capable of giving sparks several +feet in length. From the remarks already made on the analogy between +the closed organ-pipe and the Marconi aerial wire, it will be seen +that the wave which is radiated from the aerial must have a wave +length four times that of the aerial if the aerial is vibrating in its +fundamental manner. It is also possible to create electrical +oscillations in a vertical wire which are the harmonics of the +fundamental.</p> + +<p>All musicians are aware that in the case of an organ-pipe if the pipe +is blown gently it sounds a note which is called the fundamental of +the pipe. The celebrated mathematician, Daniel Bernouilli, discovered +that an organ-pipe can be made to yield a succession of musical notes +by properly varying the pressure of the current of air blown into it. +If the pipe is an open pipe, and if we call the frequency of the +primary note obtained when the pipe is gently blown, unity, then when +we blow more strongly the pipe yields notes which are the harmonics of +the fundamental one; that is to say, notes which have frequencies +represented by the numbers 2, 3, 4, 5, &c. If, however, the pipe is +closed at the top, then over-blowing the pipe makes it yield the odd +harmonics or the tones which are related to the primary tone in the +ratio of 3, 5, 7, &c., to unity. Accordingly, if a stopped pipe gives +as its fundamental the note C, its first overtone will be the fifth +above the octave or G'.</p> + +<div class="figleft" style="width: 248px;"> +<img src="images/fig10.png" width="248" height="277" alt="FIG. 10.--SEIBT'S APPARATUS FOR SHOWING STATIONARY +WAVES IN LONG SOLENOID A. I, induction coil; S, spark gap; L, +inductance coil; C_{1}C_{2}, Leyden jars; E, earth wire." title="" /> +<span class="caption smcap">Fig. 10.—Seibt's Apparatus for showing Stationary +Waves in long Solenoid A.</span><span class="caption"> I, induction coil; S, spark gap; L, +inductance coil; C<sub>1</sub>C<sub>2</sub>, Leyden jars; E, earth wire.</span> +</div> + +<p>As already remarked, the aerial wire or radiator as used in Marconi +telegraphy may be looked upon as a kind of ether organ-pipe or siren +tube, and its electrical phenomena are in every respect similar to the +acoustic phenomena of the ordinary closed organ-pipe. When the aerial +is sounding its fundamental ether note, the conditions which pertain +are that there is a current flowing into the aerial at the lower end, +but at that point the variation in potential is very small, whereas at +the upper end there is no current, but the variations of potential are +very large. Accordingly, we say that at the upper end of the aerial +there is an antinode of potential and a node of current, and at the +bottom an antinode of current and a node of potential. By altering the +frequency of the electrical impulses we can create in the aerial an +arrangement of nodes of current or potential corresponding to the +overtones of a closed organ-pipe. But whatever may be the arrangement +the conditions must always hold that there is a node of current at +the upper end and an antidote of current <span class="pagenum"><a name="Page_18" id="Page_18">[Pg 18]</a></span>at the lower end. In other +words, there are large variations of current at the place where the +aerial terminates on the spark-gap and no current at the upper end. +The first harmonic is formed where there is a node of potential at +one-third of the length of the aerial from the top. In this case we +have a node of potential not only at the lower end of the wire, but at +two-thirds of the way up. In the same way we can create in the closed +organ-pipe, by properly overblowing the pipe, a region about +two-thirds of the way up the pipe, where the pressure changes in the +air are practically no greater than they are at the mouthpiece. We can +make evident visually in a beautiful manner the existence of similar +stationary electrical waves in an aerial by means of an ingenious +arrangement devised by Dr. Georg Seibt, of Berlin. It consists of a +very long silk-covered copper wire, A (see Fig. 10), wound in a close +spiral of single layer round a wooden rod six feet long and about two +inches in diameter. This rod is insulated, and at the lower end the +wire is connected to a Leyden jar circuit, consisting of a Leyden jar +or jars and an inductance coil, L, the inductance of which can be +varied. Oscillations are set up in this jar circuit by means of an +induction-coil discharge, and the lower end of the long spiral wire is +attached to one point on the jar circuit. In this manner we can +communicate to the bottom end of the long spiral wire a series of +electric impulses, the time period of which depends upon the capacity +of the jar and the inductance of the discharge circuit. We can, +moreover, vary this frequency over wide limits. Parallel to the long +spiral wire is suspended another copper wire, E (see Fig. 10), and +between this wire and the silk-covered copper wire discharges take +place due to the potential difference between each part of the wire +and this long aerial wire. If we arrange matters so that the impulses +communicated to the bottom end of the long spiral wire correspond to +its fundamental note or periodic time, then in a darkened room we +shall see a luminous glow or discharge between the vertical wire and +the spiral wire, which increases in intensity all the way up to the +top of the spiral wire. The luminosity of this brush discharge at any +point is evidence of the potential of the spiral wire at that point, +and its distribution clearly demonstrates that the difference of +potential between the spiral wire and the aerial increases all the way +up from the bottom to the top of the spiral wire. In the next place, +by making a little adjustment and by varying the inductance of the jar +circuit, we can increase the frequency of the impulses which are +falling upon the spiral wire; and then it will be noticed that the +distribution of the brush discharge or luminosity is altered, and that +there is a <span class="pagenum"><a name="Page_19" id="Page_19">[Pg 19]</a></span>maximum now at about one-third of the height of the spiral +wire, and a dark place at about two-thirds of the height, and another +bright place at the top, thus showing that we have a node of potential +at about two-thirds the way up the wire (see Fig. 11), and we have +therefore set up in the spiral wire electrical oscillations +corresponding to the first overtone. It is possible to show in the +same way the existence of the second harmonic in the coil, but the +luminosity then becomes too faint to be seen at a distance.</p> + +<div class="figleft" style="width: 255px;"> +<img src="images/fig11.png" width="255" height="307" alt="FIG. 11.--HARMONIC OSCILLATIONS IN LONG SOLENOID SHOWN +WITH SEIBT'S APPARATUS." title="" /> +<span class="caption smcap">Fig. 11.—Harmonic Oscillations in Long Solenoid shown +with Seibt's Apparatus.</span> +</div> + +<p>An interesting form of aerial devised by Professor Slaby, of Berlin, +depends for its action entirely on the fact that the electrical +oscillations set up in it which radiate are harmonics of the +fundamental tone.</p> + +<div class="figright" style="width: 111px;"> +<img src="images/fig12.png" width="111" height="261" alt="FIG. 12.--NON-RADIATIVE CLOSED LOOP AERIAL." title="" /> +<span class="caption smcap">Fig. 12.—Non-radiative Closed Loop Aerial.</span> +</div> + +<div class="figleft" style="width: 126px;"> +<img src="images/fig13.png" width="126" height="259" alt="FIG. 13.--SLABY'S LOOP RADIATOR." title="" /> +<span class="caption smcap">Fig. 13.—Slaby's Loop Radiator.</span> +</div> + +<p>A closed vertical loop, A<sub>1</sub>A<sub>2</sub> (see Fig. 12), is formed by +erecting two parallel insulated wires vertically a few feet apart and +joining them together at the top. At the bottom these wires are +connected, with the secondary terminals of an induction coil, a +condenser, C, or Leyden jar, being bridged across the terminals and a +pair of spark balls, S, inserted in one side of the loop. It will +readily be seen that on setting the coil in action, oscillations will +take place in these vertical wires, but that if the oscillations are +simply the fundamental note of the system, then at any moment +corresponding to a current going up one side of the loop of wire there +must be a current coming down the other. Accordingly, an arrangement +of this kind, forming what is called a closed circuit, will not +radiate or radiates but very feebly. Professor Slaby found, however, +that it might be converted into a powerful radiator if we give the two +sides of the loop unequal capacity or inductance and at the same time +earth one of the lower ends of the loop, as shown in Fig. 13. By this +means it is possible to set up in the loop electrical overtones or +harmonics of the fundamental oscillation, and if we cause the system +to vibrate so as to produce its first odd harmonic, there is a +potential node at the lower end of both vertical sides of the loop, a +potential node on both vertical sides at two-thirds of the way up, and +a potential antinode at the summit of the loop; then, under these +circumstances, the closed loop of wire is in the same electrical +condition as if two simple Marconi aerials, both emitting their first +odd harmonic oscillation, were placed side by side and joined together +at the top.</p> + +<p><span class="pagenum"><a name="Page_20" id="Page_20">[Pg 20]</a></span></p> +<p>It is a little difficult without the employment of mathematical +analysis to explain precisely the manner in which earthing one side of +the loop or making the loop unsymmetrical as regards inductance has +the effect of creating overtones in it. The following rough +illustration may, however, be of some assistance. Imagine a long +spiral metallic spring supported horizontally by threads. Let this +represent a conductor, and let any movement to or fro of a part of the +spring represent a current in that conductor. Suppose we take hold of +the spring at one end, we can move it bodily to and fro as a whole. In +this case, every part of the spring is moving one way or the other in +the same manner at the same time. This corresponds with the case in +which the discharge of the condenser through the uniform loop +conductor is a flow of electricity, all in one direction one way or +the other. The current is in the same direction in all parts of the +loop at the same time, and, therefore, if the current is going up one +side of the loop it is at the same time coming down the other side. +Hence the two sides of the loop are always in exact opposition as +regards the effect of the current in them on the external space, and +the loop does not radiate. Returning again to the case of the spring. +Supposing that we add a weight to one end of the spring by attaching +to it a metal ball, and then move the other end to and fro with +certain periodic motion, it will be found quite easy to set up in the +spring a pulsatory motion resembling the movement of the air in an +open organ-pipe. Under these circumstances both ends of the spring +will be moving inwards or outwards at the same time, and the central +portions of the spring, although being pressed and expanded slightly, +are moving to and fro very little. This corresponds in the case of the +looped aerial with a current flowing up or down both sides at the same +time; in other words, when this mode of electrical oscillation is +established in the loop, its electrical condition is just that of two +simple Marconi aerials joined together at the top and vibrating in +their fundamental manner. Accordingly, if one side of the double loop +is earthed, we then have an arrangement which radiates waves. +Professor Slaby found that by giving one side of the loop less +inductance than the other, and at the same time earthing the side +having greater inductance at the bottom, he was able to make an +arrangement which radiated, not in virtue of the normal oscillations +of the condenser, but in virtue of the harmonic oscillations set up in +the conductor itself. The mathematical theory of this radiator has +been very fully developed by Dr. Georg Seibt.</p> + +<p>It will be seen, therefore, that there are several ways in which we +may start into existence oscillations in an aerial. First, the aerial +may be insulated, and we may charge it to a high potential and allow +this charge suddenly to rush out. Although this process gives rise to +a disturbance in the ether, as already explained, it is analogous to a +pop or explosion in the air, rather than to a sustained musical note. +The exact acoustic analogue would be obtained if we imagine a long +pipe pumped full of air and then suddenly opened at one end. The air +would rush out, and, communicating a blow to the outer air, would +create an atmospheric disturbance appreciated as a noise or small +explosion. This is what happens when we cut the string and let the +cork <span class="pagenum"><a name="Page_21" id="Page_21">[Pg 21]</a></span>fly out from a bottle of champagne. At the same time, the +inertia of the air rushing out of the tube would cause it to overshoot +the mark, and a short time after opening the valve the tube, so far +from containing compressed air, would contain air slightly rarefied +near its mouth, and this rarefication would travel back up the tube in +the form of wave motion, and, being reflected as condensation at the +closed end, travel down again; and so after being reflected once or +twice at the open or closed end, become damped out very rapidly in +virtue of both air friction and the radiation of the energy. In the +case, however, of the ordinary organ-pipe, we do not depend merely +upon a store of compressed air put into the pipe, but we have a store +of energy to draw upon in the form of the large amount of compressed +air contained in a wind chest, which is being continually supplied by +the bellows. This store of compressed air is fed into the organ-pipe, +with the result that we obtain a continuous radiation of sound waves. +The first case, in which the only store of energy is the compressed +air originally contained in the pipe, illustrates the operation of the +simple Marconi aerial. The second case, in which there is a larger +store of energy to draw upon, the organ-pipe being connected to a wind +chest, illustrates the Marconi-Braun method, in which an aerial is +employed to radiate a store of electric energy contained in a +condenser, gradually liberated by the aerial in the form of a series +of electrical oscillations and waves. In this arrangement the +condenser corresponds to the wind chest, and it is continually kept +full of electrical energy by means of the induction coil or +transformer, which answers to the bellows of the organ. From the +condenser, electrical energy is discharged each time the spark +discharge passes at a spark gap in the form of electrical oscillations +set up in the primary circuit of an oscillation transformer. The +secondary circuit of this transformer is connected in between the +earth and the aerial, and therefore may be considered as part of it, +and, accordingly, the energy which is radiated from the aerial is not +simply that which is stored up in it in virtue of its own small +capacity, but that which is stored up in the much larger capacity +represented by the primary condenser or, as it may be called, the +electrical wind chest. By the second arrangement we have therefore the +means of radiating more or less continuous trains of electric waves, +corresponding with each spark discharge. To create powerful +oscillations in the aerial, one condition of success is that there +shall be an identity in time-period between the circuit of the aerial +and that of the primary condenser. The aerial is an open circuit which +has capacity with respect to the earth, and it has also inductance, +partly due to the wire of the aerial and partly due to the secondary +circuit of the oscillation transformer in series with it. The primary +circuit or spark circuit has capacity—viz., the capacity of the +energy-storing condenser—and it has also inductance—viz., the +inductance of the primary circuit of the oscillation transformer. We +shall consider at a later stage more particularly the details of +syntonising arrangements, but meanwhile it may be said that one +condition for setting up powerful waves by means of the above +arrangement is that the electrical time-period of both the two +circuits mentioned shall be the same. This involves adjusting the +inductance and capacity so <span class="pagenum"><a name="Page_22" id="Page_22">[Pg 22]</a></span>that the product of conductance and +capacity for each of these two circuits is numerically the same. +Instead of employing an oscillation transformer between the condenser +circuit and the aerial, the aerial may be connected directly to some +point on the condenser circuit at which the potential oscillations are +large, and we have then another arrangement devised by Professor Braun +(see Fig. 14). In this case, in order to accumulate large potential +oscillations at the top of the aerial, it is, as we have seen, +necessary that the length of the aerial shall be one quarter the +length of the wave. If, therefore, the electrical oscillations in the +condenser circuit are at the rate of N per second, in other words, +have a frequency N, the wave-length <a name="tnd_22a" id="tnd_22a"></a><a href="#tn_22a" class="tnlink" title="printer's error, correponding for corresponding">correponding to this frequency</a> is +given by the expression,</p> + +<div class="eq"> +<img class="tex" alt="3 \times 10^{10}/N cms." src="images/eq_6.png"/> +</div> + +<div class="figleft" style="width: 237px;"> +<img src="images/fig14.png" width="237" height="204" alt="FIG. 14.--BRAUN'S RADIATOR. B, battery; I, induction +coil; K, key; S, spark-gap; L, inductance coil; C, condenser; A, +aerial." title="" /> +<span class="caption smcap">Fig. 14.—Braun's Radiator.</span><span class="caption"> B, battery; I, induction +coil; K, key; S, spark-gap; L, inductance coil; C, condenser; A, +aerial.</span> +</div> + +<p>The number 3×10<sup>10</sup> is the value in centimetres per second of the +velocity of the electromagnetic wave, and is identical with that of +light. The corresponding resonant length of the aerial is therefore +one-fourth of this wave-length, or <span class="eq"><img class="tex" alt="3 \times 10^{10}/4N" src="images/eq_7.png"/></span>. Generally speaking, +however, it will be found that with any length of aerial which is +practicable, say, 200 feet or 6,000 cms., this proportion necessitates +rather a high frequency in the primary oscillation circuit. In the +case considered—viz., for an aerial 200 feet in height—the +oscillations in the primary circuit must have a frequency of one and a +quarter million. This high frequency can only be obtained either by +greatly reducing the inductance of the primary discharge circuit, or +reducing the capacity. If we reduce the capacity, we thereby greatly +reduce the storage of energy, and it is not practicable to reduce the +inductance below a certain amount.</p> + +<p>Summing up, it may be said that there are three, and, as far as the +writer is aware, at present only three, modes of exciting the +electrical oscillations in an aerial wire. First, the aerial may +itself be used as an electrical reservoir and charged to a high +potential and suddenly discharged to the earth. This is the original +Marconi method. The second method, <a name="tnd_22b" id="tnd_22b"></a><a href="#tn_22b" class="tnlink" title="printer's error, consist for consists">due to Braun, consist of attaching</a> +the aerial to some point on an oscillation circuit consisting of a +condenser, an inductance coil and a spark gap, in series with one +another, and charging and discharging the condenser across the spark +gap so as to create alterations of potential at some point on the +oscillation circuit. The length of the aerial must then be so +proportioned as above described that it is resonant to this frequency. +Thirdly, we may employ the arrangement involving an oscillation +transformer, in which the oscillations in the primary condenser +circuit are made to induce others in the aerial circuit, the +time-period of the two circuits being the same. This method may be +called the Braun-Marconi method. Professor Slaby has combined together +in a certain way the original Marconi simple aerial with the <span class="pagenum"><a name="Page_23" id="Page_23">[Pg 23]</a></span>resonant +quarter-wave-length wire of Braun. He constructs what he calls a +<i>multiplicator</i>, which is really a wire wound into a loose spiral +connected at one point to an oscillation circuit consisting of a +condenser inductance, the length of this wire being proportioned so +that there is a great resonance or multiplication of tension or +potential at its free end. This free end is then attached to the lower +end of an ordinary Marconi aerial, and serves to charge it with a +higher potential than could be obtained by the use of the induction +coil directly attached to it.</p> + +<hr style="width: 45%;" /> + +<p>We have next to consider the appliances for creating the necessary +charging electromotive force, and for storing and releasing this +charge at pleasure, so as to generate the required electrical +oscillations in the aerial.</p> + +<p>It is essential that this generator should be able to create not only +large potential difference, but also a certain minimum electric +current. Accordingly, we are limited at the present moment to one of +two appliances—viz., the induction coil or the alternating current +transformer.</p> + +<p>It will not be necessary to enter into an explanation of the action of +the induction coil. The coil generally employed for wireless +telegraphy is technically known as a ten-inch coil—<i>i.e.</i>, a coil +which is capable of giving a ten-inch spark between pointed conductors +in air at ordinary pressure. The construction of a large coil of this +description is a matter requiring great technical skill, and is not to +be attempted without considerable previous experience in the +manufacture of smaller coils. The secondary circuit of a ten-inch coil +is formed of double silk-covered copper wire; generally speaking, the +gauge called No. 36, or else No. 34 S.W.G. is used, and a length of +ten to seventeen miles of wire is employed on the secondary circuit, +according to the gauge of wire selected. For the precautions necessary +in constructing the secondary coil, practical manuals must be +consulted.<a name="FNanchor_8_8" id="FNanchor_8_8"></a><a href="#Footnote_8_8" class="fnanchor">[8]</a></p> + +<p>Very great care is required in the insulation of the secondary circuit +of an induction coil to be used in Hertzian wave telegraphy, because +the secondary circuit is then subjected to impulsive electromotive +forces lasting for a short time, having a much higher electromotive +force than that which the coil itself normally produces.</p> + +<p>The primary circuit of a ten-inch coil generally consists of a length +of 300 or 400 feet of thick insulated copper wire. In such a coil the +secondary circuit would require about ten miles of No. 34 H.C. copper +wire, making 50,000 turns round the core. It would have a resistance +at ordinary temperatures of 6,600 ohms, and an inductance of 460 +henrys. The primary circuit, if formed of 360 turns of No. 12 H.C. +copper wire, would have a resistance of 0·36 of an ohm, and an +inductance of 0·02 of a henry.</p> + +<p><span class="pagenum"><a name="Page_24" id="Page_24">[Pg 24]</a></span></p> +<p>An important matter in connection with an induction coil to be used +for wireless telegraphy is the resistance of the secondary circuit. +The purpose for which we employ the coil is to charge a condenser of +some kind. If a constant electromotive force (V) is applied to the +terminals of a condenser having a capacity C, then the difference of +potential (<i>v</i>) of the terminals of the condenser at any time that the +contact is made is given by the expression:</p> + +<div class="eq"> +<img class="tex" alt="v = V(1 - e^{-t/RC})." src="images/eq_8.png"/> +</div> + +<p>In the above equation, the letter e stands for the number 2·71828, the +base of the Napierian logarithms, and R is the resistance in series +with the condenser, of which the capacity is C, to which the +electromotive force is applied. This equation can easily be deduced +from first principles,<a name="FNanchor_9_9" id="FNanchor_9_9"></a><a href="#Footnote_9_9" class="fnanchor">[9]</a> and it shows that the potential difference +<i>v</i> of the terminals of the condenser does not instantly attain a +value equal to the impressed electromotive force V, but rises up +gradually. Thus, for instance, suppose that a condenser of one +microfarad is being charged through a resistance of one megohm by an +impressed voltage of 100 volts, the equation shows that at the end of +the first second after contact, the terminal potential difference of +the condenser will be only 63 volts, at the end of the second second, +86 volts, and so on.</p> + +<p>Since e<sup>-10</sup> is an exceedingly small number, it follows that in 10 +seconds the condenser would be practically charged with a voltage +equal to 100 volts. The product CR in the above equation is called the +<i>time-constant</i> of the condenser, and we may say that the condenser is +practically charged after an interval of time equal to ten times the +time-constant, counting from the moment of first contact between the +condenser and the source of constant voltage. The time-constant is to +be reckoned as the product of the capacity (C) in microfarads, by the +resistance of the charging circuit (R) in megohms. To take another +illustration. Supposing we are charging a condenser having a <a name="tnd_24" id="tnd_24"></a><a href="#tn_24" class="tnlink" title="printer's error, one-hundreth for one-hundredth">capacity +of one-hundreth of a microfarad,</a> through a resistance of ten thousand +ohms. Since ten thousand ohms is equal to one-hundredth of a megohm, +the time-constant would be equal to one-ten-thousandth of a second, +and ten times this time-constant would be equal to a thousandth of a +second. Hence, in order to charge the above capacity through the above +resistance, it is necessary that the contact between the source of +voltage and the condenser should be maintained for at least +one-thousandth part of a second.</p> + +<p>In discussing the methods of interrupting the circuit, we shall return +to this matter, but, meanwhile, it may be said that in order to secure +a small time-constant for the charging circuit, it is desirable that +the secondary circuit of the induction coil should have as low a +resistance as possible. This, of course, involves winding the +secondary circuit with a rather thick wire. If, however, we employ a +wire larger in size than No. 34, or at the most No. 32, the bulk and +the cost of the induction coil began to rise very rapidly. Hence, as +in all other <span class="pagenum"><a name="Page_25" id="Page_25">[Pg 25]</a></span>departments of electrical construction, the details of +the design are more or less a matter of compromise. Generally +speaking, however, it may be said that the larger the capacity which +is to be charged, the lower should be the resistance of the secondary +circuit of the induction coil.</p> + +<p>In the practical construction of induction coils for wireless +telegraphy, manufacturers have departed from the stock designs. We are +all familiar with the appearance of the instrument maker's induction +coil; its polished mahogany base, its lacquered brass fittings, and +its secondary bobbin constructed of and covered with ebonite. But such +a coil, although it may look very pretty on the lecture table, is yet +very unsuited to positions in which it may be used in connection with +Hertzian wave telegraphy.</p> + +<p>Three important adjuncts of the induction coil are the primary +condenser, the interrupter and the primary key. The interrupter is the +arrangement for intermitting the primary current. We have in some way +or other to rapidly interrupt the primary current, and the torrent of +sparks that then appears between the secondary terminals of the coil +is due to the electromotive force set up in the secondary circuit at +each break or interruption of the primary circuit. We may divide +interrupters into five classes.</p> + +<p>We have first the well-known hammer interrupter which Continental +writers generally attribute to Neef or Wagner.<a name="FNanchor_10_10" id="FNanchor_10_10"></a><a href="#Footnote_10_10" class="fnanchor">[10]</a> In this +interrupter, the magnetisation of the iron core of the coil is caused +to attract a soft-iron block fixed at the top of a brass spring, and +by so doing to interrupt the primary circuit between two platinum +contacts. Mr. Apps, of London, added an arrangement for pressing back +the spring against the back contact, and the form of hammer that is +now generally employed is therefore called an Apps break.</p> + +<p>As the ten-inch coil takes a primary current of ten amperes at sixteen +volts when in operation, it requires very substantial platinum +contacts to withstand the interruption of this current continuously +without damage. The small platinum contacts that are generally put on +these coils by instrument makers are very soon worn out in practical +wireless telegraph work. If a hammer break is used at all, it is +essential to make the contacts of very stout pieces of platinum, and +from time to time, as they get burnt away or roughened, they must be +smoothed up with a fine file. It does not require much skill to keep +the hammer contacts in good order and prevent them from sticking +together and becoming damaged by the break spark.</p> + +<p>By regulating the pressure of the spring against the back contact, by +means of an adjusting screw, the rate at which the break vibrates can +be regulated, but as a rule it is not possible, with a hammer break, +to obtain more than about 800 interruptions per minute, or, say, +twelve a second. The hammer break is usually operated by the magnetism +of the iron core of the coil, but for some reasons it is better to +separate the break from the coil altogether, and to work it <span class="pagenum"><a name="Page_26" id="Page_26">[Pg 26]</a></span>by an +independent electromagnet, which, however, may be excited by a current +from the same battery supplying the induction coil. For coils up to +the ten-inch size the hammer break can be used when very rapid +interruptions are not required. It is not in general practicable to +work coils larger than the ten-inch size with a platinum contact +hammer break, as such a butt contact becomes overheated and sticks if +more than ten amperes is passed through it. In the case of larger +coils, we have to employ some form of interrupter in which mercury or +a conducting liquid forms one of the contact surfaces.</p> + +<p>The next class of interrupter is the vibrating or hand-worked mercury +break, in which a platinum or steel pin is made to vibrate in and out +of mercury. This movement may be effected by the attraction of an iron +armature by an electromagnet, or by the varying magnetism of the core +of the coil, or it may be effected more slowly by hand.</p> + +<p>The mercury surface must be covered with water, alcohol, paraffin or +creosote oil to prevent oxidation and to extinguish the break spark. +The interruption of the primary current obtained by the mercury break +is more sudden than that obtained by the platinum contact in air, in +consequence of the more rapid extinction of the spark; hence the +sparks obtained from coils fitted with mercury interrupters are +generally from twenty to thirty per cent. longer than those obtained +from the same coil under the same conditions, with platinum contact +interrupters. The mercury breaks will not, however, work well unless +cleaned at regular intervals by emptying off the oil and rinsing well +with clean water, and hence they require rather more attention than +platinum interrupters. It is not generally possible to obtain so many +interruptions per minute with the simple vibrating mercury interrupter +as with the ordinary hammer interrupter. The mercury interrupter has, +however, the advantage that the contact time during which the circuit +is kept closed may be made longer than is the case with the hammer +break. Also, if fresh water is allowed to flow continuously over the +mercury surface, it can be kept clean, and the break will then operate +for considerable periods of time without attention. The mercury +interrupter may be worked by a separate electromagnet or by the +magnetism of the core of the induction coil.</p> + +<p>The third class of interrupter may be called the motor interrupter, of +which a large number have been invented in recent years. In this +interrupter some form of a continuously-rotating electromotor is +employed to make and break a mercury or other liquid contact. In one +simple form the motor shaft carries an eccentric, which simply dips a +platinum point into mercury, or else a platinum horseshoe into two +mercury surfaces, making in this manner an interruption of the primary +circuit at one or two places. As a small motor can easily be run at +twelve hundred revolutions per minute, or twenty per second, it is +possible to secure easily in this manner a uniform rate of +interruption of the primary current at the rate of about twenty per +second. If, however, much higher speeds are employed, then the time of +contact becomes abbreviated, and the ability of the coil to charge a +capacity is diminished.</p> + +<p><span class="pagenum"><a name="Page_27" id="Page_27">[Pg 27]</a></span></p> +<p>Professor J. Trowbridge has described an effective form of motor break +for large coils, in which the interruption is caused by withdrawing a +stout platinum wire from a dilute solution of sulphuric acid, and by +this means he increased the spark given by a coil provided with hammer +break and condenser from fifteen inches to thirty inches when using +the liquid break and no condenser.<a name="FNanchor_11_11" id="FNanchor_11_11"></a><a href="#Footnote_11_11" class="fnanchor">[11]</a></p> + +<p>A good form of motor-interrupter, due to Dr. Mackenzie Davidson, +consists of a slate disc bearing pin contacts fixed on the prolonged +steel axle of a motor placed in an inclined position; the disc and the +lower part of the axle lie in a vessel filled one-third with mercury +and two-thirds with paraffin oil. The circuit is made and broken by +the revolution of the disc causing the pins to enter and leave the +mercury. The speed of the motor can be regulated by a small +resistance, and can be adapted to the electromotive force used in the +primary circuit. When the motor is running slowly the interrupter can +be used with a low electromotive force, that is to say, something +between twelve and twenty volts, but with a higher speed a large +electromotive force can be used without danger of overheating the +primary coil, and with an electromotive force of about fifty volts, +the interruptions may be so rapid that an unbroken arc of flame, +resembling an alternating-current arc, springs between the secondary +terminals of the coil.</p> + +<p>Mr. Tesla has devised numerous forms of rotating mercury break. In +one, a star-shaped metal disc revolves in a box so that its points dip +into mercury covered with oil, and make and break contact. In another +form, a jet of mercury plays against a similar shaped rotating wheel. +For details, the reader must consult the fuller descriptions in <i>The +Electrical World</i> of New York, Vol. XXXII., p. 111, 1898; also Vol. +XXXIII., p. 247; or <i>Science Abstracts</i>, Vol. II., pp. 46 and 47, +1898.</p> + +<p>The fourth class of interrupter is called a turbine interrupter. In +this appliance, a jet of mercury is forced out of a small aperture by +means of a centrifugal pump, and is made to squirt against a metal +plate, and interrupted intermittently by a toothed wheel made of +insulating material rotated by the motor which drives the pump. The +current supplying the coil passes through or along this jet of +mercury, and is therefore rendered intermittent when the wheel +revolves. In the case of this interrupter, the duration of the +contacts, as well as the number of interruptions per second, is under +control, and for this reason better results are probably obtained with +it than with any other form of break.</p> + +<p>A description of a turbine mercury break devised by M. Max Levy was +given in the <i>Elektrotechnische Zeitschrift</i>, Vol. XX., p. 717, +October 12, 1899 (see also <i>Science Abstracts</i>, Vol. III., p. 63, +abstract No. 165) as follows:—</p> + +<p>A toothed wheel made of insulating material carries from 6 to 24 +teeth, and can be made to rotate from 300 to 1,000 times per minute, +the interruptions being thus regulated between 5 and 400 per second. +By raising or lowering the position of the jet of mercury and that of +the plate against which it strikes, the duration of the contact can +be <span class="pagenum"><a name="Page_28" id="Page_28">[Pg 28]</a></span>varied, so that it is possible to regulate this period without +disturbing the number of interruptions per second.</p> + +<p>The sparks obtained from a coil worked with a turbine interrupter have +more quantity than the sparks obtained with any other interrupter +under similar conditions, and the coil can be worked with a far higher +voltage than is possible when using the hammer break. In this manner, +the appearance of the secondary sparks can be varied from the thin +snappy sparks given by the hammer break to the thick flame-like arc +sparks given by the electrolytic break. This break can be adapted for +any voltage from twelve to two hundred and fifty volts, and the +primary circuit cannot be closed before the interrupter is acting. The +mercury in the break is generally covered with alcohol or paraffin oil +to reduce oxidation, and the appliance is nearly noiseless when in +operation. The mercury has to be cleaned at intervals, if the +interrupter is much used. If alcohol is used to cover the mercury, the +cleaning is very simple; the break requires only to be rinsed under a +water tap. When paraffin oil is used, the cleaning is generally +effected with the help of a few ounces of sulphuric acid in a very few +minutes. It is best, however, to clean the mercury continuously by +allowing the water to flow over it.</p> + +<p>The motor driving the centrifugal pump and the fan can be wound for +any voltage, and it is best to have it so arranged that this motor +works on the same battery which supplies the primary circuit of the +coil, the two circuits working parallel together. A rheostat can be +added to the motor circuit to regulate the speed.</p> + +<p>The turbine break driven by an independent motor, which is kept always +running, has another advantage over the hammer break in practical +wireless telegraphy, viz., that a useful secondary spark can be +secured with a shorter time of closure of the primary circuit, since +there is no inertia to overcome as <a name="tnd_28" id="tnd_28"></a><a href="#tn_28" class="tnlink" title="printer's error, missing full stop at end of sentence added">in the case of the hammer break.</a> +This latter form has only continued in use because of its simplicity +and ease of management by ordinary operators.</p> + +<p>The mercury turbine interrupter has been extensively adopted both in +the German and British navies in connection with induction coils used +for wireless telegraphy.</p> + +<p>Lastly we have the electrolytic interrupters, the first of which was +introduced by Dr. Wehnelt, of Charlottenburg, in the year 1899, and +modified by subsequent inventors. In its original form, a glass vessel +filled with dilute sulphuric acid (one of acid to five or else ten +parts of water) contains two electrodes of very different sizes; one +is a large lead electrode formed of a piece of sheet lead laid round +the interior of the vessel, and the other is a short piece of platinum +wire projecting from the end of a glass or porcelain tube. The smaller +of these electrodes is made the positive, and the large one the +negative. If this electrolytic cell is connected in series with the +primary circuit of the induction coil (the condenser being cut out) +and supplied with an electromotive force from forty to eighty volts, +an electrolytic action takes place which interrupts the current +periodically.<a name="FNanchor_12_12" id="FNanchor_12_12"></a><a href="#Footnote_12_12" class="fnanchor">[12]</a> An enormous number of interruptions can, by suitable +adjustment, be <span class="pagenum"><a name="Page_29" id="Page_29">[Pg 29]</a></span>produced per second, and the appearance of a discharge +from the secondary terminals of the coil, while using the Wehnelt +break, more resembles an alternate-current arc than the usual +disruptive spark.</p> + +<p>At the time when the Wehnelt break was first introduced, great +interest was excited in it, and the technical journals in 1899 were +full of discussions as to the theory of its operation.<a name="FNanchor_13_13" id="FNanchor_13_13"></a><a href="#Footnote_13_13" class="fnanchor">[13]</a> The general +facts concerning the Wehnelt break are that the electrolyte must be +dilute sulphuric acid in the proportion of one of acid to five or ten +of water. The large lead plate must be the cathode or negative pole, +and the anode or positive pole must be a platinum wire, about a +millimetre in diameter, and projecting one or two millimetres from the +pointed end of a porcelain, glass or other acid-proof insulating tube. +The aperture through which the platinum wire works must be so tight +that acid cannot enter, yet it is desirable that the platinum wire +should be capable of being projected more or less from the aperture by +means of an adjusting screw. The glass vessel which contains these two +electrodes should be of considerable size, holding, say, a quart of +fluid, and it is better to include this vessel in a larger one in +which water can be placed to cool the electrolyte, as the latter gets +very warm when the break is used continuously. If such an electrolytic +cell has a continuous electromotive force applied to it tending to +force a current through the electrolyte from the platinum wire to the +lead plate, we can distinguish three stages in its operation, which +are determined by the electromotive force and the inductance in the +circuit. First, if the electromotive force is below sixteen or twenty +volts, then ordinary and silent electrolysis of the liquid proceeds, +bubbles of oxygen being liberated from the platinum wire and hydrogen +set free against the lead plate. If the electromotive force is raised +above twenty-five volts, then if there is no inductance in the +circuit, the continuous flow of current proceeds, but if the circuit +of the electrolyte possesses a certain minimum inductance, the +character of the current flow changes, and it becomes intermittent, +and the cell acts as an interrupter, the current being interrupted +from 100 to 2,000 times per second, according to the electromotive +force and the inductance of the circuit. Under these conditions, the +cell produces a rattling noise and a luminous glow appears round the +tip of the platinum wire. Thus, in a particular case, with an +inductance of 0·004 millihenry in the circuit of a Wehnelt break, no +interruption of the circuit took place, but with one millihenry of +inductance in the circuit, and with an electromotive force of 48 +volts, the current became intermittent at the rate of 930 per second, +and by increasing the voltage to 120 volts, the intermittency rose to +1,850 a second.</p> + +<p>The Wehnelt break acts best as an interrupter with an electromotive +force from 40 to 80 volts. At higher voltages a third stage sets in: +the luminous glow round the platinum wire disappears, and it <span class="pagenum"><a name="Page_30" id="Page_30">[Pg 30]</a></span>becomes +surrounded with a layer of vapour, as observed by MM. Violle and +Chassagny; the interruptions of current cease, and the platinum wire +becomes red hot. If there is no inductance in the circuit, the +interrupter stage never sets in at all, but the first stage passes +directly into the third stage. In the first stage bubbles of oxygen +rise steadily from the platinum wire, and in the interrupted stage +they rise at longer intervals, but regularly. The cell will not, +however, act as a break at all unless some inductance exists in the +circuit.</p> + +<p>In applying the Wehnelt break to an induction coil, the condenser is +discarded and also the ordinary hammer break, and the Wehnelt break is +placed in circuit with the primary coil. In some cases, the inductance +of the primary coil alone is sufficient to start the break in +operation, but with voltages above 50 or 60, it is generally necessary +to supplement the inductance of the primary coil by another inductive +coil. The best form of Wehnelt break for operating induction coils is +the one with multiple anodes (see Dr. Marchant, <i>The Electrician</i>, +Vol. XLII., p. 841, 1899), and when it has to be used for long +periods, the cathode may advantageously be formed of a spiral of lead +pipe, through which cold water is made to circulate.</p> + +<p>Another form of electrolytic break was introduced by Mr. Caldwell. In +this, a vessel containing dilute sulphuric acid is divided into two +parts. In the partition is a small hole, and in the two compartments +are electrodes of sheet lead. The small hole causes an intermittency +in the current which converts the arrangement into a break. Mr. +Campbell Swinton modified the above arrangement by making the +partition to consist of a sort of porcelain test-tube with a hole in +the bottom. This hole can be more or less plugged up by a glass rod +drawn out to a point, and this is used to more or less close the hole. +This porcelain vessel contains dilute acid and stands in a larger +vessel of acid, and lead electrodes are placed in both compartments. +The current and intermittency can be regulated by more or less closing +the aperture between the two regions.</p> + +<p>When the Wehnelt break is applied to an ordinary ten-inch induction +coil, and the inductance of the primary circuit and the electromotive +force varied until the break interrupts the current regularly and with +the frequency of some hundred a second, the character of the secondary +discharge is entirely different from its appearance with the ordinary +hammer break. The thin blue lightning-like sparks are then replaced by a +thicker mobile flaming discharge, which resembles an alternating-current +arc, and, when carefully examined or photographed, is found to consist +of a number of separate discharges superimposed upon one another in +slightly different positions.</p> + +<p>Many theories have been adopted as to the action of the break, but +time will not permit us to examine these. Professor S. P. Thompson and +Dr. Marchant have suggested a theory of resonance.<a name="FNanchor_14_14" id="FNanchor_14_14"></a><a href="#Footnote_14_14" class="fnanchor">[14]</a> One difficulty +in explaining the action of the break is created by the fact that it +will not work if the platinum wire is made a cathode.</p> + +<p>Although the Wehnelt break has some advantages in connection with the +use of the induction coil for Röntgen ray work, its utility <span class="pagenum"><a name="Page_31" id="Page_31">[Pg 31]</a></span>as far as +regards Hertzian wave telegraphy is not by any means so marked. It has +already been explained that, in order to charge a condenser of a given +capacity at a constant voltage, the electromotive force must be +applied for a certain minimum time, which is determined by the value +of the capacity and the resistance of the secondary circuit of the +induction coil. If the coil is a ten-inch coil and has a secondary +resistance of, say, 6,000 ohms, and if the capacity to be charged has +a value, say, of one-thirtieth of a microfarad, then the time-constant +of the circuit is 1/5,000 of a second. Therefore, the contact with the +condenser must be maintained for at least 1/500 of a second, during +the time that the secondary electromotive force of the coil is at its +maximum, so that the condenser may become charged to a voltage which +the coil is then capable of producing.</p> + +<p>In the induction coil, the electromotive force generated in the +secondary coil at the "break" of the primary current is higher than +that at the "make," and this electromotive force, other things being +equal, depends upon the rate at which the magnetism of the iron core +dies away, and its duration is shorter in proportion as the whole time +occupied in the disappearance of the magnetism is less. The Wehnelt +break does not increase the actual secondary electromotive force, nor +apparently its duration, but it greatly increases the number of times +per second this electromotive force makes it appearance. Hence this +break increases the current, but not the electromotive force in the +secondary coil. It, therefore, does not assist us in the direction +required—viz., in prolonging the duration of the secondary +electromotive force to enable larger capacities to be charged.</p> + +<p>The important point in connection with the working of a coil used for +charging a condenser is not the length of spark which the coil can +give alone, but the length of spark which can be obtained between +small balls attached to the secondary terminals, when these terminals +are also connected to the two surfaces of the condenser. Thus, a coil +may give a ten-inch spark if worked alone, but on a capacity of +one-thirtieth of a microfarad it may not be able to give more than a +five-millimetre spark. Hence, in describing the value of a coil for +wireless telegraph purposes, it is not the least use to state the +length of spark which the coil will give between the pointed +conductors in air, but we must know the spark length which it will +give between brass balls, say, 1 centimetre in diameter, connected to +the secondary terminals, when these terminals are also short-circuited +by a stated capacity, the spark not exceeding that length at which it +becomes non-oscillatory.</p> + +<p>A good way of describing the value of an induction coil for wireless +telegraph purposes is to state the length of oscillatory spark which +can be produced between balls one centimetre in diameter connected to +the secondary terminals, when these balls are short-circuited by a +condenser having a capacity, say, of one-hundredth of a microfarad, +and also one-tenth of a microfarad.</p> + +<p>If a hammer or motor interrupter is employed with the coil, then a +primary condenser must be connected across the points between which +the primary circuit is broken. This condenser generally <span class="pagenum"><a name="Page_32" id="Page_32">[Pg 32]</a></span>consists of +sheets of tinfoil alternated with sheets of paraffin paper, and for a +ten-inch coil may have a capacity of about 0·4 or 0·5 of a +microfarad.<a name="FNanchor_15_15" id="FNanchor_15_15"></a><a href="#Footnote_15_15" class="fnanchor">[15]</a></p> + +<p>Lord Rayleigh discovered that if the interruption of the primary +circuit is sufficiently sudden and complete, as when the primary +circuit is severed by a bullet from a gun, the primary condenser can +be removed and yet the sparks obtained from the secondary circuit are +actually longer than those obtained with the condenser and the +ordinary break.<a name="FNanchor_16_16" id="FNanchor_16_16"></a><a href="#Footnote_16_16" class="fnanchor">[16]</a></p> + +<p>In the use, however, of the coil for Hertzian wave telegraphy, with +all interrupters except the Wehnelt break a condenser of suitable +capacity must be joined across the break points.</p> + +<p>Turning in the next place to the primary key, or signalling +interrupter, it is necessary to be able to control the torrent of +sparks between the secondary terminals of the coil, and to cut them up +into long and short periods in accordance with the letters of the +Morse alphabet. This is done by means of the primary key. The primary +key generally consists of an ordinary massive single contact key with +heavy platinum contacts. As the current to be interrupted amounts to +about ten amperes and is flowing in a highly inductive circuit, the +spark at break is considerable. If the attempt is made to extinguish +this spark by making the contacts move rapidly away from one another +through a long distance, in other words, by using a key with a wide +movement, then the speed at which the signals can be set is greatly +diminished. The speed of sending greatly depends upon the time taken +to move the key up and down between sending two dots, and hence a +short range key sends quicker than a long range key. If it is desired +to use a short range key, then some method must be employed to +extinguish the spark at the contacts. This is done in one of three +ways: Either by using a high resistance coil to short-circuit these +contacts, or by a condenser, or by a magnetic blow-out, as in the case +of an electric tramcar circuit controller. Of these, the magnetic +blow-out is probably the best.</p> + +<p>Mr. Marconi has designed a signalling key which performs the function +not only of interrupting the primary circuit, but at the same time +breaks connection between the receiving appliance and the aerial.</p> + +<p>The author has designed for signalling purposes a multiple contact key +which interrupts the circuit simultaneously in ten or twelve different +places. The particular point about this break is the means which are +taken to make the twelve interruptions absolutely simultaneous. If +these interruptions are not simultaneous, the spark always takes place +at the contact which is broken first, but if the circuit is +interrupted in a dozen places quite simultaneously, then the spark is +cut up into a dozen different portions, and the spark at each contact +is very much diminished. By this break, voltages up to two thousand +volts may be quite easily dealt with.</p> + +<p>Various forms of break have been devised in which the circuit is +<span class="pagenum"><a name="Page_33" id="Page_33">[Pg 33]</a></span> +broken under oil or insulating fluids, but, generally speaking, these +devices are not very portable, and a dry contact between platinum +surfaces with appropriate means for cutting up the spark and blowing +it out so that the mechanical movement of the switch may be small is +the best thing to use.</p> + +<p>The signalling key is really a very important part of the transmitting +arrangement, because whatever may be the improvements in receiving +instruments, it is not possible to receive faster than we can send. A +great many statements have appeared in the daily papers as to the +possibility of receiving hundreds of words a minute by Hertzian wave +telegraphy, but the fact remains that whatever may be the sensibility +of the receiving appliance, the rate at which telegraphy of any kind +can be conducted is essentially dependent upon the rate at which the +signals can be sent, and this in turn is largely dependent upon the +mechanical movement which the key has to make to interrupt the primary +circuit, and so interrupt the secondary discharge.</p> + +<p>In order to make the separation of the contact points of the switch as +small as possible, and yet prevent an arc being established, various +blow-out devices have been employed. The simplest arrangement for this +purpose is a powerful permanent magnet so placed that its inter-polar +field embraces the contact points and is at right angles to them.</p> + +<p>As already explained, the applicability of the induction coil in +wireless telegraphy is limited by the fact of the high resistance of +the secondary circuit and the small current that can be supplied from +it. Data are yet wanting to show what is the precise efficiency of the +induction coil, as used in Hertzian wave telegraphy, but there are +reasons for believing that it does not exceed 50 or 60 per cent.</p> + +<p>Where large condensers have to be charged—in other words, where we +have to deal with larger powers—we are obliged to discard the +induction coil and to employ the alternating-current transformer. But +this introduces us to a new class of difficulties. If an +alternating-current transformer wound for a secondary voltage, say, of +20,000 or 30,000 volts, has its primary circuit connected to an +alternator, then if the secondary terminals, to which are connected +two spark balls, are gradually brought within striking distance of one +another, the moment we do this an alternating-current arc starts +between these balls. If the transformer is a small one, there is no +difficulty in extinguishing this arc by withdrawing the secondary +terminals, but if the transformer is a large one, say, of ten or +twenty kilowatts, dangerous effects are apt to ensue when such an +experiment is tried. The short circuiting of the secondary circuit +almost entirely annuls the inductance of the primary circuit. There +is, therefore, a rush of current into the transformer, and if it is +connected to an alternator of low armature resistance the fuses are +generally blown and other damage done.</p> + +<p><a name="tnd_33" id="tnd_33"></a><a href="#tn_33" class="tnlink" title=" printer's error, supppse for suppose">Let us supppse</a>, then, that the secondary terminals of the transformer +are also connected to a condenser. On bringing together the spark +balls connected with the secondary terminals we may have one or more +oscillatory discharges, but the process will not be continuous, +because the moment that the alternating-current arc starts between the +spark balls it reduces their difference of potential to a +<span class="pagenum"><a name="Page_34" id="Page_34">[Pg 34]</a></span> +comparatively low value, and hence the charge taken by the condenser +is very small, and, moreover, the circuit is not interrupted +periodically so as to re-start a train of oscillations.</p> + +<p>When, therefore, we desire to employ an alternating-current +transformer as a source of electromotive force, although it may have +the advantage that the resistance of the secondary circuit of the +transformer is generally small compared with that of the secondary +circuit of an induction coil, yet, nevertheless, we are confronted +with two practical difficulties: (1) How to control the primary +current flowing into the transformer, and (2) how to destroy the +alternating-current arc between the spark balls and reduce the +discharge entirely to the disruptive or oscillatory discharge of the +condenser.</p> + +<p>The control over the current can be obtained, in accordance with a +plan suggested by the author, by inserting in the primary circuit of +the transformer two variable choking coils. The form in which it is +preferred to construct these is that of a cylindrical bobbin standing +upon a laminated cross-piece of iron. These bobbins can have let down +into them an <b>E</b>-shaped piece of laminated iron, so as to complete the +magnetic circuit, and thus raise the inductance of the bobbin. By +placing two of these variable choking coils in series with the primary +circuit, the current is under perfect control. We can fix a minimum +value below which the current shall not fall, by adjusting the +position of the cores of these two choking coils, and we can then +cause that current to be increased up to a certain limit which it +cannot exceed, by short-circuiting one of these choking coils by an +appropriate switch. Several ways have been suggested for extinguishing +the alternating current arc which forms between the spark balls +connected to the secondary terminals when these are brought within a +certain distance of one another. One of these is due to Mr. Tesla. He +places a strong electromagnet so that its lines of magnetic flux pass +transversely between the spark balls. When the discharge takes place +the electric arc is blown out, but if the balls are short-circuited by +a condenser the oscillatory discharge of the condenser still takes +place across the spark gap. Professor Elihu Thomson achieves the same +result by employing a blast of air thrown on the spark gap. This has +the effect of destroying the alternating-current arc, but still leaves +the oscillating discharge of the condenser. The action is somewhat +tedious to explain in words, but it can easily be understood that the +blast of air, by continually breaking down the alternating-current arc +which tends to form, allows the condenser connected to the spark balls +to become charged with the potential of the secondary circuit of the +transformer, and that this condenser then discharges across the spark +gap, producing an oscillatory discharge in the usual manner. The +author has found that, without the use of any air blast or +electromagnet, simple adjustment of the double choking coil in the +primary circuit of the transformer, as above described, is sufficient +to bring about the desired result, when the capacity of the condenser +is adjusted to be in resonance.</p> + +<p>Another method, which has been adopted by M. d'Arsonval, is to cause +the spark to pass between two balls placed at the extremities of +<span class="pagenum"><a name="Page_35" id="Page_35">[Pg 35]</a></span> +metal rods, which are in rapid rotation like the spokes of a wheel. In +this case, the draught of air produced by the passage of the spark +balls blows out the arc and performs the same function as the blast of +air in Professor Elihu Thomson's method. When these adjustments are +properly made, it is possible, by means of a condenser and an +alternating-current transformer supplied with current from an +alternator, to create a rapidly intermittent oscillatory discharge, +the sparks of which succeed one another so quickly that it appears +almost continuous. When using a large transformer and condenser, the +noise and brilliancy of these sparks are almost unbearable, and the +eyes may be injured by looking at this spark for more than a moment. +In the construction of transformers intended to be used in this +manner, very special precautions have to be taken in the insulation of +the primary and secondary circuits, and the insulation of these from +the core.</p> + +<p>It may be remarked in passing that experimenting with large +high-tension transformers coupled to condensers of large capacity is +exceedingly dangerous work, and the greatest precautions are necessary +to avoid accident. In the light, however, of sufficient experience +there is no difficulty in employing high-tension transformers in the +above-described manner, and in obtaining electromotive forces of +upwards of a hundred thousand volts supplied through transformers +capable of yielding any required amount of current.</p> + +<p>On occasions where continuous current alone is available, a motor +generator has to be employed converting the continuous current into an +alternating current. This is best achieved by the employment of a +small alternator directly coupled to a continuous-current motor; or by +providing the shaft of a continuous-current motor with two rings +connected to two opposite portions of its armature, so that when +continuous current is supplied to the brushes pressing against the +commutator, an alternating current can be drawn off from two other +brushes touching the above-mentioned insulated rings.</p> + +<p>The next element of importance in the transmitting arrangement is the +spark gap. In the case of those transmitters employing an ordinary +induction coil, the secondary spark, or the discharge of any condenser +connected to the secondary terminals can be taken between the brass +balls about half an inch or one inch in diameter, with which the +terminals of the secondary coil are usually furnished; and it is +generally the custom to allow this spark discharge to take place in +air at ordinary pressure. In the very early days of his work Mr. +Marconi adopted the discharger devised by Professor Rhigi, in which +the spark takes place between two brass balls placed in vaseline or +other highly insulating oil.<a name="FNanchor_17_17" id="FNanchor_17_17"></a><a href="#Footnote_17_17" class="fnanchor">[17]</a> But whatever advantage may accrue +from using oil as the dielectric in which the spark discharge takes +place, when carrying out simple laboratory experiments on Hertzian +waves, there is no advantage in the case of wireless telegraphy. The +Rhigi discharger <span class="pagenum"><a name="Page_36" id="Page_36">[Pg 36]</a></span>was, therefore, soon discarded. If discharges having +large quantity are passed through oil, it is rapidly decomposed or +charred, and ceases to retain the special insulating and +self-restoring character which is necessary in the medium in which an +oscillating spark is formed. The conditions when the discharges of +large condensers are passed between spark balls are entirely different +from those when the quantity of the spark, or to put it in more exact +language, the current passing, is very small. In the case of Hertzian +experiments it is necessary, as shown by Hertz, to maintain a high +state of polish on the spark balls when they are employed for the +production of short waves of small energy, but when we are dealing +with large quantities of energy at each discharge, those methods which +succeed for laboratory experiments are perfectly impracticable. The +conditions necessary to be fulfilled by a discharger for use in +Hertzian wave telegraphy are that the surfaces shall maintain a +constant condition and not be fused or eaten away by the spark, and, +next, that the medium in which the discharge takes place shall not be +decomposed by the passage of the spark, but shall maintain the +property of giving way suddenly when a certain critical pressure is +reached, and passing instantly from a condition in which it is a very +perfect insulator to one in which it is a very good conductor; and, +thirdly, that on the cessation of the discharge, the medium shall +immediately restore itself to its original condition.</p> + +<p>When using the ordinary ten-inch induction coil, and when the capacity +charged by it does not exceed a small fraction of a microfarad, it is +quite sufficient to employ brass or steel balls separated by a certain +distance in air, at the ordinary pressure, as the arrangement of the +discharger. When, however, we come to deal with the discharges of very +large condensers, at high electromotive forces, then it is necessary +to have special arrangements to prevent the destruction of the +surfaces between which the spark passes, or their continual +alteration, and many devices have been invented for this purpose. The +author has devised an arrangement which fulfils the above conditions +very perfectly for use in large power stations, but the details of +this cannot be made public at the present time.</p> + +<hr style="width: 45%;" /> + +<p>We have to consider in connection with this part of the subject the +dielectric strength of air under different pressures and for different +thicknesses. It was shown by Lord Kelvin, in 1860, that the dielectric +strength of very thin layers of air is greater than that of thick +layers.<a name="FNanchor_18_18" id="FNanchor_18_18"></a><a href="#Footnote_18_18" class="fnanchor">[18]</a> The electric force, reckoned in volts per centimetre, +required to pierce a thickness of air from two to ten millimetres in +thickness, at atmospheric pressure, may be taken at 30,000 volts per +centimetre. The same force in electrostatic units is represented by +the number 100, since a gradient of 300 volts per centimetre +corresponds to a force of one electrostatic unit. It appears also that +for air and other gases there is a certain minimum voltage +(approximately 400 volts) below which no discharge takes place, +however near the conducting surfaces <span class="pagenum"><a name="Page_37" id="Page_37">[Pg 37]</a></span>may be approximated. In this +particular practical application, however, we are only concerned with +spark lengths which are measured in millimetres or centimetres, lying, +say, between one or two millimetres and five or six centimetres. Over +this range of spark length we shall not generally be wrong in +reckoning the voltage required to produce a spark between metal balls +in air at the ordinary pressure to be given by the rule:</p> + +<div class="eq"> +<img class="tex" alt="\textrm{Disruptive voltage} = 3,000 \times \textrm{spark-gap length in millimetres}." src="images/eq_9.png"/> +</div> + +<p>If, however, the air pressure is increased above the normal by +including the spark balls in a vessel in which air can be compressed, +then the spark length, corresponding to a given potential difference, +very rapidly decreases. Mr. F. J. Jervis-Smith<a name="FNanchor_19_19" id="FNanchor_19_19"></a><a href="#Footnote_19_19" class="fnanchor">[19]</a> found that by +increasing the air pressure from one atmosphere to two atmospheres +round a pair of spark balls he reduced the spark length given by a +certain voltage from 2·5 to 0·75 centimetre.</p> + +<p>Professor R. A. Fessenden has also made some interesting observations +on the effect of using compressed air round spark gaps. He found that +if a certain voltage between metal surfaces would yield a spark four +inches in length, at the ordinary pressure of the air, if the spark +balls were enclosed in a cylinder, the air round them compressed at +50lb. per square inch, the spark length for the same potential +difference of the balls was only one quarter of an inch, or +one-sixteenth of its former value.</p> + +<p>The writer has also made experiments with an apparatus designed to +study the effect of compressed air round the spark gap. The +experimental arrangements are as follows: A ten-inch induction coil +has one of its terminals connected to the internal coating of a +battery of Leyden jars. The external coating is connected through the +primary coil of an oscillation transformer with the other secondary +terminal of the coil, and these secondary terminals are also connected +to a spark gap consisting of two brass balls enclosed in a glass +vessel into which air can be forced by a pump, the air pressure being +measured by a gauge. The balls in the glass vessel are set at a +distance of about three millimetres apart. The secondary circuit of +the oscillation transformer is connected to another pair of spark +balls, the distance of which can be varied.</p> + +<p>Suppose we begin with the air in the glass vessel containing the balls +connected to the secondary terminals of the induction coil, which may +be called the secondary balls, at atmospheric pressure, and create +oscillatory discharges in the primary coil of the oscillation +transformer, we have a spark between the balls, which may be called +the tertiary balls, connected to the secondary terminals of the +oscillation transformer. If the secondary balls are placed, say, three +millimetres apart, the air in the glass vessel enclosing them being at +the ordinary atmospheric pressure, then with one particular +arrangement of jars <span class="pagenum"><a name="Page_38" id="Page_38">[Pg 38]</a></span>used, a spark twenty-five or twenty-six +millimetres long between the tertiary balls will take place. Suppose, +then, we increase the pressure of the air round the secondary balls, +pumping it by degrees to 10, 20, 30, 40 and 50lb. per square inch +above the atmospheric pressure. We find that the spark between the +tertiary balls will gradually leap a greater and greater distance, and +when the pressure of the air is 50lb. per square inch, we can obtain a +fifty-millimetre spark between the tertiary balls, whereas when the +air in the glass vessel is at atmospheric pressure, we can only obtain +a spark between the tertiary balls of half that length.</p> + +<p>This experiment demonstrates that the effect of compressing the air +round the secondary terminals of the induction coil is to greatly +increase the difference of potential between these balls before the +spark passes. In fact, it requires about double the voltage to force a +spark of the same length through air compressed at 50lb. on the square +inch that it does to make a spark of identical length between the same +balls in air at normal pressure. This shows that there is a very great +advantage in taking the discharge spark in compressed air. A better +effect can be produced by substituting dry gaseous hydrochloric acid +for air at ordinary pressures.</p> + +<p>One other incidental advantage is that the noise of the spark is very +much reduced. The continual crackle, of the discharge spark of the +induction coil in connection with wireless telegraphy is very annoying +to sensitive ears, but in this manner we can render it perfectly +silent.</p> + +<p>Professor Fessenden also states that when the spark balls are +surrounded by compressed air, and if one of the balls is connected +with a radiator, the compression of the air, although it shortens the +spark-gap corresponding to a given voltage, does not in any way +increase the radiation. When, however, the air in the spark-ball +vessel is compressed to 60lb. in the square inch, there is a marked +increase in the effective radiation, and at 80lb. per square inch the +energy emitted in the form of waves is nearly three and a-half times +greater than at 50lb., the potential difference between the balls +remaining the same.</p> + +<p>This effect is no doubt connected with the fact that the production of +a wave, whether in ether or in any other material, is not so much +dependent upon the absolute force applied as upon the suddenness of +its application. To translate it into the language of the electronic +theory, we may say that the electron radiates only whilst it is being +accelerated, and that its radiating power, therefore, depends not so +much upon its motion as upon the rate at which its motion is changing.</p> + +<p>The advantage in using compressed air round the spark gap is that we +can increase the effective potential difference between the balls +without rendering the spark non-oscillatory. In air of the ordinary +pressure there is a certain well-defined limit of spark length for +each voltage, beyond which the discharge becomes non-oscillatory, but +by the employment of spark balls in compressed air, we can increase +the potential difference between the balls corresponding to a given +distance apart before a discharge takes place, or employ higher +potentials with <span class="pagenum"><a name="Page_39" id="Page_39">[Pg 39]</a></span>the same length of spark gap. In addition to this, we +have, perhaps, the production of a more effective radiation, as +asserted by Fessenden, when the air pressure exceeds a certain +critical value.</p> + +<p>The next element which we have to consider in the transmitting +arrangements is a condenser of some kind for storing the energy which +is radiated at intervals. Where a condenser other than the aerial is +employed for storing the electric energy which is to be radiated by +the aerial, some form of it must be constructed which will withstand +high potentials. As the dielectric for such a condenser, only two +materials seem to be of any practical use, viz., glass and micanite. +Glass condensers in the form of Leyden jars have been extensively +employed, but they have the disadvantage that they are very bulky in +proportion to their electrical capacity. The instrument maker's quart +Leyden jar has a capacity of about one-five hundredth of a microfarad, +but it occupies about 150 cubic inches or more. Professor Braun has +employed in his transmitting arrangements condensers consisting of +small glass tubes like test tubes, lined on the inside and outside +with tinfoil, which are more economical in space. The author has found +that condensers for this purpose are best made of sheet glass about +one-eighth or one-tenth of an inch in thickness, coated to within one +inch of their edge on both sides with tinfoil, and arranged in a +vessel containing resin or linseed oil, like the plates of a storage +battery. M. d'Arsonval has employed micanite, but although this +material has a considerably higher dielectric strength than glass, it +is much more expensive to obtain a given capacity by means of micanite +than by glass, although the bulk of the condenser for a given capacity +is less.</p> + +<p>To store up a certain amount of electric energy in a condenser, we +require a certain definite volume of dielectric, no matter how we may +arrange it, and the volume required per unit of energy is determined +by the dielectric strength of the material. Thus, for instance, +ordinary sheet glass cannot be safely employed with a greater electric +force than is represented by 20,000 volts for one-tenth of an inch in +thickness, or, say, a potential gradient of 160,000 volts per +centimetre. This is equivalent to an electric force of about 500 +electrostatic units. This may be called the safe-working force. The +electrostatic capacity of a condenser formed of two metal surfaces a +foot square separated by glass three millimetres in thickness is +between 1/360 and 1/400 of a microfarad. If this condenser is charged +to 20,000 volts, we have stored up in it half a joule of electric +energy, and the volume of the dielectric is 270 cubic centimetres. +Hence, to store up in a glass condenser electric energy represented by +one joule at a pressure of 20,000 volts, we require 500 cubic +centimetres of glass, and it will be found that if we double the +pressure and double the thickness of the glass, we still require the +same volume.<a name="FNanchor_20_20" id="FNanchor_20_20"></a><a href="#Footnote_20_20" class="fnanchor">[20]</a> Hence, in the construction of high-tension condensers +to store up a given amount of energy, the economical problem is how to +obtain the greatest energy-storing capacity for the <span class="pagenum"><a name="Page_40" id="Page_40">[Pg 40]</a></span>least money. +Glass fulfils this condition better than any other material. Although +some materials may have very high dielectric strength, such as paper +saturated with various oils, or resins, yet they cannot be used for +the purpose of making condensers to yield oscillatory discharges, +because the oscillations are damped out of existence too soon by the +dielectric.</p> + +<p>In arranging condensers to attain a given capacity, regard has to be +taken of the fact that for a given potential difference there must be +a certain total thickness of dielectric, and that if condensers of +equal size are being arranged in parallel it adds to their capacity, +whilst joining them in series divides their capacity. If N equal +condensers or Leyden jars have each a capacity represented by C, and +if they are joined <i>n</i> in series and <i>m</i> in parallel, the joint +capacity of the whole number is <i>m</i>C/<i>n</i>, where the product <i>mn</i> = N.</p> + +<p>Passing on next to the consideration of oscillation transformers of +various kinds—these are appliances of the nature of induction coils +for transforming the current or electromotive force of electrical +oscillations in a required ratio. These coils are, however, destitute +of any iron core, and they generally consist of coils of wire wound on +a fibre, wooden or ebonite frame, and must be immersed in a vat of oil +to preserve the necessary insulation. No dry insulation of the nature +of indiarubber or gutta-percha will withstand the high pressures that +are brought to bear upon the circuits of an oscillation transformer. +In constructing these transformers we have to set aside all previous +notions gathered from the design of low-frequency iron-core +transformers. The chief difficulty we have to contend against in the +construction of an effective oscillation transformer is the inductance +of the primary circuit and the magnetic leakage that takes place. In +other words, the failure of the whole of the flux generated by the +primary circuit to pass through or be linked with the secondary +circuit. Mr. Marconi has employed an excellent form of oscillation +transformer, in the design of which he was guided by a large amount of +experience. In this transformer the two circuits are wound round a +square wooden frame. The primary circuit consists of a number of +strands of thick insulated cable laid on in parallel, so that it +consists of only one turn of a stranded conductor. The secondary +circuit consists of a number of turns, say, ten to twenty, of thinner +insulated wire laid over the primary circuit and close to it, so that +the transformer has the transformation ratio of one to ten or one to +twenty. In the arrangements devised and patented by Mr. Marconi, these +two circuits, with their respective capacities in series with them, +are tuned to one another, so that the time-period of each circuit is +exactly the same, and without this tuning the device becomes +ineffective as a transformer.<a name="FNanchor_21_21" id="FNanchor_21_21"></a><a href="#Footnote_21_21" class="fnanchor">[21]</a> There is no advantage in putting a +number of turns on the primary circuit, because such multiplication +simply increases the inductance, and, therefore, diminishes the +primary current in the same ratio which it multiplies the turns, and +hence the magnetic field due to the primary circuit remains the same. +Where it is desired to <span class="pagenum"><a name="Page_41" id="Page_41">[Pg 41]</a></span>put a number of turns upon a coil, and yet at +the same time keep the inductance down, the writer has adopted the +device of winding a silk or hemp rope well paraffined between the +turns of the circuit, so as to keep them further apart from one +another, and as the inductance depends on the turns per centimetre, +this has the effect of reducing the inductance.</p> + +<p>The next and most important element in any transmitting station is the +aerial or radiator, and it was the introduction of this element by Mr. +Marconi which laid the foundation for Hertzian wave telegraphy as +opposed to mere experiments with the Hertzian waves. We may consider +the different varieties of aerial which have been evolved from the +fundamental idea. The simple single Marconi aerial consists of a bare +or insulated wire, generally about 100ft. or 150ft. in length, +suspended from a sprit attached to a tall mast. As these masts have +generally to be erected in exposed positions, considerable care has to +be taken in erecting them with a large margin of strength. To the end +of a sprit is attached an insulator of some kind, which may be a +simple ebonite rod, or sometimes a more elaborate arrangement of oil +insulators, and to the lower end of this insulator is attached the +aerial wire. As at the top of the aerial we have to deal with +potentials capable sometimes of giving sparks several feet in length, +the insulation of the upper end of the aerial is an important matter.</p> + +<p>In the original Marconi system, the lower end of the aerial was simply +attached to one spark ball connected to one terminal of the induction +coil, and the other terminal and spark ball were connected to the +earth. In this arrangement, the aerial acted not only as radiator, but +as energy-storing capacity, and as already explained, its radiating +power was on that account limited. The earth connection is an +important matter. For long distance work, a good earth is essential. +This earth must be made by embedding a metal plate in the soil, and +many persons are under the impression that the efficiency of the earth +plate depends upon its area, but this is not the fact. It depends much +more upon its shape, and principally upon the amount of its "edge." It +has been shown by Professor A. Tanakadate, of Japan, that if a metal +plate of negligible resistance is embedded in an infinite medium +having a resistivity <i>r</i>, the electrical conductance of this plate is +equal to <span class="eq"><img class="tex" alt="4 \pi /r" src="images/eq_10.png"/></span> times the electrostatic capacity of the same +plate placed in a dielectric of infinite extent. Hence in designing an +earth plate, we have to consider not how to give it the utmost amount +of surface, but how to give it the greatest electrostatic capacity, +and for this purpose it is far better to divide a given amount of +metal into long strips radiating out in different directions, rather +than to employ it in the form of one big square or circular plate. The +importance of the "good earth" will have been seen from our discussion +on the mode of formation of electric waves. There must be a perfectly +free access for the electrons to pass into and out of the aerial. +Hence, if the soil is dry, or badly conductive in the neighbourhood, +we have to go down to a level at which we get a good moist earth. In +fact, the precautions which have to be taken in making a <span class="pagenum"><a name="Page_42" id="Page_42">[Pg 42]</a></span>good earth +for Hertzian wave telegraphy are exactly those which should be taken +in making a good earth for a lightning conductor.</p> + +<p>Whilst on the subject of aerials, a word may be said on the +localisation of wireless telegraph stations on the Marconi system. For +reasons which were explained previously, the transmission of signals +is effected more easily over water than over dry land, and it is +hindered if the soil in the neighbourhood of the sending station is a +poor conductor. Hence, all active Hertzian wave telegraph stations, +like all active volcanoes, are generally found near the sea. In those +cases in which a multiple aerial has to be put up consisting of many +wires, one mast may be insufficient to support the structure, and +several masts arranged in the form of a square or a circle have to be +employed. The illustrated papers have reproduced numerous pictures of +the Marconi power stations at Poldhu in Cornwall, Glace Bay in Nova +Scotia, and Cape Cod in the United States. In these stations, after +preliminary failures to obtain the necessary structural strength with +ordinary masts, tall lattice girder wooden towers have been built, +about 215 feet in height, well stayed against wind pressure, and which +so far have proved themselves capable of withstanding any storm of +wind which has come against them.</p> + +<p>An important question in connection with the sending power of an +aerial is that of the relation of its height to the distance covered. +Some time ago Mr. Marconi enunciated a law, as the result of his +experiments, connecting these two quantities, which may be called +Marconi's Law. He stated that the height of the aerial to cover a +given distance, other things remaining the same, varies as the square +root of the distance. Let D be the distance and let L be the length of +the aerial, then if both the transmitting and receiving aerial are the +same height, we may say that D varies as L<sup>2</sup>. This relation may be +theoretically deduced as follows:—Any given receiving apparatus for +Hertzian wave telegraphy requires a certain minimum energy to be +imparted to it to make it yield a signal. If the resistance and the +capacity of the receiver is taken as constant, this minimum working +energy is proportional to the square of the electromotive force set up +in the receiving aerial by the impact on it of the electric waves. +This electromotive force varies as the length of the receiving aerial +and as the magnetic force due to the wave cutting across it, and the +magnetic force varies as the current in the transmitting aerial, and +therefore, for any given voltage varies as the capacity, and therefore +as the length of the transmitting aerial. If, therefore, the +transmitting and receiving aerial have the same length, the minimum +energy varies as the square of the electromotive force in the +receiving aerial, and therefore as the fourth power of the length of +either aerial, since the electromotive force varies as the product of +the lengths of the aerials. Hence, when the distance between the +aerials is constant, the minimum working energy varies as the fourth +power of the height of either aerial, but when the lengths of the +aerials are constant, the energy caught up by the receiving aerial +must vary inversely as the square of the distance D between the +aerials. Hence, if we call <i>e</i> this minimum working energy, <i>e</i> must +vary as 1/D<sup>2</sup> when L is constant, <span class="pagenum"><a name="Page_43" id="Page_43">[Pg 43]</a></span>or as L<sup>4</sup> when D is constant, and +since <i>e</i> is a constant quantity for any given arrangements of +receiver and transmitter, it follows that when the height of aerial +and distance vary, the ratio L<sup>4</sup>/D<sup>2</sup> is constant, or, in other words, +D<sup>2</sup> varies as L<sup>4</sup> or D varies as L<sup>2</sup>—<i>i.e.</i>, distance varies as the +square of the height of the aerial, which is Marconi's Law. The curve, +therefore, connecting height of aerial with sending distance for given +arrangements is a portion of a parabola.</p> + +<p>Otherwise, the law may be stated in the form <span class="eq"><img class="tex" alt="L=a\sqrt{D}" src="images/eq_11.png"/></span>, where <i>a</i> +is a numerical coefficient. If L and D are both measured in metres, +then, for recent Marconi apparatus as used on ships, <i>a</i>=0·15 roughly. +(See a report on experiments made for the Italian Navy, 1900-1901, by +Captain Quintino Bonomo—"Telegrafia senza fili," Rome, 1902.)</p> + +<p>This law, however, must not be used without discretion. After Mr. +Marconi had transmitted signals across the British Channel, some +people, forgetting that a little knowledge is a dangerous thing, +predicted that aerials a thousand feet in height would be required to +signal across the Atlantic, but Mr. Marconi has made such improvements +of late years in the receiving arrangements that he has been able to +receive signals over three thousand miles in 1903 with aerials only +thirty-three per cent. longer than those which, in 1899, he employed +to cover twenty miles across the English Channel.</p> + +<div class="figcenter" style="width: 432px;"> +<img src="images/fig15.png" width="432" height="220" alt="FIG. 15.--ALTERNATING-CURRENT DOUBLE-TRANSFORMATION +POWER PLANT FOR GENERATING ELECTRIC WAVES (Fleming). _a_, alternator; +H_{1}H_{2}, choking coil; K, signalling key; T, step-up transformer; +S_{1}S_{2} spark-gap; C_{1}C_{2} condensers; T_{1}T_{2}, oscillation +transformers; A, aerial; E, earthplate." title="" /> +<span class="caption smcap">Fig. 15.—Alternating-current Double-transformation +Power Plant for Generating Electric Waves</span><span class="caption"> (Fleming). <i>a</i>, alternator; +H<sub>1</sub>H<sub>2</sub>, choking coil; K, signalling key; T, step-up transformer; +S<sub>1</sub>S<sub>2</sub> spark-gap; C<sub>1</sub>C<sub>2</sub> condensers; T<sub>1</sub>T<sub>2</sub>, oscillation +transformers; A, aerial; E, earthplate.</span> +</div> + +<p>We turn, in the next place, to the consideration of those devices for +putting more power into the aerial than can be achieved when the +aerial itself is simply employed as the reservoir of energy. Professor +Braun, of Strassburg, in 1899, described a method for doing this by +inducing oscillations in the aerial by means of an oscillation +transformer, these oscillations being set up by the discharges from a +Leyden jar or battery of Leyden jars, which formed the reservoir of +energy. The induction coil is employed to produce a rapidly +intermittent series of electrical oscillations in the primary coil of +an oscillation transformer by the discharge through it of a Leyden +jar. Mr. Marconi immensely improved this arrangement, as described by +him in a lecture given before the Society of Arts on May 17, 1901, by +syntonising the two circuits and making the circuit, consisting of the +capacity of the aerial and the inductance of the secondary circuit of +the oscillation transformer, have the same time-period as the circuit +consisting <span class="pagenum"><a name="Page_44" id="Page_44">[Pg 44]</a></span>of the Leyden jars, or energy-storing condenser, and the +primary circuit of the oscillation transformer, and by so doing +immensely added to the power and range of the apparatus.</p> + +<p>Starting from these inventions of Braun and Marconi, the author +devised a double transmission system in which the oscillations are +twice transformed before being generated in the aerial, each time with +a multiplication of electromotive force and a multiplication of the +number of groups of oscillations per second. This arrangement can best +be understood from the diagram (see Fig. 15).</p> + +<p>In this case a transformer, T, or transformers receive alternating +low-frequency current from an alternator, <i>a</i>, being regulated by +passing through two variable choking coils, H<sub>1</sub> and H<sub>2</sub>, so as to +control it. This alternating current is transformed up from a +potential of two thousand to twenty, forty or a hundred thousand, and +is employed to charge a large condenser, C<sub>1</sub>, which discharges across +a primary spark-gap, S<sub>1</sub>, through the primary coil of an oscillation +transformer, T<sub>1</sub>. The secondary circuit of the oscillation transformer +is connected to a second pair of spark balls, S<sub>2</sub>, which in turn are +connected by a secondary condenser, C<sub>2</sub>, and the primary circuit of a +third transformer, T<sub>2</sub> and the secondary circuit of this last +transformer are inserted between a Marconi aerial, A, and the earth E. +When all these circuits are tuned to resonance by Mr. Marconi's +methods, we have an enormously powerful arrangement for creating +electric waves, or rather trains of electric waves, sent out from the +aerial, and the oscillations are controlled and the signals made by +short-circuiting one of the choking coils.</p> + +<p>Another transmitting arrangement, which involves a slightly different +principle, and employs no oscillation transformer, is one due also to +Professor Braun. In this case, a condenser and inductance are connected +in series to the spark balls of an induction coil, and oscillations are +set up in this circuit. Accordingly, there are rapid fluctuations of +potential at one terminal of the condenser. If to this we connect a long +aerial, the length of which has been adjusted to be one quarter of the +length of wave corresponding to the frequency, in other words, to make +it a quarter-wave resonator, then powerful oscillations will be +accumulated in this rod. The relation between the height (H) of the +aerial and the frequency is given by the equation 3 × 10<sup>10</sup>=4<i>n</i>H, +where <i>n</i> is the frequency of the oscillations and H the height of the +aerial in centimetres. The frequency of the oscillations is determined +by the capacity (C) and inductance (L) of the condenser circuit, and can +be calculated from the formula</p> + +<div class="eq"> +<img class="tex" alt="n = \frac{5,000,000}{\sqrt{C \textrm{(in mfds.)} \times L \textrm{(in cms.)}}}." src="images/eq_12.png"/> +</div> + +<p>That is, the frequency is obtained by dividing into the number +5,000,000, the square root of the product of the capacity in +microfarads, and inductance in centimetres, of the condenser circuit. +It will be found, on applying these rules, that it is impossible to +unite together any aerial of a length obtainable in practice with a +condenser circuit of more than a very moderate capacity. It has been +shown that <span class="pagenum"><a name="Page_45" id="Page_45">[Pg 45]</a></span>for an aerial two hundred feet in height the corresponding +resonating frequency is about one and a quarter million.<a name="FNanchor_22_22" id="FNanchor_22_22"></a><a href="#Footnote_22_22" class="fnanchor">[22]</a> As we are +limited in the amount to which we can reduce the inductance of a +discharge circuit, probably to something like a thousand centimetres, +a simple calculation shows that the largest capacity we can employ is +about a sixtieth of a microfarad. This capacity, even if charged at +60,000 volts, would only contain thirty joules of energy, or about +22·5 foot-pounds, which is a small storage compared to that which can +be achieved when we are employing the above-described methods, which +involve the use of an oscillation transformer. In such a case, +however, it is an advantage to employ a spark-gap in compressed air, +because we can then raise the voltage to a much higher value than in +air of ordinary pressure without lengthening the spark so much as to +render it non-oscillatory.</p> + +<p>When employing methods involving the use of an oscillation +transformer, it is possible to use multiple aerials having large +capacity, and hence to store up a very large amount of energy in the +aerial, which is liberated at each discharge. The most effective +arrangement is one in which the radiator draws off gradually a large +supply of energy from a non-radiating circuit, and so sends out a true +train of waves, and not mere impulses, into the ether, and as we shall +see later on, it is only when the radiation takes place in the form of +true wave trains that anything like syntony can be obtained.</p> + +<p>There are a number of variants of the above methods of arranging the +radiator and associated energy-storing in circuit. Descriptions of +these arrangements will be found in patents by Mr. Marconi, Professor +Slaby and Count von Arco, Sir Oliver Lodge, Dr. Muirhead, Professor +Popoff, Professor Fessenden and others. In all cases, however, they +are variations of the three simple forms of radiator already +described.</p> + +<p>Returning to the analogy with the air or steam siren suggested at the +commencement of this article, the reader will see in the light of the +explanations already given, that all parts of the air-wave producing +apparatus have their analogues in the electrical radiator as used in +Hertzian wave telegraphy. The object in the one case is to produce +rapid oscillations of air particles in a tube, which result in the +production of an air wave in external space; in the other case, the +arrangement serves to produce oscillations of electrons or electrical +particles in a wire, the movements of which create a disturbance in +the ether called an electrical wave. Comparing together, item by item, +it will be seen, therefore, that the induction coil or transformer +used in connection with electric-wave apparatus is analogous to the +air pump in the siren plant. In the electrical apparatus, this +electron pump is employed to put an electrical charge into a +condenser; in the air wave apparatus, the air pump is employed to +charge an air vessel with high <span class="pagenum"><a name="Page_46" id="Page_46">[Pg 46]</a></span>pressure air. From the electrical +condenser the charge is released in the form of a series of electrical +oscillations, and in the air wave producing appliance, the compressed +air is released in the form of a series of intermittent puffs or +blasts. In the electrical wave producing apparatus, these electrical +oscillations in the condenser circuit are finally made to produce +other oscillations in an air wire or open circuit, just as the puffs +of air finally expend themselves in producing aerial oscillations in +the siren tube. Finally, in the one case we have a series of air waves +and in the other case, a series of electrical waves. These trains of +electric waves or air waves, as the case may be, are intermitted into +long and short groups, in accordance with the signals of the Morse +alphabet, and, therefore, the Hertzian wave transmitter, in whatever +form it may be employed, when operated by means of a Marconi aerial, +is in fact an electrical siren apparatus, the function of which is to +create periodic disturbances in the universal ether of the same +character as those which the siren produces in atmospheric air.</p> + +<hr style="width: 45%;" /> + +<p>We have to consider in the next place the arrangements of the +receiving station and the various forms of receivers that have been +devised for effecting telegraphy by Hertzian waves. Just as the +transmitting station consists essentially of two parts, viz., a part +for creating electrical oscillations and a part for throwing out or +radiating electric waves, so the receiving-station appliances may be +divided into two portions; the function of one being to catch up a +portion of the energy of the passing wave, and that of the other to +make a record or intelligible signal in some manner in the form of an +audible or visible sign.</p> + +<p>Accordingly, there must be at the receiving station an arrangement +called a receiving aerial, which in general takes the form of a long +vertical wire or wires, similar in form <a name="tnd_46" id="tnd_46"></a><a href="#tn_46" class="tnlink" title="printer's error, comma rather than full stop at end of +sentence">to the transmitting aerial,</a> +There is, however, a distinct difference in the function of the +transmitting aerial and the receiving aerial. The function of the +first is effective radiation, and for this purpose the aerial must +have associated with it a store of energy to be released as wave +energy; but the function of the receiving aerial is to be the seat of +an electromotive force which is created by the electric force and the +magnetic force of the incident electric wave.</p> + +<p>In tracing out the mode of operation of the transmitting aerial, it +was pointed out that the electric waves emitted consisted of +alternations of electric force in a direction which is perpendicular +to the surface of the earth, and magnetic force parallel to the +surface of the earth. These two quantities, the electric force and the +magnetic force, are called the <i>wave vectors</i>, and they both lie in a +plane perpendicular to the direction in which the wave is travelling +and at right angles to one another, the electric force being +perpendicular to the surface of the earth. In optical language, the +wave sent out by the aerial would be called a plane polarised wave, +the plane of polarisation being parallel to the magnetic force. Hence, +if at any point in the path of the wave we erect a vertical conductor, +as the wave passes over it, it is cut transversely by the magnetic +force of the wave and longitudinally <span class="pagenum"><a name="Page_47" id="Page_47">[Pg 47]</a></span>by the electric force. Both of +these operations result in the creation of an alternating +electromotive force in the receiving aerial wire.</p> + +<p>As in all other cases of oscillatory motion, the principle of +resonance may here be brought into play to increase immensely the +amplitude of the current oscillations thereby set up in the receiving +aerial. As already explained, any vertical insulated wire placed with +its lower end near the earth has capacity with respect to the earth, +and it has also inductance, the value of these factors depending on +its shape and height. Accordingly, it has a natural electrical +time-period of its own, and if the periodic electromotive impulses +which are set up in it by the passage of the waves over it agree in +period with its own natural time-period, then the amplitude of the +current vibrations in it may become enormously greater than when there +is a disagreement between these two periods. Before concluding these +articles we shall return to this subject of electric resonance and +syntony, and discuss it with reference to what is called the tuning of +Hertzian wave stations. Meanwhile, it may be said that for the sake of +obtaining, at any rate in an approximate degree, this coincidence of +time-period, it is generally usual to make the receiving aerial as far +as possible identical with the transmitting aerial. If the receiving +aerial is not insulated, but is connected to the earth at its lower +end through the primary coil of an oscillation transformer, we can +still set up in it electrical oscillations by the impact on it of an +electric wave of proper period; and if the oscillation transformer is +properly constructed we can draw from its secondary circuit electric +oscillations in a similar period.</p> + +<p>One problem in connection with the design of a receiving aerial is +that of increasing its effective length and capacity so as to increase +correspondingly the electromotive force or current oscillations in it. +It is clear that if we put a number of receiving wires in parallel so +that each one of them is operated upon by the wave separately, +although we can increase in this way the magnitude of the alternating +current which can be drawn off from the aerial, we cannot increase the +electromotive force in it except by increasing the actual height of +the wires. Unfortunately, there is a limit to the height of the +receiving aerial. It has to be suspended, like the transmitting +aerial, from a mast or tower, and the engineering problem of +constructing such a permanent supporting structure higher than, say, +two hundred feet is a difficult one.</p> + +<p>Since any one station has to send as well as receive, it is usual to +make one and the same aerial wire or wires do double duty. It is +switched over from the transmitting to the receiving apparatus, as +required. This, however, is a concession to convenience and cost. In +some respects it would be better to have two separate aerials at each +station, the one of the best form for sending, and the other of the +best form for receiving.</p> + +<p>In Mr. Marconi's early arrangements, the so-called coherer or +sensitive wave-detecting appliance, to be described more in detail +presently, was inserted between the base of the insulated receiving +aerial and the earth, but it was subsequently found by him to be a +great improvement to act upon the receiving device, not directly by +the electromotive force set up in the aerial, but by the induced +<span class="pagenum"><a name="Page_48" id="Page_48">[Pg 48]</a></span> +electromotive force of a special form of step-up oscillation +transformer he calls a "jigger," the primary circuit of which was +inserted in between the receiving aerial and the earth plate, and the +secondary circuit was connected to the sensitive organ of the +telegraphic receiving arrangements.<a name="FNanchor_23_23" id="FNanchor_23_23"></a><a href="#Footnote_23_23" class="fnanchor">[23]</a> A suggestion to employ +transformed oscillations in affecting a coherer, had also been +described in a patent specification by Sir Oliver Lodge, in 1897, but +the essence of success in the use of this device is not merely the +employment of a transformer, but of a transformer constructed +specially to transform electrical oscillations.</p> + +<p>Turning, then, to the consideration of the relation existing between +the transmitting and receiving aerials, we note that in their simplest +form these consist of two similar tall rods of metal placed upright, +with their feet in good connection with the earth at two places. We +may think of them as two identical lightning conductors, well earthed +at the bottom, and supported by non-conducting masts or towers. These +rods must be in good connection with the earth, and therefore with it +form, as it were, one conductor. If, as usual, these aerials are +separated by the sea, the intermediate portion of this circuit is an +electrolyte. The operations which take place when a signal is sent are +as follows:—</p> + +<p>At the transmitting station, we set up in the transmitting aerial +electric oscillations, of which the frequency may be of the order of a +million, <i>i.e.</i>, the oscillations as long as they last are at the rate +of a million a second. Each spark discharge at the transmitter +results, however, only in the production of a train of a dozen or two +oscillations, and these trains succeed each other at a rate depending +upon the transmitting arrangements used. Each oscillation in the +transmitting aerial is accompanied by the detachment from it of +semi-loops of electric strain, as already explained. The <a name="tnd_48" id="tnd_48"></a><a href="#tn_48" class="tnlink" title="possible printer's error, alterations for alternations">alterations +of electric strain</a> directed perpendicularly to the earth, and of the +associated magnetic force parallel to the earth, constitute an +electric wave in the ether, just as the alternations of pressure and +motion of air molecules constitute an air wave. Associated with these +physical actions above ground, there is a propagation through the +earth of electric action, which may consist in a motion or atomic +exchange of electrons. Each change or movement of a semi-loop of +electric strain above ground has its equivalent below ground in +inter-atomic exchanges or movements of the electrons, on which the +ends of these semi-loops of electric strain terminate. The earth must +play, therefore, a very important part in so-called "wireless +telegraphy," and we might also say the earth does as much as the ether +in its production.</p> + +<p>The function of the receiving aerial is to bring about a union between +these two operations above and below ground. When the electric waves +fall upon it, they give rise to electromotive force in the receiving +aerial, and, therefore, produce oscillations in it which, in fact, +<span class="pagenum"><a name="Page_49" id="Page_49">[Pg 49]</a></span> +are electric currents flowing into and out of the receiving aerial. We +may say that the transmitting aerial, the receiving aerial and the +earth form one gigantic Hertz oscillator. In one part of this system, +electric oscillations of a certain period are set up by the discharge +of a condenser and are propagated to the other part. In the earth, +there is a propagation of electric oscillations; in the space above +and between the aerials, there is a propagation of electric waves. The +receiving aerial <i>feels</i>, therefore, what is happening at the distant +aerial and can be made to record it.<a name="FNanchor_24_24" id="FNanchor_24_24"></a><a href="#Footnote_24_24" class="fnanchor">[24]</a></p> + +<p>We have next to consider the question of the wave-detecting devices +which enable us to appreciate and record the impact of a wave or wave +train against the aerial. At the very outset it will be necessary to +coin a new word to apply generally to these appliances. Most readers +are probably familiar with the term "coherer," which was applied by +Sir Oliver Lodge, in the first instance, to an electric wave-detecting +device of one particular kind—viz., that in which a metal point was +lightly pressed against another metal surface and caused to stick to +it when an electric wave fell upon it. As our knowledge increased, it +was found that there were many cases in which the effect of the +electric radiation was to cause a severance and not a coherence, and +hence such clumsy phrases as "anticoherer" and "self-decohering +coherer" have come into use. Moreover, we have now many kinds of +electric wave detectors based on quite different physical principles. +At the risk of incurring reprobation for adding to scientific +nomenclature, the author ventures to think that the time has arrived +when a simple and inclusive term will be found useful to describe all +the devices, whatever their nature, which are employed for detecting +the presence of an electric wave. For this purpose the term +<i>kumascope</i>, from the Greek κυμα (a wave), is suggested. The +scientific study of waves has already been called <i>kumatology</i>, and in +view of our familiarity with such terms as <i>microscope</i>, +<i>electroscope</i> and <i>hygroscope</i>, there does not seem to be any +objection to enlarging our vocabulary by calling a wave-detecting +appliance a <i>kumascope</i>. We are then able to look at the subject +broadly and to classify kumascopes of different kinds.</p> + +<p>We may, in the first place, arrange them according to the principle on +which they act. Thus, we may have electric, magnetic, thermal, +chemical and physiological operations involved; and finally, we may +divide them into those which are self-restoring, in the sense that +after the passage or action of a wave upon them they return to their +original sensitive condition; and those which are non-restoring, in +that they must be subjected to some treatment to bring them back again +to a condition in which they are fit to respond again to the action of +a wave.</p> + +<p>We have no space to refer to the whole of the steps of discovery which +led up to the invention of all the various forms of the modern +electric kumascope or wave detector. Suffice it to say that the +researches of Hertz in 1887 threw a flood of light upon many +previously obscure phenomena, and enabled us to see that an electric +spark, <span class="pagenum"><a name="Page_50" id="Page_50">[Pg 50]</a></span>and especially an oscillatory spark, creates a disturbance in +the ether, which has a resemblance in Nature to the expanding ripples +produced by a stone hurled into water. Scientific investigation then +returned with fresh interest to previously incomprehensible effects, +and a new meaning was read into many old observations. Again and again +it had been noticed that loose metallic contacts, loose aggregations +of metallic filings or fragments, had a mysterious way of altering +their conductivity under the action of electric sparks, lightning +discharges and high electromotive forces.</p> + +<p>As far back as 1852, Mr. Varley had noticed that masses of powdered +metals had a very small conductivity, which increased in a remarkable +way during thunderstorms;<a name="FNanchor_25_25" id="FNanchor_25_25"></a><a href="#Footnote_25_25" class="fnanchor">[25]</a> and in 1866, C. and S. A. Varley +patented a device for protecting telegraphic instruments from +lightning, which consisted of a small box of powdered carbon in which +were buried two nearly touching metal points, and they stated that +"powdered conducting matter offers a great resistance to a current of +moderate tension, but offers but little resistance to currents of high +tension."<a name="FNanchor_26_26" id="FNanchor_26_26"></a><a href="#Footnote_26_26" class="fnanchor">[26]</a></p> + +<p>We then pass over a long interval and find that the next published +account of similar observations was due to Professor T. +Calzecchi-Onesti, who described in an Italian journal, <i>Il Nuovo +Cimento</i> (see Vol. XVI., p. 58, and Vol. XVII., p. 38), in 1884 and +1885 his observations on the decrease in resistance of metal powders +when the spark from an induction coil was sent through them.<a name="FNanchor_27_27" id="FNanchor_27_27"></a><a href="#Footnote_27_27" class="fnanchor">[27]</a> These +observations did not attract much attention until Professor E. Branly, +of Paris, in 1890 and 1891, repeated them on an extended scale and +with great variations, making the important observation that an +electric spark <i>at a distance</i> had a similar effect in increasing the +conductivity of metallic powders.<a name="FNanchor_28_28" id="FNanchor_28_28"></a><a href="#Footnote_28_28" class="fnanchor">[28]</a> Branly, however, noticed that in +some cases of conductors in powder, such as the peroxide of lead or +antimony, the effect of the spark was to cause a decrease of +conductivity.</p> + +<p>To Professor E. Branly unquestionably belongs the honour of giving to +science a new weapon in the shape of a tube containing metallic +filings or powder rather loosely packed between metal plugs, and of +showing that when the pressure on the powder was adjusted such a tube +may be a conductor of very high resistance, but that the electrical +conductivity is enormously increased if an electric spark is made in +its neighbourhood. He also proved that the same effect occurred in the +case of two slightly oxidised steel or copper wires laid across one +another with light pressure, and that this loose or imperfect contact +was extraordinarily sensitive to an electric spark, dropping in +resistance from thousands of ohms to a few ohms when a spark was made +many yards away.</p> + +<p>It is curious to notice how long some important researches take to +become generally known. Branly's work did not attract much <span class="pagenum"><a name="Page_51" id="Page_51">[Pg 51]</a></span>attention +in England until 1892, when Dr. Dawson Turner described his own +repetition of Branly's experiments with the metallic filings tube at a +meeting of the British Association in Edinburgh. In the discussion +which followed, Professor George Forbes made an important remark. He +asked whether it was possible that the decrease in resistance could be +brought about by Hertz waves.<a name="FNanchor_29_29" id="FNanchor_29_29"></a><a href="#Footnote_29_29" class="fnanchor">[29]</a></p> + +<p>This question shows that even in 1892 the idea that the effect of the +spark on the Branly tube was really due to Hertzian waves was only +just beginning to arise. The following year, however, Mr. W. B. Croft +repeated Branly's experiment with copper filings before the Physical +Society of London, and entitled his short Paper "Electric Radiation on +Copper Filings."<a name="FNanchor_30_30" id="FNanchor_30_30"></a><a href="#Footnote_30_30" class="fnanchor">[30]</a> He exhibited a tube containing copper filings +loosely held between two copper plugs and joined in series with a +galvanometer and cell. The effect of an electric spark at a distance, +in causing increase of conductivity, was shown, and the return of the +tube to its non-conducting state when tapped was also noticed.</p> + +<p>In the discussion which followed the reading of this Paper, Professor +Minchin described the effects of electric radiation on his impulsion +cells. He followed up this by reading a Paper to the Physical Society +on November 24, 1893, on the action of Hertzian radiation on films +containing metallic powders, and expressed the opinion that the change +in resistance of the Branly tube was due to electric radiation.<a name="FNanchor_31_31" id="FNanchor_31_31"></a><a href="#Footnote_31_31" class="fnanchor">[31]</a></p> + +<p>Thus, at the end of 1893, a few physicists clearly recognised that a +new means had been given to us for detecting those invisible ether +waves, the chief properties of which Hertz had unravelled with +surpassing skill six years before, by means of a detector consisting +of a ring of wire having a small spark-gap in it.</p> + +<p>In June, 1894, Sir Oliver Lodge delivered a discourse at the Royal +Institution, entitled "The Work of Hertz," and at this lecture use was +made of the Branly tube as a Hertz wave detector. The chief object of +the lecture was to describe the properties of Hertzian waves and their +reflection, absorption and transmission, and many brilliant +quasi-optical experiments were exhibited. Although a Branly tube, or +imperfect metallic contact, then named by him a <i>coherer</i>, was +employed by Sir Oliver Lodge to detect an electric wave generated in +another room, there was no mention in this lecture of any use of the +instrument for telegraphic purposes.<a name="FNanchor_32_32" id="FNanchor_32_32"></a><a href="#Footnote_32_32" class="fnanchor">[32]</a></p> + +<p>As we are here concerned only with the applications in telegraphy, <span class="pagenum"><a name="Page_52" id="Page_52">[Pg 52]</a></span>we +shall not spend any more time discussing the purely scientific work +done with laboratory forms of this wave detector.</p> + +<p>Without attempting to touch the very delicate question as to the +precise point at which laboratory research passed into technical +application, we shall briefly describe the forms of kumascope which +have been devised with special reference to Hertzian wave telegraphic +work. A very exact classification is at present impossible, but we may +say that telegraphic kumascopes may be roughly divided into six +classes. The first class includes all those that depend for their +action on the "coherer principle" or the reduction of the resistance +of a metallic microphone by the action of electromotive force. As they +depend upon an imperfect contact, they may be called <i>contact +kumascopes</i>. This class is furthermore subdivided into the +self-restoring and the non-self-restoring varieties. The second class +comprises the <i>magnetic kumascopes</i> which depend upon the action of an +electrical oscillation as a magnetising or demagnetising agency. The +third class comprises the <i>electrolytic responders</i>, in which the +action of electric oscillations either promotes or destroys the +results of electrolysis. The fourth class consists of the +<i>electrothermal detectors</i>, in which the power of an electrical +oscillation as a high frequency electric current to heat a conductor +is utilised. The fifth class comprises the <i>electromagnetic</i> or +<i>electro-dynamic</i> instruments, which are virtually very sensitive +alternating-current ammeters, adapted for immensely high frequency. +The sixth class must be made to contain all those which cannot be well +fitted at present into any of the others, such as the sensitive +responder of Schäfer, the action of which is not very clearly made +out.</p> + +<p>We may proceed briefly to describe the construction of the principal +forms of kumascope coming under the above headings. In the first +place, let us consider those which are commonly called the "coherers" +or, as the writer prefers to call them, the <i>contact kumascopes</i>. The +simplest of these is the crossed needle or single contact, which +originated with Professor E. Branly.<a name="FNanchor_33_33" id="FNanchor_33_33"></a><a href="#Footnote_33_33" class="fnanchor">[33]</a> The pressure of the point of +a steel needle against an aluminium plate was subsequently found by +Sir Oliver Lodge to be a very sensitive arrangement when so adjusted +that a single cell sends little or no current through the contact.<a name="FNanchor_34_34" id="FNanchor_34_34"></a><a href="#Footnote_34_34" class="fnanchor">[34]</a> +When an electric wave passes over it, good conducting contact ensues. +The point is, in fact, welded to the plate, and can only be detached +by giving the plate or needle a light shock or vibration. A variation +of the above form is a pair of crossed needles, one resting on the +other.</p> + +<p>Professor Branly found, in 1891, that if a pair of slightly-oxidised +copper wires rest across one another the contact-resistance may fall +from 8,000 to 7 ohms by the impact of an electric wave. He has +recently devised a tripod arrangement, in which a light metal stool +with three slightly-oxidised legs stands on a polished plate of steel. +The contact points must be oxidised, but not too heavily, and the +<span class="pagenum"><a name="Page_53" id="Page_53">[Pg 53]</a></span> +stool makes a bad electrical contact until a wave falls upon it.<a name="FNanchor_35_35" id="FNanchor_35_35"></a><a href="#Footnote_35_35" class="fnanchor">[35]</a> +The decoherence is effected by giving the stool a tilt by means of an +electromagnet.</p> + +<p>These single or multiple-point kumascopes labour under the +disadvantage that only a very small current can be passed through the +variable contact when used as a relay arrangement, without welding +them together so much that a considerable mechanical shock is required +to break the contact and reset the trap.</p> + +<p>The logical development of the single contact is, therefore, the +infinite number of contacts existing in the tube of metallic filings, +which has been the form of kumascope most used for many years. In its +typical form it consists of a tube of insulating material with +metallic plugs at each end, and between them a mass of metallic +powder, filings, borings, granules or small spheres, lightly touching +one another. Imperfect contact must be arranged by light pressure, and +in the majority of cases the resistance is very large until an +electric wave falls upon the tube, when it drops suddenly to a small +value and remains there until the tube is given a slight shake or the +granules disturbed in any way, when the resistance suddenly rises +again. This type of responder is a non-restoring kumascope, and +requires the continual operation of some external agency to keep it in +a condition in which it is receptive or sensitive to electric waves.</p> + +<p>Much discussion and considerable research have taken place in +connection with the action and improvement of these metallic powder +kumascopes. As regards materials, the magnetic metals, nickel, iron +and cobalt, in the order named, appear to give the best results. The +noble metals, gold, silver and platinum, are too sensitive, and the +very oxidisable metals too insensitive, for telegraphic work, but an +admixture may be advantageously made.</p> + +<p>Omitting the intermediate developments of invention, it may be said +that Mr. Marconi was the first to recognise that to secure great +sensibility in an electric wave detector of this type the following +conditions must be fulfilled: An exceedingly small mass of metallic +filings must be placed in a very narrow gap between two plugs, the +whole being contained in a vessel which is wholly or partly exhausted +of its air. Mr. Marconi devoted himself with great success to the +development of this instrument, and in a very short time succeeded in +transforming it from an uncertain laboratory appliance, capable of +yielding results only in very skilled hands, into an instrument +certain and simple in its operations as an ordinary telegraphic relay. +He did this, partly by reducing its size, and partly by a most +judicious selection of materials for its construction. As made at +present, the Marconi metallic filings tube consists of a small glass +tube, the interior diameter of which is not much more than one-eighth +of an inch, which has in it two silver plugs which are bevelled off +obliquely. These are placed opposite to each other, so as to form a +wedge-shaped gap, about a millimetre in width at the bottom and <span class="pagenum"><a name="Page_54" id="Page_54">[Pg 54]</a></span>two, +or at most three, millimetres in width at the top (see Fig. 16). The +silver plugs exactly fill the aperture of the tube, and are connected +to platinum wires sealed through the glass. The tube has a lateral +glass tube fused into it, by which the exhaustion is made, which is +afterwards sealed off, and this tube projects on the side of the wider +portion of the gap between the silver plugs. The sensitive material +consists of a mixture of metallic filings, five per cent. silver and +ninety-five per cent. nickel, being carefully mixed and sifted to a +certain standard fineness. In the manufacture of these tubes, great +care is taken to make them as far as possible absolutely identical. +Each tube when finished is exhausted, but not to a very high vacuum. +The tube so finished is attached to a bone holder, by which it can be +held in a horizontal position. The object of bevelling off the plugs +in the Marconi tube is to enable the sensitiveness of the tube to be +varied by turning it round, so that the small quantity of filings lie +in between a wider or narrower part of the gap.<a name="FNanchor_36_36" id="FNanchor_36_36"></a><a href="#Footnote_36_36" class="fnanchor">[36]</a></p> + +<div class="figleft" style="width: 261px;"> +<img src="images/fig16.png" width="261" height="97" alt="FIG. 16.--MARCONI SENSITIVE TUBE OR METALLIC FILINGS +KUMASCOPE. PP, silver plugs; TT, platinum wires; F, nickel and silver +filings." title="" /> +<span class="caption smcap">Fig. 16.—Marconi Sensitive Tube or Metallic Filings +Kumascope.</span><span class="caption"> PP, silver plugs; TT, platinum wires; F, nickel and silver +filings.</span> +</div> + +<p>Other ways of adjusting the quantity of the filings to the width of +the gap have been devised. Sometimes one of the plugs is made movable. +In other cases, such as the tubes devised by M. Blondel and Sir Oliver +Lodge, there is a pocket in the glass receptacle to hold square +filings, from which more or less can be shaken into the gap.</p> + +<p>An interesting question, which we have not time to discuss in full, is +the cause of the initial coherence of the metallic filings in a Branly +tube. It does not seem to be a simple welding action due to heat, and +it certainly takes place with a difference of potential, which is very +far indeed below that which we know is required to produce a spark. On +the other hand, it seems to be <a name="tnd_54" id="tnd_54"></a><a href="#tn_54" class="tnlink" title="printer's error, Banly for Branly">proved that in a Banly tube,</a> when acted +upon by electric waves, chains of metallic particles are produced. The +effect is not peculiar to electric waves. It can be accomplished by +the application of any high electromotive force. Thus Branly found +that coherence may be produced by the application of an electromotive +force of twenty or thirty volts, operating through a very high water +resistance, and thus precluding the passage of any but an excessively +small current. Again, the coherence seems to take place in some cases +when metallic particles are immersed in a liquid, or even in a solid, +insulator. Processor Branly has, therefore, preferred to speak of +masses of metallic granules as <i>radio-conductors</i>, and Professor Bose +has divided substances into positive and negative, according as the +operation of electromotive force is to increase the coherence of the +particles or to decrease it.</p> + +<p>It has been asserted that for every particular Branly tube, there is a +critical electromotive force, in the neighbourhood of two or three +volts <span class="pagenum"><a name="Page_55" id="Page_55">[Pg 55]</a></span>which causes the tube to break down and pass instantly from a +non-conductive to a conductive condition, and that this critical +electromotive force may become a measure of the utility of the tube +for telegraphic purposes. Thus, C. Kinsley (<i>Physical Review</i>, Vol. +XII., p. 177, 1901) has made measurements of this supposed critical +potential for different "coherers," and subsequently tested the same +as receivers at a wireless telegraph station of the U.S.A. Signal +Corps. The average of twenty-four experiments gave in one case 2·2 +volts as the breaking down potential of one of these coherers or +Branly tubes, 3·8 volts for a second and 5·5 volts for the third. +These same instruments, tested as telegraphic kumascopes, showed that +the first of the three was most sensitive.</p> + +<p>On the other hand, W. H. Eccles (<i>Electrician</i>, Vol. XLVII., pp. 682 +and 715, 1901) has made experiments with Marconi nickel-silver +sensitive tubes, using a liquid potentiometer made with copper +sulphate, to apply the potential so that infinitesimal spark contacts +might be avoided and the changes in potential made without any +abruptness. He states that if the coherer tube is continuously tapped, +say at the rate of fifty vibrations per second, whilst at the same +time an increasing potential is applied to its terminals and the +current passing through it measured on a galvanometer, there is no +abrupt change in current at any point. He found that when the current +and voltage were plotted against each other, a regular curve was +obtained, which after a time becomes linear. A decided change occurs +in the conductivity of the mass of metallic filings when treated in +this manner at voltages lower than the critical voltage obtained by +previous methods. He ascertained that there was a complete +correspondence between the sensitiveness of the tubes used as +telegraphic instruments and the form of the characteristic curve of +current and voltage drawn by the above-described method.</p> + +<p>In the same manner, K. E. Guthe and A. Trowbridge (<i>Physical Review</i>, +Vol. II., p. 22, 1900) investigated the action of a simple ball +coherer formed of half a dozen steel, lead or phosphor-bronze balls in +slight contact. They measured the current <i>i</i> passing through the +series under the action of a difference of potential <i>v</i> between the +ends, and found a relation which could be expressed in the form</p> + +<div class="eq"> +<img class="tex" alt="v = V(1 - e^{ki})," src="images/eq_13.png"/> +</div> + +<p>where V and <i>k</i> are constants.</p> + +<p>The current through this ball coherer is, therefore, a logarithmic +function of the potential difference between its ends, of the form</p> + +<div class="eq"> +<img class="tex" alt="i = log(v - V)" src="images/eq_14.png"/> +</div> + +<p>and exhibits no discontinuity.</p> + +<p>The inference was drawn that the "resistance" is due to films of water +adhering to the metallic particles through which electrolytic action +occurs.</p> + +<p>A good metallic filings tube for use as a receiver in Hertzian wave +telegraphy should exhibit a constancy of action and should cohere and +decohere, to use the common terms, sharply, at the smallest possible +<span class="pagenum"><a name="Page_56" id="Page_56">[Pg 56]</a></span> +tap. It should not have a current passed through it by the external +cell of more than a fraction of a milliampere, or else it becomes +<a name="tnd_56" id="tnd_56"></a><a href="#tn_56" class="tnlink" title="variation in spelling, unsensitive for insensitive">wounded and unsensitive.</a></p> + +<p>The investigations which have already taken place seem to show pretty +clearly that the agency causing the masses of filings to pass from a +non-conductive to a conductive condition is electromotive force, and +that, therefore, it is the electromotive force set up in the aerial by +the incident waves which is the effective agent in causing the change +in the metallic filings tube, when this is used as a telegraphic +kumascope. This transformation of the tube from a non-conductor to a +conductor is made to act as a circuit-closer, completing the circuit, +by means of which a single cell of a local battery is made to send +current through an ordinary telegraph relay, and so by the aid of a +second battery operate a telegraphic printer or recorder of any kind. +Hence it is clear that after one impact, the metallic filings tube has +to be brought back to its non-conductive condition, and this may be +achieved in several ways. (1) By the administration of +carefully-regulated taps or shocks or by rotating the tube on its +axis; (2) by the aid of an alternating current; (3) in those cases +where filings of magnetic metals are employed, by magnetism.</p> + +<p>The decoherence by taps was discovered by Branly,<a name="FNanchor_37_37" id="FNanchor_37_37"></a><a href="#Footnote_37_37" class="fnanchor">[37]</a> and Popoff, +following the example of Sir Oliver Lodge, employed an electric bell +arrangement for this purpose.<a name="FNanchor_38_38" id="FNanchor_38_38"></a><a href="#Footnote_38_38" class="fnanchor">[38]</a></p> + +<p>Mr. Marconi, in his original receiving instruments, placed an +electromagnet under the coherer tube with a vibrating armature like an +electric bell.<a name="FNanchor_39_39" id="FNanchor_39_39"></a><a href="#Footnote_39_39" class="fnanchor">[39]</a> This armature carries a small hammer or tapper, +which, when set in action, hits the tube on the under side, and +various adjusting screws are arranged for regulating exactly the force +and amplitude of the blows. This tapper is actuated by the same +current as the Morse printer, or other telegraphic recorder, so that +when the signal is received and the metallic filings tube passes into +the conductive condition and closes the relay circuit, this latter in +turn closes the circuit of the Morse printer or other recorder, and at +the same time, a current passes through the electromagnet of the +tapper and the tube is tapped back. This sequence of operations +requires a certain time which limits the speed of receiving. The +tapper has to be so arranged that it is possible to receive and to +record not only the <i>dot</i> but a <i>dash</i> on the Morse system. The <i>dash</i> +is really a series of closely adjacent dots, which run together in +virtue of the inertia and inductance of the different parts of the +whole receiving apparatus. The adjustment has so to be made that, +whilst the <i>dash</i> is being recorded and a continuous tapping is kept +up, yet, nevertheless the continued electromotive force in the aerial, +due to the continually arriving trains of waves, is able to act +against the tapping and to keep the filings in the tube in the +conductive condition. Hence, the successful operation of the +arrangement requires attention to a number of adjustments, but <span class="pagenum"><a name="Page_57" id="Page_57">[Pg 57]</a></span>these +are not more difficult, or even as difficult, as those involved in the +use of many telegraphic receivers employed in ordinary telegraphy with +wires.</p> + +<p>Mr. Marconi also introduced devices for preventing the sparks at the +contacts of the electromagnetic hammer from directly affecting the +tube, and also to prevent the electric oscillations which are set up +in the aerial from being partly shunted through other circuits than +that of the sensitive tube. We pass on to notice the remaining devices +for restoring the metallic filings tube to a condition of +sensitiveness or receptiveness.</p> + +<p>A method for doing this by alternating currents is due to Mr. S. G. +Brown.<a name="FNanchor_40_40" id="FNanchor_40_40"></a><a href="#Footnote_40_40" class="fnanchor">[40]</a> The pole pieces of the coherer tube are made of iron, and +they are enveloped in magnetising coils traversed by an alternating +electric current. Between these pole pieces is placed a small quantity +of nickel or iron filings, and under the action of the electromotive +force, due to an electric wave acting on them, may be made to cohere +in the usual fashion; but the moment that the wave ceases, the +alternating magnetism of the electrodes causes the filings to drop +apart or decohere. In place of the alternating current, Mr. Brown +finds that a revolving permanent magnet can be used to produce the +alternating magnetisation of the pole pieces of the sensitive tube or +coherer.</p> + +<p>The third method of causing the decoherence of the filings is that due +to T. Tommasina. He found that when a Branly tube is made with filings +of a magnetic metal, such as iron, nickel and cobalt, the decoherence +of the filings can be produced by means of an electromagnet placed in +a suitable position under the tube.<a name="FNanchor_41_41" id="FNanchor_41_41"></a><a href="#Footnote_41_41" class="fnanchor">[41]</a> The explanation of this fact +seems to be that, when an electric wave falls upon the tube or when +any other source of electromotive force acts upon it, chains of +metallic particles are formed, stretching from one electrode to the +other. Tommasina contends that he has proved the existence of these +chains of particles by experiments made with iron filings; and R. +Malagoli,<a name="FNanchor_42_42" id="FNanchor_42_42"></a><a href="#Footnote_42_42" class="fnanchor">[42]</a> in referring to Tommasina's assertion, states that it +can be witnessed in the case of brass filings placed between two +plates of metal and immersed in vaseline oil, when a difference of +potential is made between the plates.</p> + +<p>T. Sundorph<a name="FNanchor_43_43" id="FNanchor_43_43"></a><a href="#Footnote_43_43" class="fnanchor">[43]</a> says he has confirmed Tommasina's discovery of the +formation of these chains of metallic particles in the coherer. The +filings do not all cling together, but certain chains are formed which +afford a conducting path for the current subsequently passed through +the coherer from an external source. Accordingly, Tommasina's method +of causing decoherence in the case of filings of magnetic metals is to +pull them apart by an external magnetic field; and he stated that +decoherence can be effected more easily and regularly in this way than +by tapping. Whilst on this point, it may be mentioned that C. +Tissot<a name="FNanchor_44_44" id="FNanchor_44_44"></a><a href="#Footnote_44_44" class="fnanchor">[44]</a> says that he has found that the sensitiveness of a coherer +formed of nickel and iron filings can be increased by placing it in +the magnetic <span class="pagenum"><a name="Page_58" id="Page_58">[Pg 58]</a></span>field, the lines of which are parallel to the axis of +the tube. According to MM. A. Blondel and G. Dobkevitch, this is +merely the result of an increased coherence of the particles.</p> + +<hr style="width: 45%;" /> + +<p>Quite recently, Sir Oliver Lodge and Dr. Muirhead have employed as a +self-restoring coherer or kumascope a steel disc revolved by +clockwork, the edge of which just touches a globule of mercury covered +with a thin film of paraffin oil. The contact is made between the +mercury and the steel by the electric wave generating an electromotive +force in the aerial, sufficient to break through the thin film of oil. +When the wave stops, the circuit is again interrupted automatically.</p> + +<p>This device is used without a relay to actuate directly a syphon +recorder as used in submarine telegraphy. The working battery employed +with it must only have an electromotive force of about a tenth of a +volt. It may be used also with a telephone in circuit and can, +therefore, be employed either for telegraphic or telephonic +reception.<a name="FNanchor_45_45" id="FNanchor_45_45"></a><a href="#Footnote_45_45" class="fnanchor">[45]</a></p> + +<div class="figleft" style="width: 211px;"> +<img src="images/fig17.png" width="211" height="202" alt="FIG. 17.--ITALIAN NAVY SELF-RESTORING KUMASCOPE. C, +carbon plug; I, iron plug; M, mercury globule; A, aerial; B, battery; +T, telephone; S, adjusting screw." title="" /> +<span class="caption smcap">Fig. 17.—Italian Navy Self-restoring Kumascope.</span><span class="caption"> C, +carbon plug; I, iron plug; M, mercury globule; A, aerial; B, battery; +T, telephone; S, adjusting screw.</span> +</div> + +<p>One of the most sensitive of these self-restoring kumascopes is the +carbon-steel-mercury coherer, the invention of which has been +attributed to Castelli, a signalman in the Italian Navy,<a name="FNanchor_46_46" id="FNanchor_46_46"></a><a href="#Footnote_46_46" class="fnanchor">[46]</a> but also +stated on good authority to have been the invention of officers in the +Royal Italian Navy, and has, therefore, been called the Italian Navy +coherer.<a name="FNanchor_47_47" id="FNanchor_47_47"></a><a href="#Footnote_47_47" class="fnanchor">[47]</a> This instrument has been arranged in several forms, but +in the simplest of these it consists of a glass tube, having in it a +plug of iron and a plug of arc-lamp carbon, or two plugs of iron with +a plug of carbon between them. The plugs of iron, or of iron and +carbon, are separated by an exceedingly small globule of mercury, the +size of which should be between one and a-half and three millimetres. +The plugs closing the tube must be capable of movement, one of them by +means of a screw, as shown in the diagram (Fig. 17), taken from a +patent specification communicated to Mr. Marconi by the Marchese Luigi +Solari, of the Royal Italian Navy. One of the plugs of this tube is +connected to the aerial and the other to the earth, and they are also +connected through another circuit composed of a single dry cell and a +telephone. The arrangement then forms an extremely sensitive detector +of electric waves or of small electromotive forces, or, if a wave +falls on the aerial, the electromotive force at once improves the +contact between the mercury and the plugs, and therefore causes a +sudden increase in the current through the telephone, giving rise to a +sound; but when the wave ceases, or the electromotive force is +withdrawn, the <span class="pagenum"><a name="Page_59" id="Page_59">[Pg 59]</a></span>resistance falls back again to its origin value, and +the arrangement is, therefore, self-acting, requiring no tapping or +other device for restoring it to receptivity.</p> + +<p>A very ingenious form of combined telephone and coherer has been +devised by T. Tommasina.<a name="FNanchor_48_48" id="FNanchor_48_48"></a><a href="#Footnote_48_48" class="fnanchor">[48]</a> In this instrument the diaphragm of an +ordinary Bell telephone carries upon it a very small carbon or +metallic coherer. This coherer is connected in between the aerial and +the earth, and is also in circuit with a battery and the electromagnet +of a telegraphic relay. When this relay operates it closes the circuit +of another battery which is placed in series with the telephone coil. +The moment the current passes through the telephone coil it attracts, +and therefore vibrates, the diaphragm and shakes up the metallic +filings. If an observer, therefore, places the telephone to his ear, +he hears a sound corresponding to every train of waves incident upon +the aerial. With this arrangement, one can obtain two different kinds +of results, according to the nature of the cohering powder placed in +the cavity in the diaphragm. First, if the powder consists of a +non-magnetic metal, gold, silver, platinum or the like, the receiver +will be very sensitive; and at the same time the current passing +through it when it is cohered will be sufficient <a name="tnd_59a" id="tnd_59a"></a><a href="#tn_59a" class="tnlink" title="possible printer's error, sensive for sensitive">to work a sensive +recording apparatus</a> in series with the telephone coil. Secondly, if +the metallic powder placed in the cavity is a magnetic metal, the +receiver will be somewhat less sensitive, but will work with more +precision, because of the magnetic action of the magnet of the +telephone upon the cohering powder. If no recording apparatus is used, +the observer must write down the signals as heard in the telephone, +since corresponding to a short spark at the transmitting station, a +single tick or short sound is heard at the telephone, and +corresponding to a series of rapidly successive sparks, a more +prolonged sound is heard in the telephone. These two sounds, as +already explained, constitute the dot and the dash of the Morse +signals.</p> + +<p>We may, in the next place, refer to that form of kumascope in which +the action of the wave or of electromotive force is not to decrease +the resistance of a contact, but to increase that of an imperfect +contact. As already mentioned, Professor Branly discovered long ago +that peroxide of lead acts in an opposite manner to metallic filings, +in that when placed in a Branly tube it increases its resistance under +the action of an electric spark, instead of decreasing it. Again, +Professor Bose has found that fragments of metallic potassium in +kerosene oil behave in a similar manner, and that certain varieties of +silver, antimony and of arsenic, and a few other metals, have a +similar property. Branly tubes, therefore, made with these materials, +or any arrangements which act in a similar manner, have been called +"anti-coherers." The <a name="tnd_59b" id="tnd_59b"></a><a href="#tn_59b" class="tnlink" title=" possible printer's error, arragement for arrangement">most interesting arragement</a> which has been called +by this name is that of Schäfer.<a name="FNanchor_49_49" id="FNanchor_49_49"></a><a href="#Footnote_49_49" class="fnanchor">[49]</a> Schäfer's kumascope is made in +the following manner: A very thin film of silver is deposited upon +glass, and a strip of this silver is scratched across with a diamond, +making a fine transverse cut or gap. If the resistance of this divided +strip of silver is <span class="pagenum"><a name="Page_60" id="Page_60">[Pg 60]</a></span>measured, it will be found not to be infinite, but +may have a resistance as low as forty or fifty ohms if the strip is +thirty millimetres wide. On examining the cut in the strip with a +microscope, it will be found that the edges are ragged and that there +are little particles of silver lying about in the gap. If, then, an +electromotive force of three volts or more is put on the two separated +parts of the strip, these little particles of silver fly to and fro +like the pith balls in a familiar electrical experiment, and they +convey electricity across from side to side. Hence a current passes +having a magnitude of a few milliamperes. If, however, the strip is +employed as a kumascope and connected at one end to the earth and at +the other end to an aerial, when electric waves fall upon the aerial, +the electrical oscillations thereby excited seem to have the property +of stopping this dance of silver particles and the resistance of the +gap is increased several times, but falls again when the wave ceases. +If, therefore, a telephone and battery are connected between two +portions of the strip, the variation of this battery current will +affect the telephone in accordance with the waves which fall upon the +aerial, and the arrangement becomes therefore a wave-detecting device. +It is said to have been used in wireless telegraph experiments in +Germany up to a distance of ninety-five kilometres.</p> + +<p>We must next direct attention to those wave-detecting devices which +depend upon magnetisation of iron, and here we are able to record +recent and most interesting developments. More than seventy years ago +Joseph Henry, in the United States, noticed the effect of an electric +spark at a distance upon magnetised needles.<a name="FNanchor_50_50" id="FNanchor_50_50"></a><a href="#Footnote_50_50" class="fnanchor">[50]</a> Of recent times, the +subject came back into notice through the researches of Professor E. +Rutherford,<a name="FNanchor_51_51" id="FNanchor_51_51"></a><a href="#Footnote_51_51" class="fnanchor">[51]</a> who carried out at Cambridge, England, in 1896, a +valuable series of experiments on this subject. He found that if a +magnetised steel needle or a very small bundle of extremely thin iron +wires is magnetised and placed in the interior of a small coil, the +ends of which are connected to two long collecting wires, then an +electric wave started from a Hertz oscillator at a distance causes an +immediate demagnetisation of the iron. This demagnetisation he +detected by means of the movement of the needle of a magnetometer +placed near one end of the iron wire. Although Rutherford's wave +detector has been much used in scientific research, it was not, in the +form in which he used it, a telegraphic instrument, and could not +record alphabetic signals.</p> + +<div class="figright" style="width: 230px;"> +<img src="images/fig18.png" width="230" height="195" alt="FIG. 18.--MARCONI MAGNETIC RECEIVER. W_{1}W_{2}, wheels; +I, iron wire band; P, primary coil; S, secondary coil; T, telephone; +A, aerial; E, earthplate." title="" /> +<span class="caption smcap">Fig. 18.—Marconi Magnetic Receiver</span><span class="caption">. W<sub>1</sub>W<sub>2</sub>, wheels; I, +iron wire band; P, primary coil; S, secondary coil; T, telephone; +A, aerial; E, earthplate.</span> +</div> + +<p>Not long ago Mr. Marconi invented, however, a telegraphic instrument +based upon his discovery that the magnetic hysteresis of iron can be +annulled by electric oscillations. In one form, Mr. Marconi's magnetic +receiver is constructed as follows<a name="FNanchor_52_52" id="FNanchor_52_52"></a><a href="#Footnote_52_52" class="fnanchor">[52]</a> (see Fig. 18): An endless band +of thin iron wire composed of several iron wires about No. 36 gauge, +arranged in parallel, is made to move slowly round on two pulleys, +like the driving belt of a machine. In one part of its <span class="pagenum"><a name="Page_61" id="Page_61">[Pg 61]</a></span>path the wire +passes through a glass tube, on which are found two coils of wire, one +a rather short, thick coil, and the other a very fine, long one. The +fine, long coil is connected with a telephone, and the shorter coil is +connected at one end to the earth and the other to the aerial. Two +permanent horse-shoe magnets are placed <a name="tnd_61" id="tnd_61"></a><a href="#tn_61" class="tnlink" title="printer's error, missing letter i">as shown n Fig. 18,</a> with their +similar poles together, and, as the iron band passes through their +field, a certain length of it is magnetised, and owing to the +hysteresis of the material, it retains this magnetism for a short time +after it has passed out of the centre of the field. If then an +electric oscillation, coming down from the aerial, is passed through +the shorter coil, it changes the position of the magnetised portion of +the iron and, so to speak, brings the magnetised portion of iron back +into the position it would have occupied if the iron had had no +hysteresis. This action, by varying the magnetic flux through the +secondary coil, creates in it an electromotive force which causes a +sound to be heard in the telephone connected to it. If at a distant +place a single wave or train of waves is started and received by the +aerial, this will express itself by making an audible tick in the +telephone, and if several groups of closely adjacent wave trains are +sent, these will indicate themselves by producing a rapid series of +ticks in the telephone, heard as a short continuous noise and taken as +equivalent to the Morse <i>dash</i>.</p> + +<p>It was by means of this remarkably ingenious instrument that Mr. +Marconi was able, in the summer of 1902, to detect the waves sent out +from Poldhu on the coast of Cornwall, and receive messages as far as +Cronstadt in the Baltic, in one direction, and as far as Spezzia in +the Mediterranean in another direction, and also to receive messages +across the Atlantic from the power stations situated in Glace Bay, +Nova Scotia, and from one at Cape Cod in Massachusetts, U.S.A., in +December, 1902.</p> + +<p>There can be no question that this magnetic detector of Mr. Marconi's, +used in connection with a good telephone and an acute human ear, is +the most sensitive device yet invented for the detection of electric +waves and their utilisation in telegraphy without continuous wires. It +is marvellously simple, ingenious and yet effective, as a Hertzian +wave telegraphic receiver.</p> + +<p>Whilst on the subject of magnetic wave detectors, the author may +describe experiments that he has been recently making to construct a +Hertzian wave detector on the Rutherford principle, which shall be +strictly quantitative. All the receivers of the coherer type and +electrolytic type give no indications that are at all proportional to +the energy of the incident wave. Their indications are more or less +accidental and depend upon the manner in which the receiver was last +left. There is a great need for a quantitative wave detector, the +indications <span class="pagenum"><a name="Page_62" id="Page_62">[Pg 62]</a></span>of which shall give us a measure of the energy of the +arriving wave. It is only by the possession of such an instrument that +we can hope to study properly the sending powers of various +transmitters or the efficiency of different forms of aerial or devices +by which the wave is produced. This magnetic receiver is constructed +as follows:</p> + +<p>A coil of fine wire is constructed in sections like the secondary coil +of an induction coil, and in the instrument already made, this coil +contains thirty or forty thousand turns of wire. In the interior of +this coil are placed a number of little bundles of fine iron wire +wound round with two coils, a fine wire coil which is a magnetising +coil, and a thicker wire coil which is a demagnetising coil. These +sets of coils are joined up, respectively, in series or in parallel. +Then, associated with this form of induction coil is a commutator of a +peculiar kind, which performs the following functions when a battery +is connected to it and when it is made to revolve by a motor or by +clockwork. First, during part of the revolution, the commutator closes +the battery circuit and magnetises the iron cores, and whilst this is +taking place the secondary circuit of the induction coil is +short-circuited and the galvanometer is disconnected from it. +Secondly, the magnetising current is stopped, and soon after that the +secondary coil is unshort-circuited and connected to the galvanometer, +and remains in this condition during the remainder of the revolution. +This cycle of operations is repeated at every revolution. If then an +electrical oscillation is sent into the demagnetising coils, and if it +continues longer than one revolution of the commutator, it will +demagnetise the iron core during that period of time in which the +battery is disconnected and the galvanometer connected. The +demagnetisation of the iron which ensues produces an electromotive +force in the secondary coil and causes a deflection of the +galvanometer, and this deflection will continue and remain steady if +the oscillation persists. Moreover, since this deflection is due to +the passage through the galvanometer of a rapid series of discharges, +it is large when the oscillations continue for a long time and are +powerful, and small when they continue for a short time or are weak. +We can, therefore, with this arrangement, receive on the galvanometer, +just as on the mirror galvanometer used in submarine cable work, a dot +or dash, and, moreover, the magnitude of these deflections is a +measure of the energy of the wave.</p> + +<p>It is probable that when this arrangement is perfected it will become +exceedingly useful for making all kinds of tests and measurements in +connection with Hertzian telegraphy, even if it is not sensitive +enough to use as a long distance receiver.</p> + +<p>Of late years a variety of wave-detecting devices have been brought +forward which depend upon electrolysis. One of the best known of these +is that by De Forest and Smythe.<a name="FNanchor_53_53" id="FNanchor_53_53"></a><a href="#Footnote_53_53" class="fnanchor">[53]</a> In this arrangement, a tube +contains two small electrodes like plugs, which may be made of tin, +silver or nickel, or other metal. The ends of these plugs are flat and +separated from each other by about one two-hundredth of an inch. +Sometimes the end of one of these plugs is made cup shaped and the cup +or recess is filled with a mass of peroxide of lead and glycerine. <span class="pagenum"><a name="Page_63" id="Page_63">[Pg 63]</a></span>In +the interval between the electrodes is placed an electrolyzable +mixture, which consists of glycerine or vaseline mixed with water or +alcohol, and a small quantity of litharge and metallic filings. These +metallic filings act as secondary electrodes. When a small +electromotive force is applied between the terminals of the electrodes +of this tube through a very high resistance of twenty or thirty +thousand ohms, an exceedingly small current passes through this +mixture, and it causes an electrolytic action which results in the +production of chains of metallic particles connecting the two +electrodes together. If, in addition to this, one terminal or +electrode of the arrangement is connected to an aerial wire and the +other terminal to the earth, then on the arrival of an electric wave +creating oscillations in the wire, these oscillations pass down into +the electrolytic cell, where they break up the chains of metallic +particles and thus interrupt the current passing through the telephone +quite suddenly, which is heard as a slight tick by an ear applied to +it. As soon as the wave ceases, the chain of metallic particles is +re-established, so that the appliance is always in a condition to be +affected by a wave. It is said that this breaking up and reformation +of the chains of metallic particles is so rapid that a short spark +made at the transmitting station is heard as a tick in the telephone, +but a rapid succession of oscillatory sparks is heard as a short +continuous sound; hence the two signals necessary for alphabetical +conversation can be transmitted.</p> + +<p>Another receiver which has some resemblance to the above, although +different in principle, is that of Neugschwender.<a name="FNanchor_54_54" id="FNanchor_54_54"></a><a href="#Footnote_54_54" class="fnanchor">[54]</a> In this +arrangement, which to a certain extent resembles the Schäfer detector, +a glass plate has upon it a deposit of silver in the form of a strip, +which is cut across at one place, thus interrupting it. If the cut is +breathed upon or placed in a moist atmosphere, a little dew is +deposited upon the glass, which bridges over the cut in the metal and +creates an electric continuity. Hence a small current can be passed +across the gap and through a telephone by one or two cells of a +battery. If, however, an electric oscillation passes across the gap on +its way from an aerial to the earth, then the continuity of the liquid +film is destroyed, and the current is interrupted and a sound created +in the telephone.</p> + +<p>The opinion has been expressed by Sir Oliver Lodge that in this case +the interruption of the circuit which occurs is really due to the +coalescence of minute water particles into larger drops, as when +vapour is condensed into rain, and hence the continuity of the +material is interrupted.</p> + +<p>We must then make a brief reference to other kumascopes which depend +upon the heating power of an electrical oscillation, which it +possesses in common with every other form of electric current. +Professor R. A. Fessenden<a name="FNanchor_55_55" id="FNanchor_55_55"></a><a href="#Footnote_55_55" class="fnanchor">[55]</a> has constructed a very ingenious thermal +receiver in the following manner: An extremely fine platinum wire, +about 0·003 of an inch in diameter, is embedded in the middle of a +silver wire about one tenth of an inch in diameter, like the wick of +a <span class="pagenum"><a name="Page_64" id="Page_64">[Pg 64]</a></span>candle. This compound wire is then drawn down until the diameter of +the silver wire is only 0·002 of an inch, and hence the platinum wire +in its interior, being reduced in the same ratio, will have been drawn +to a diameter of 0·00006 of an inch. A short piece of this drawn wire +is then bent into a loop and the ends fixed to wires. The tip of the +loop is then immersed in nitric acid and dissolved in the silver, +leaving an exquisitely fine platinum wire a few hundredths of an inch +in length and having a resistance of about thirty ohms. This little +loop is sealed into a glass bulb like a very small incandescent lamp, +or it may be enclosed in a small silver bulb and the air may be +exhausted. If an electrical oscillation is sent through this +exceedingly fine platinum wire it heats it and rapidly increases its +resistance. The electrical oscillations produced in an aerial are sent +through a number of these loops arranged in parallel, and the loops +are short-circuited by a telephone, joined in series with a source of +very small electromotive force produced by shunting a single cell or +opposing to one another two cells of nearly equal electromotive force. +Any variation of resistance of the little platinum loops due to the +heat produced by the oscillations, by suddenly altering the current +flowing through the telephone, will cause a sound to be heard in it. +The electrical oscillations when passing through the loops are +therefore detected by the heat which they generate in these +exquisitely fine platinum wires.</p> + +<p>Finally, one word must be said on the subject of electrodynamic +receivers, due to the same inventor. An exceedingly small silver ring +is suspended by a quartz fibre and has a mirror attached to it in the +manner of a galvanometer. This ring is suspended between two coils +joined in series, which are placed either in the circuit of the aerial +or in the secondary circuit of the small air core transformer inserted +between the aerial and the earth. When electrical oscillations travel +down the aerial they induce other electrical oscillations in the +silver ring, and if the ring is so placed that its normal position is +with its plane inclined at an angle of forty-five degrees to the plane +of the fixed coils, then the ring will be slightly deflected every +time an oscillation occurs in the aerial.</p> + +<p>Omitting further mention of the details of the kumascopes in use and +the receiving aerial, we must next proceed to consider the receiving +arrangements taken as a whole.</p> + +<p>In the original Marconi system, the sensitive tube or coherer was +inserted between the bottom of the receiving aerial and the earth.<a name="FNanchor_56_56" id="FNanchor_56_56"></a><a href="#Footnote_56_56" class="fnanchor">[56]</a> +Accordingly, when the incident electric wave strikes the receiving +aerial and creates in it an oscillatory electromotive force, this last +will, if of sufficient amplitude, cause the particles of the coherer +to cohere and become conductive. This sudden change from a nearly +perfect non-conductivity to a conductive condition is made to act as a +switch or relay, closing or completing the circuit of a single cell, +and so sending a current through an ordinary telegraphic relay, +closing or completing the circuit of a single cell, which may in turn +actuate another recording telegraphic instrument, such as a Morse +printer. To <span class="pagenum"><a name="Page_65" id="Page_65">[Pg 65]</a></span>prevent the oscillations from passing into the relay +circuit, small choking or inductance coils are inserted between the +ends of the sensitive tube and the relay and cell and serve to confine +the oscillations to the tube.</p> + +<p>It has already been pointed out that in the transmitting aerial the +amplitude at the potential vibrations increases from the bottom to the +top, and when vibrating in its fundamental manner there is a potential +node at the earth connection and a potential loop or antinode at the +top. The same is true of the receiving aerial. Hence, if the kumascope +employed is a Branly metallic filings tube and is inserted near the +base of the aerial, the difference of potential between its two ends +will be small.</p> + +<p>It has also been mentioned that a receiver of this type acts in virtue +of electromotive force or potential difference, and hence the proper +place to insert the coherer is not at the base of the aerial, but +between the top of the aerial and the earth. This, however, could not +be done by running up another wire from the earth, as that would +amount to putting the coherer between the tops of two identical +aerials, and between its ends there would be no difference of +potential. Professor Slaby, in conjunction with Count von Arco, has +given an ingenious solution of this problem. If we take two equal +lengths of wire, bent at right angles, and connect the point of +intersection with the earth, placing one of these wires vertically and +the other horizontally, we then have an arrangement which responds to +the impact of electric waves, and has electrical oscillations set up +in it in such fashion that the common point of the two wires has a +very small amplitude of potential, but the two extremities have equal +and large variations. If, then, we insert a coherer tube between the +earth and the outer extremity of the horizontal wire, it is influenced +in the same manner as it would be by the potential variations at the +top of the vertical wire. In other words, it is acted upon by a large +difference of potential instead of a small one. It is not found +necessary to stretch the horizontal wire out straight; it may be +coiled into a spiral with open turns, and the slight decrease in +capacity and increase in inductance resulting from this can be +compensated by cutting off a short piece of it.</p> + +<div class="figright" style="width: 222px;"> +<img src="images/fig19.png" width="222" height="168" alt="FIG. 19.--SLABY RECEIVER. A, aerial; E, earth plate; F, +coherer; M, multiplier; C, condenser; R, relay; B, battery; E, earth +plate." title="" /> +<span class="caption smcap">Fig. 19.—Slaby Receiver.</span><span class="caption"> A, aerial; E, earth plate; F, +coherer; M, multiplier; C, condenser; R, relay; B, battery; E, earth +plate.</span> +</div> + +<p>In this way we have an arrangement (see Fig. 19) in which the outer +extremity of this open spiral experiences variations or potential +which exactly correspond with those at the summit of the vertical +aerial. The receiving arrangements are then completed as in Fig. 19, +one end of the coherer being attached to the outer end of the spiral +and the other end through a condenser to the earth, a relay and a +voltaic cell being arranged as shown in the diagram. The mode of +operation of this receiver is as follows: When the wave strikes the +aerial it sets up in it electrical oscillations with a potential +antinode at the summit, and at the same time a potential antinode is +created at the outer end of the spiral attached near the base of the +aerial, this spiral being called by Professor Slaby a +<i>multiplicator</i>. <span class="pagenum"><a name="Page_66" id="Page_66">[Pg 66]</a></span>As long as the coherer tube remains non-conductive, +the local cell cannot send a current through the relay, but, as soon +as the resistance is broken down by the impact of a wave, the local +cell sends a current through the coherer tube which, passing down to +the earth through the base of the aerial and up through the earth +connection to the condenser, completes its circuit through the relay. +Many variations of this arrangement have been made by Slaby and Von +Arco and by the Allgemeine Elektricitäts Gesellschaft of Berlin.</p> + +<p>In 1898, Mr. Marconi made a great advance in the construction of his +receiving apparatus by the insertion of his "jigger" or oscillation +transformer in the aerial receiving circuit.<a name="FNanchor_57_57" id="FNanchor_57_57"></a><a href="#Footnote_57_57" class="fnanchor">[57]</a> In this arrangement, +the primary coil of an air core transformer wound in a particular way +is inserted between the receiving aerial and the earth, and the +secondary circuit is cut in the middle and connected to the two +surfaces of a condenser, these surfaces being also connected through +the circuit of an ordinary telegraphic relay and a single cell (see +Fig. 20). The ends of the secondary circuit of this oscillation +transformer are also connected to the terminals of the coherer tube, +and these again are short-circuited by a small condenser.</p> + +<div class="figleft" style="width: 242px;"> +<img src="images/fig20.png" width="242" height="195" alt="FIG. 20.--MARCONI RECEIVER. A, aerial; J, jigger; CC, +condensers; F, filings tube; T, tapper; R, relay; B, battery; M, Morse +printer." title="" /> +<span class="caption smcap">Fig. 20.—Marconi Receiver.</span><span class="caption"> A, aerial; J, jigger; CC, +condensers; F, filings tube; T, tapper; R, relay; B, battery; M, Morse +printer.</span> +</div> + +<p>The operation of this receiver is as follows: The oscillations set up +in the aerial pass through the primary circuit of the jigger, and +these induce other oscillations in the secondary circuit; the +electromotive force or difference of potential between the primary +terminals being transformed up in any desired ratio. It is this +exalted electromotive force which is made to act on the coherer tube, +and, inasmuch as the jigger operates in virtue of a current passing +through its primary circuit and this current is at a maximum at the +lower end of the aerial, the arrangement is exceedingly effective, +because it, so to speak, converts current into voltage. At the lower +end of the aerial, although the amplitude of the potential +oscillations is a minimum, the amplitude of the current oscillations +is a maximum, and the jigger transforms these large current +oscillations into large potential oscillations, <i>provided it is +constructed in the right manner</i>. We can also transform up or increase +the amplitude of the small potential variations near the bottom of the +aerial by employing the principle of resonance. Many devices of this +kind, due to Professor Slaby and others, have been suggested and tried +but the details are rather too technical to be fully described here.</p> + +<p>It will be noticed that the receiving aerial may be arranged in one of +two ways—it may be either earthed at the lower end or it may be +insulated. It has been claimed that there is a great advantage in +earthing the receiving aerial directly in that it eliminates +atmospheric disturbances.</p> + +<p><span class="pagenum"><a name="Page_67" id="Page_67">[Pg 67]</a></span></p> +<p>We shall allude to this point more particularly later on. Meanwhile it +may be mentioned that the receiving arrangements, as a whole, +constitute a sensitive arrangement, as shown by Popoff, Tommasina and +by all the large experience of Mr. Marconi himself for detecting +changes in the electrical condition of the atmosphere, which are +doubtless of the nature of electrical oscillations. On the other hand, +the receiving arrangements may be perfectly insulated and some +experimentalists have asserted that by this method the greatest +freedom is secured from atmospheric disturbances. Amongst the +non-earthed arrangements the system invented by Professor F. Braun, of +Strassburg, and worked by Messrs. Siemens, of Berlin, may be +mentioned.<a name="FNanchor_58_58" id="FNanchor_58_58"></a><a href="#Footnote_58_58" class="fnanchor">[58]</a></p> + +<div class="figright" style="width: 217px;"> +<img src="images/fig21.png" width="217" height="204" alt="FIG. 21.--BRAUN'S NON-EARTHED RECEIVER. I, induction +coil; CC, condensers; S, spark gap; J, transmitting jigger; K, +receiving jigger; F, filings tube; R, relay; B, battery." title="" /> +<span class="caption smcap">Fig. 21.—Braun's Non-earthed Receiver.</span><span class="caption"> I, induction +coil; CC, condensers; S, spark gap; J, transmitting jigger; K, +receiving jigger; F, filings tube; R, relay; B, battery.</span> +</div> + +<p>Professor Braun's arrangements are indicated in the diagram in Fig. +21. In this case an induction coil is used to create a discharge +between two spark balls, and to these two balls are connected the two +outer coatings of two condensers, the inner coatings of which are +connected together through the primary coil of an air core +transformer. The secondary coil of this transformer is connected to +two extension wires forming a Hertz resonator, and the length of these +wires is so adjusted with reference to the time period of the primary +circuit that they resonate to it, the whole length from end to end of +the secondary circuit being half a wave-length. The receiver, as shown +in the diagram, consists of a pair of quarter wave-length receiving +wires connected through two condensers, which are short-circuited by +the primary coil of an oscillation transformer. The secondary circuit +of this last oscillation transformer has two extension wires to it, +turned in the same manner, to respond to the primary oscillator; and +in the circuit of one of these extension wires is placed a coherer +tube, short-circuited by a relay and a local battery.</p> + +<p>It will thus be seen that there is an entire abolition of ground +connection, which, Professor Braun claims, practically avoids all +atmospheric disturbances.<a name="FNanchor_59_59" id="FNanchor_59_59"></a><a href="#Footnote_59_59" class="fnanchor">[59]</a> The details of the receiving arrangement +are as follows:—The coherer tube consists of an ebonite tube +containing hard steel particles of a uniform size, placed in the +adjustable space between two polished steel electrodes. It is found +that with this steel coherer, a small amount of magnetism in the +particles increases its sensitiveness, and to obtain this, a ring +magnet is employed in connection with a coherer tube. Receiving +apparatus arranged on this system is said to have been used for +telegraphing between Heligoland and Cuxhaven, a distance of thirty-six +miles.</p> + +<p><span class="pagenum"><a name="Page_68" id="Page_68">[Pg 68]</a></span></p> +<p>All the immense experience, however, gained by Mr. Marconi and those +who have worked with his system, is in favour of using the earth +connection. There is no doubt that Hertzian wave telegraphy can be +conducted over short distances by means of totally insulated aerials, +but for long distances the earth connection is essential, for the +reasons that have been explained previously.</p> + +<p>There are many of the details of the receiving arrangements which +remain to be considered. If the communication is received by a +telegraphic instrument like the Morse printer, which requires a +current of anything like ten milliamperes to work it, then an +important element in the receiving arrangement is the relay. The relay +that is generally used is a modified form of the Siemens polarised +relay, which is so adjusted as to make a single contact. For marine +work on board ship, it is essential that this relay shall be balanced +so that variations in position shall not affect it. Sometimes the +relay is hung in gimbals like a compass, and at other times suspended +from a support by elastic bands, so as to avoid jolting. In any case, +the relay must be so adjusted that no change of position will cause it +to close the circuit of the telegraphic printer or recorder. Its +sensibility ought to be such that it is actuated by a tenth of a +milliampere, and, if possible, even by less. The alteration of +sensibility in the ordinary contact form of relay is the pressure that +is necessary to bring the platinum points of the circuit closer +together, so as to pass the minimum current which will work the +telegraph printer.</p> + +<p>The important matter, however, in connection with the use of the relay +in Hertzian wave telegraphy, is that it should be capable of +adjustment without extraordinary skill. It is no use to put into the +hands of an operator a relay which requires abnormal dexterity to make +it work at all.</p> + +<hr style="width: 45%;" /> + +<p>It remains, then, to consider some of the questions connected with +practical Hertzian wave telegraphy and the problem of the limitation +of communication. These matters at the present moment very much occupy +the public attention, and many conflicting opinions are expressed +concerning them.</p> + +<p>It may be observed at the outset that the difficulty of dealing with +the subject as freely as many desire is that Hertzian wave telegraphy +is no longer merely a subject of scientific investigation, but has +developed into a business and involves, therefore, other interests +than the simple advancement of scientific knowledge. We can, however, +discuss in a general manner some of the scientific problems which +present themselves for solution. The first of these is the +independence of communication between stations. It is desirable, at +the outset, to clear up a little misunderstanding. There is a great +difference between preventing the reception of communication when it +is not desired by the recipient, and preventing it when it is the +object of the latter to overhear if he can. It is, therefore, +necessary to distinguish between isolation and overhearing. We may say +that a station is <i>isolated</i> when it is not affected by Hertzian waves +other than those it desires to receive; but that a station <i>overhears</i> +when it can, if it chooses, pick up <span class="pagenum"><a name="Page_69" id="Page_69">[Pg 69]</a></span>communications not intended for +it, or cannot help receiving them against its will.</p> + +<p>This distinction is a perfectly fair one. Any telegraph or telephone +wire can be tapped, if it is desired, but unless there is some fault +on the line, no station will receive a message against its desires. +Moreover, it may be noted that there are penalties attached to tapping +a telegraph wire, and at present there are none connected with the +misappropriation of an ether wave.</p> + +<p>We shall, therefore, consider in the first place the methods so far +proposed for preventing any given receiver from being affected by +Hertzian waves sent out from other stations, except that of those from +which it is desired to receive them. The first method is that which +has been called the method of <i>electrical syntony</i>, and consists in +adjusting the electrical capacity and inductance of the various open +and closed circuits of the receiving and transmitting stations to be +put in communication so that they have the same electrical +time-period.<a name="FNanchor_60_60" id="FNanchor_60_60"></a><a href="#Footnote_60_60" class="fnanchor">[60]</a></p> + +<p>In the Cantor Lectures before the Society of Arts in 1900, on +electrical oscillations and electric waves, the author has discussed +at length the conditions under which powerful electrical oscillations +can be set up in a circuit. It was there shown that every electric +circuit having capacity and inductance has a particular or natural +time-period of electrical oscillation depending on the product of +these qualities, and that, to accumulate powerful electrical +oscillations in it, the electromotive impulses on it must be delivered +at this rate. Illustrations were drawn from mechanics, such as the +examples furnished by vibrating pendulums and springs, and from +acoustics, as illustrated by the phenomena of resonance, to show that +small or feeble blows or impulses delivered at the proper time +intervals have a cumulative effect in setting up vibrations in a body +capable of oscillation. It is a familiar fact that if we time our +blows, we can achieve that which no single blow, however powerful, can +accomplish in throwing into vibration a body such as a pendulum, which +is capable of oscillation under the action of a restoring force. +Precisely the same is true of an electric circuit. We have already +seen that the receiving aerial has an alternating electromotive force +set up in it by the impact of the successive electric waves sent out +from the transmitter. It must, however, be remembered that the +transmitter sends out a series of trains of waves, not by any means a +continuous train, but one cut up into groups of probably ten to fifty +waves, each separated by intervals of silence, long, compared with the +duration of a single train of waves.</p> + +<div class="figleft" style="width: 235px;"> +<img src="images/fig22.png" width="235" height="166" alt="FIG. 22.--SEIBT'S APPARATUS FOR EXHIBITING ELECTRIC +RESONANCE. I, induction coil; S, spark gap; CC, condensers; L, +variable inductance; E, earth plate; WW, wire spirals; VV, vacuum +tubes." title="" /> +<span class="caption smcap">Fig. 22.—Seibt's Apparatus for Exhibiting Electric +Resonance.</span><span class="caption"> I, induction coil; S, spark gap; CC, condensers; L, +variable inductance; E, earth plate; WW, wire spirals; VV, vacuum +tubes.</span> +</div> + +<p>If, however, by a suitable adjustment of capacity and inductance, we +make the natural time-period of oscillation of the receiving aerial +circuits agree with those of the transmitting aerial, within certain +limits the former will only be receptive for waves of the frequency +sent out by the transmitter. It is quite easy to illustrate this +principle by numerous experiments. It can be done by means of an +apparatus <span class="pagenum"><a name="Page_70" id="Page_70">[Pg 70]</a></span>devised by Dr. Georg Seibt for showing in an interesting +manner the syntonisation or tuning of two electric circuits. This +consists of two bobbins, each consisting of one layer of insulated +wire wound on a wooden rod (see Fig. 22). Each of these bobbins has a +certain electrical capacity with respect to the earth, when considered +as an insulated conductor, and it has also a certain inductance. If, +therefore, electromotive impulses are applied to one end of the bobbin +at regular intervals, electrical oscillations will be set up in it, +and, as already explained, if these are timed at a certain rate, the +bobbin will act like a closed organ-pipe to air impulses and +oscillations of potential will be accumulated at the opposite end, +which have much greater amplitude than the impressed oscillations at +the end at which they are applied. We can make the existence of the +amplitude oscillations of potential evident by attaching to one end of +the bobbin a vacuum tube, which will be illuminated thereby, or by +terminating it by a pointed piece of wire, so that an electrical brush +can be formed at the point, if the potential variations have +sufficient amplitude. We arrange also another closed oscillation +circuit, consisting of two Leyden jars and a variable inductance coil +and a pair of spark balls which are connected to an induction coil. In +this manner we can set up oscillations in the discharge circuit of +these Leyden jars, and we can vary the time period by altering the +inductance and the capacity. If we denote the capacity of the jars in +the microfarads by the letter C and the inductance in centimetres of +the discharge circuit of the jars by the letter L, it can then be +shown that the number of oscillations per second denoted by <i>n</i> is +given by the expression—<a name="FNanchor_61_61" id="FNanchor_61_61"></a><a href="#Footnote_61_61" class="fnanchor">[61]</a></p> + +<div class="eq"> +<img class="tex" alt="n = \frac{5,000,000,000}{\sqrt{CL}}." src="images/eq_15.png"/> +</div> + +<p>If now we adjust the Leyden jar circuit to a particular rate of +oscillation, we have between the terminals of the jar or condenser an +alternating difference of potential or electromotive force. If we +connect one side of the jars to the earth and the other side to the +foot of one of the spirals or bobbins above described, we shall find +perhaps that the vacuum tube at the other end is not rendered +luminous. When, however, we adjust the inductance in the discharge +circuit of the jar to a certain value to make the frequency of the +condenser oscillations agree with the natural time period of the +bobbin terminated by the vacuum tube, this latter at once lights up +brilliantly. Again, if we connect both these bobbins at the same time +to the discharge circuit of the <span class="pagenum"><a name="Page_71" id="Page_71">[Pg 71]</a></span>Leyden jar, we shall find that we can +make an adjustment of the inductance of that circuit, such that either +of the bobbins at pleasure can be made to respond and be set in +electrical vibration, as shown by the illumination of the vacuum tube +at its upper end or by an electrical brush being formed at the +terminal. In making this adjustment of inductance, we are <i>tuning</i>, as +it is called, the Leyden jar discharge circuit to the resonating +bobbin. A very small variation of the inductance of the jar circuit +causes the vacuum tube to change in luminosity. If, however, the +natural time periods of these bobbins do not lie very far apart, then +a faint luminosity will make its appearance in both the vacuum tubes. +Supposing, therefore, that we connect to the oscillating circuit of +the jar a number of bobbins having different time periods of +oscillation, like organ-pipes, and supply them all with one common +alternating electromotive force. These bobbins, whose natural time +period is very different <a name="tnd_71a" id="tnd_71a"></a><a href="#tn_71a" class="tnlink" title="printer's error, osciilating for oscillating">to that of the osciilating circuit</a> or +impressed electromotive force, will not respond, but those bobbins of +which the natural time period lies near to, even if not quite exactly +the same as, that of the impressed electromotive force will give +evidence of being set in oscillation. A very violent electromotive +force will cause them all to respond to some slight extent, no matter +whether <a name="tnd_71b" id="tnd_71b"></a><a href="#tn_71b" class="tnlink" title="printer's error, impluse for impulse">the period of that impluse</a> is tuned to their common period +precisely or not.</p> + +<p>At this point questions arise of great practical importance. A matter +which has been in dispute in connection with practical Hertzian wave +telegraphy is how far this electrical tuning is a sufficient solution +of the practical problem of isolation. It is not denied that +experiments such as those made with Seibt's apparatus can be shown on +a small scale; and, on a still larger scale, Mr. Marconi gave to the +author in September, 1900, a demonstration in practical telegraphic +work of sending two independent Hertzian wave messages and receiving +them on two independent receivers attached to the same aerial.</p> + +<p>Since that date much experience has been gained and large power +stations erected, and a statement has been frequently made that +syntony is no protection against interference when one of the stations +is sending out very powerful waves. The contention has been raised +that large power stations producing electric waves will therefore play +havoc with Hertzian wave telegraphy on a smaller scale, such as the +ship to shore and intermarine communication. Under these +circumstances, it appeared to the author important to subject the +matter to a special test, and Mr. Marconi, therefore, offered to give +a demonstration, with this object, in support of the opinion that he +has expressed positively that waves from his power stations do not +interfere with the working of his ship installations. This matter is +vital to the whole question of practical Hertzian wave telegraphy, for +the ship to shore communication is of stupendous importance; and if +Mr. Marconi had done nothing else except to render this possible and +effective, he would have earned, as he has done, the gratitude of +humanity for all time. Accordingly, the author embraced the +opportunity of making some careful tests to settle the question +whether the powerful waves sent out from a station such as Poldhu did +or did not affect the exchange of messages between ship <span class="pagenum"><a name="Page_72" id="Page_72">[Pg 72]</a></span>and shore +stations in proximity, equipped with Marconi apparatus of a suitable +type.</p> + +<p>These experiments were carried out on the eighteenth of March last, at +Poldhu, in Cornwall, and a programme was arranged by the author of the +following kind. Close to the Poldhu station is an isolated mast, which +was equipped by Mr. Marconi with a Hertzian wave apparatus, similar to +that he places on ships. Six miles from Poldhu is the Lizard receiving +station, with which ships proceeding up or down the English Channel +communicate. It was arranged that a series of secret messages, some of +them in cipher, should be delivered simultaneously at certain known +times, both to the power station at Poldhu and to the small adjacent +ship station; and it was arranged that these messages should be sent +off simultaneously, the operators being kept in ignorance up to the +moment of sending as to the nature of the messages. At the Lizard, Mr. +Marconi connected two of his receiving instruments to the aerial, one +of them tuned to the waves proceeding from the power station at +Poldhu, and the other to those proceeding from the small ship station. +At the appointed time, these two sets of messages were received +simultaneously in the presence of the author, each message being +printed down independently on its own receiver; and Mr. Marconi read +off and interpreted all these messages perfectly correctly, not having +known before what was the message that was about to be sent. In +addition to this, precautions were taken to prove that the power +station at Poldhu was really emitting waves sufficiently powerful to +cross the Atlantic and not being made to sing small for the occasion. +To assist in proving this, the messages sent out from the power +station were also received at a station at Poole, two hundred miles +away, and the assistant there was instructed to telegraph back these +messages by wire as soon as he received them. These messages came back +perfectly correctly, thus demonstrating that the power station was +sending out power waves. The whole programme was carried out with the +greatest care to avoid any mistakes on the part of the assistants, and +provided an absolute demonstration of the truth of Mr. Marconi's +assertion that the waves from one of his power stations, such as +Poldhu, do not in the least degree interfere with the transmission and +reception of messages between ship and shore, effected by means of +certain forms of Marconi apparatus for producing and detecting waves +of a different wave length.<a name="FNanchor_62_62" id="FNanchor_62_62"></a><a href="#Footnote_62_62" class="fnanchor">[62]</a> This complete independence of +transmission, however, is entirely due to the employment of a +receiving circuit of a certain type in Mr. Marconi's receivers. It +does not at all follow that a receiving circuit of any kind, even a +Marconi receiver not especially arranged, set up in proximity to a +power station would not be affected. This, however, is not an +important matter. Far more important is it to show, as has been shown, +that practically perfect isolation can be achieved if it is desired.</p> + +<p>It must be noted, however, that, although the fact that electric +circuits have a natural time-period of oscillation of their own is a +scientific principle which carries us a considerable way towards a +<span class="pagenum"><a name="Page_73" id="Page_73">[Pg 73]</a></span> +solution of what is called syntonic Hertzian wave telegraphy, it is +not in itself alone in every respect an entire solution of the +practical problem. The degree to which it is a solution depends to a +considerable extent upon the nature of the detecting device, or +kumascope, which we are employing. The coherer, or Branly filings +tube, has the peculiarity that its passage from a non-conductive to a +conductive condition follows immediately when the difference of +potential between its ends is made sufficiently great. In other words, +if the tube is acted upon by a sufficient electromotive force, it is +not necessary that electromotive force should be repeated at intervals +to make this particular form of kumascope responsive. Again, if we +consider the nature of the oscillations which are sent out from any +transmitting aerial, we find that each group of oscillations +corresponding to a single spark consists of waves gradually decreasing +in amplitude. In other words, the first wave of the group is the +strongest, and the decay in amplitude is often very rapid. Supposing, +then, we construct a simple receiver consisting of an aerial having +inserted in its circuit a sensitive Branly filings tube. Such a +receiver is almost entirely non-syntonic; that is to say, it is +affected by any wave passing over it which is sufficiently powerful. +We may look upon it that if the first wave of the series is +sufficiently powerful to affect the kumascope, the conductive change +takes place whether or not the first wave is followed by others. +Accordingly, it is perfectly certain that if a transmitter is sending +out trains of waves of any period, a simple combination of coherer and +aerial will be influenced, if it is placed near enough to the +transmitter. On the other hand, it is possible to combine a kumascope +of a certain type with a receiving aerial and other circuits in such a +manner that when the waves that reach it are feeble it shall not +respond at all unless those waves have very nearly a time period of a +certain value.</p> + +<p>At this stage, it may be perhaps well to explain a little in detail +what is meant by an easily responsive circuit, and, on the other hand, +by an irresponsive circuit, or, as we may call it, a <i>stiff</i> circuit. +Supposing that we consider an aerial consisting of a simple straight +wire having small capacity and small inductance, such a circuit admits +of being sent into electrical oscillation, not only by waves of its +own natural time-period, but by the sudden application of any violent +electromotive impulse. If, on the other hand, we bestow upon the +circuit in any way considerable inductance, we then obtain what may be +called a stiff or irresponsive circuit, which is one in which +electrical oscillations can be accumulated only by the prolonged +action of impulses tuned to a particular period.</p> + +<p>A mechanical analogue of this difference may be found in considering +the different behaviour of elastic bodies to mechanical blows. Take, +for instance, a piece of elastic steel and fix the bottom end in a +vice. The steel strip may be thrown into vibration by deflecting the +upper end. It has, however, a very small mass, and therefore any +violent blow or blows, even although not repeated, will set it in +oscillation. If, however, we add mass to it by fixing at the other end +a heavy weight, such as a ball of lead, and at the same time make the +<span class="pagenum"><a name="Page_74" id="Page_74">[Pg 74]</a></span> +spring stiffer, we have an arrangement which is capable of being sent +into considerable oscillation only by the action of a series of +impulses or blows which are timed at a particular rate.</p> + +<p>Returning then to the electrical problem, we see that in order to +preserve a kumascope or wave detector from being operated on by any +vagrant wave or waves having a period very different to an assigned +period, it must be associated with an electrical circuit of the kind +above called a stiff circuit.</p> + +<p>We will now consider the manner in which the problem has been +practically attacked by Mr. Marconi, Dr. Slaby, Sir Oliver Lodge and +others, who have invented forms of receiver and transmitter, which are +syntonic or sympathetic to one another.</p> + +<p>Some of the methods which Mr. Marconi has devised for the achievement +of syntonic wireless telegraphy were fully described by him in a Paper +read before the Society of Arts on <a name="tnd_74" id="tnd_74"></a><a href="#tn_74" class="tnlink" title="possible printer's error, extra comma in date">May, 17, 1901.</a><a name="FNanchor_63_63" id="FNanchor_63_63"></a><a href="#Footnote_63_63" class="fnanchor">[63]</a></p> + +<div class="figcenter" style="width: 503px;"> +<img src="images/fig23.png" width="503" height="183" alt="FIG. 23.--MARCONI TRANSMITTER AND RECEIVER. I, +induction coil; A, aerial; E, earth plate; HH, choking coils; S, spark +gap; J, transmitting jigger; K, receiving jigger; R, relay; C, +condenser; F, filings tube; B, battery. Many practical details are +omitted." title="" /> +<span class="caption smcap">Fig. 23.—Marconi Transmitter and Receiver.</span><span class="caption"> I, +induction coil; A, aerial; E, earth plate; HH, choking coils; S, spark +gap; J, transmitting jigger; K, receiving jigger; R, relay; C, +condenser; F, filings tube; B, battery. Many practical details are +omitted.</span> +</div> + +<p>On referring to his Paper, it will be seen that in one form his +transmitter consists of an aerial, near the base of which is inserted +the secondary circuit of an oscillation transformer or transmitting +jigger. One end of this secondary circuit is attached to the aerial +and the other end is connected to the earth through a variable +inductance coil. The primary circuit of this oscillation transformer +is connected in series with a condenser, consisting of a battery of +Leyden jars, and the two together are connected across to the spark +balls which close the secondary circuit of an induction coil, having +the usual make and break key in the primary circuit. Mr. Marconi so +adjusts the induction of the aerial and the capacity of the condenser, +or battery of Leyden jars, that the two circuits, consisting +respectively of this battery of Leyden jars and the primary circuit of +the transformer, and on the other hand of the capacity of the aerial +and the inductance in series with it, and that of the secondary +circuit of the transformer have the same time period. In other words, +these two inductive circuits are tuned together. At the receiving end, +the aerial is connected in series with a variable inductance and with +the primary circuit of another oscillation transformer, the second +terminal of which is connected to the earth. The secondary circuit of +this last oscillation transformer is cut in the middle and is +connected to the terminals of a small condenser. The outer terminals +<span class="pagenum"><a name="Page_75" id="Page_75">[Pg 75]</a></span> +of this secondary circuit are connected to the metallic filings tube +or other sensitive receiver and to a small condenser in parallel with +it (see Fig. 23). The terminals of the condenser which is inserted in +the middle of the secondary circuit of the oscillation transformer are +connected through two small inductance coils with a relay and a single +cell. This relay in turn actuates a Morse printer by means of a local +battery. The two circuits of the oscillation transformer are tuned or +syntonised to one another, and also to the similar circuit of the +transmitting arrangement. When this is the case, the transmitter +affects the co-resonant receiving arrangement, but will not affect any +other similar arrangement, unless it is within a certain minimum range +of distance. Owing to the inductance of the oscillation transformer +forming part of the receiving arrangements, the receiving circuit is, +as before stated, very stiff or irresponsive; the sensitive tube is +therefore not acted upon in virtue merely of the impact of the single +wave against the aerial, but it needs repeated or accumulated effects +of a great many waves, coming in proper time, to break down the +coherer and cause the recording mechanism to act. An inspection of the +diagram will show that as soon as the secondary electromotive force in +the small oscillation transformer or jigger of the receiving +instrument is of sufficient amplitude to break down the resistance of +the coherer, the local cell in circuit with the relay can send a +current through it and cause the relay to act and in turn make the +associated telegraphic instrument record or sound.</p> + +<p>Mr. Marconi described in the above-mentioned Paper some other +arrangements for achieving the same result, but those mentioned all +depend for their operation upon the construction of a receiving +circuit on which the time-period of electrical oscillations is +identical with that of a transmitting arrangement. By this means he +showed experiments during the reading of his Paper, illustrating the +fact that two pairs of transmitting and receiving arrangements could +be so syntonised that each receiver responded only to its particular +transmitter and not to the other.</p> + +<p>With arrangements of substantially the same nature, he made +experiments in the autumn of 1900 between Niton, in the Isle of Wight, +and Bournemouth, a distance of about thirty miles, in which +independent messages were sent and received on the same aerial.</p> + +<p>Dr. Slaby and Count von Arco, working in Germany, have followed very +much on the same lines as Mr. Marconi, though with appliances of a +somewhat different nature. As constructed by the General Electric +Company, of Berlin, the Slaby-Arco syntonic system of Hertzian +telegraphy is arranged in one form as follows:—The transmitter +consists of a vertical rod like a lightning conductor, say, 100 or 150 +feet in height. At a point six or nine feet above the ground, a +connection is made to a spark ball (see Fig. 24), and the +corresponding ball is connected through a variable inductance with one +terminal of a condenser, the other terminal of which is connected to +the earth. The two spark balls are connected to an induction coil, or +alternating current transformer, and by variation of the inductance +and capacity the frequency is so arranged that the wave-length +corresponding to it is equal to four <span class="pagenum"><a name="Page_76" id="Page_76">[Pg 76]</a></span>times the length of that portion +of the aerial which is above the spark ball connection. The method by +which this tuning is achieved is to insert in the portion of the +aerial below the spark balls, between it and the earth, a hot wire +ammeter of some form. It has already been shown that in the case of +such an earthed aerial, when electrical oscillations are set up in it, +there is a potential node at the earth and a potential anti-node or +loop at the summit, if it is vibrating in its fundamental manner; +also, there is a node of current at the summit of the aerial and an +anti-node at the base. This amounts to saying that the amplitude of +the potential vibrations is greatest at the top end of the aerial, and +the amplitude of the current vibrations is greatest at the bottom or +earthed end. Accordingly, the inductance and capacity of the lateral +branch of the transmitter is altered until the hot wire ammeter in the +base of the aerial shows the largest possible current.</p> + +<div class="figcenter" style="width: 511px;"> +<img src="images/fig24.png" width="511" height="199" alt="FIG. 24.--SLABY-ARCO SYNTONIC TRANSMITTER AND RECEIVER. +I, induction coil; M, multiplier; B, battery; A, aerial; F, filings +tube; R, relay; E, earth plate; C, condenser." title="" /> +<span class="caption smcap">Fig. 24.—Slaby-Arco Syntonic Transmitter and Receiver.</span><span class="caption"> +I, induction coil; M, multiplier; B, battery; A, aerial; F, filings +tube; R, relay; E, earth plate; C, condenser.</span> +</div> + +<p>The corresponding receiver is constructed in a very similar manner. A +lightning conductor or long vertical rod of the same height as the +transmitting aerial is set up at the receiving station, and at a point +six or nine feet from the ground a circuit is taken off, consisting of +a wire loosely coiled in a spiral, the length of which is nearly equal +to, although a little shorter than, the height of the vertical wire +above the point of connection. The outer end of this loose spiral is +connected to one terminal of the coherer tube, and the other terminal +of the coherer is connected to the earth through a condenser of rather +large capacity. The terminals of this last condenser are +short-circuited by a relay and a single cell. When the adjustments are +properly made, it is claimed that the receiver responds only to waves +coming from its own syntonised or tuned transmitter. In this case the +length of the receiving aerial above the point of junction with the +coherer circuit is one quarter the length of the wave. A <a name="tnd_76" id="tnd_76"></a><a href="#tn_76" class="tnlink" title="printer's error, arangements for arrangements">variation of +the above arangements</a> consists in making this lateral circuit equal in +length to one-half of a wave, and connecting the coherer to its centre +through a condenser to the earth. The outer end of this lateral +circuit is also connected to the earth (see Fig. 24).<a name="FNanchor_64_64" id="FNanchor_64_64"></a><a href="#Footnote_64_64" class="fnanchor">[64]</a></p> + +<p>Dr. Slaby claims that this arrangement is not affected by atmospheric +electricity, and that the complete and direct earthing of the <span class="pagenum"><a name="Page_77" id="Page_77">[Pg 77]</a></span>aerial +and also in the second arrangement, of the receiver of the outer end +of the lateral conductor, conduces to preserve the receiver immune +from any electrical disturbances except those having a period to which +it is tuned.</p> + +<p>A method has also been arranged by him for receiving on the same +aerial two messages from different transmitting stations +simultaneously. In this case, two lateral wires of different lengths +are connected to the receiving aerial, and to the outer end of each of +these is connected a coherer tube, the other end of which is earthed +through a condenser. One of these lateral wires is made equal, or +nearly equal, in length to the aerial, and the other is made longer to +fulfil the following condition.<a name="FNanchor_65_65" id="FNanchor_65_65"></a><a href="#Footnote_65_65" class="fnanchor">[65]</a> If we call H the height of the +receiving aerial above point of junction of the lateral wires, then +the length of one lateral wire is made equal to H, and the height of +the aerial is adjusted to be equal to one-quarter of the wave length +of one incident wave. The other lateral wire may then be made of a +length equal to one-third of H, and it will then respond to the first +odd harmonic of that wave, of which the fundamental is in syntony with +the vertical wire. By suitably choosing the relation between the +wave-lengths of the two transmitting stations, it is possible to +receive in this manner two different messages at the same time on the +same aerial. Subsequently to the date of the above-mentioned +demonstration of multiplex wireless telegraphy by Mr. Marconi an +exhibition of a similar nature was given by Professor Slaby in a +lecture given in Berlin on December 22, 1900.<a name="FNanchor_66_66" id="FNanchor_66_66"></a><a href="#Footnote_66_66" class="fnanchor">[66]</a></p> + +<div class="figcenter" style="width: 493px;"> +<img src="images/fig25.png" width="493" height="231" alt="FIG. 25.--LODGE-MUIRHEAD SYNTONIC RECEIVER. I, +induction coil; S, spark gap; A, aerial; CC, condensers; E, earth +plate; R, relay; L, variable inductance; F, filings tube; B, battery." title="" /> +<span class="caption smcap">Fig. 25.—Lodge-Muirhead Syntonic Receiver.</span><span class="caption"> I, +induction coil; S, spark gap; A, aerial; CC, condensers; E, earth +plate; R, relay; L, variable inductance; F, filings tube; B, battery.</span> +</div> + +<p>Both the above-described syntonic systems of Mr. Marconi and Dr. Slaby +are "earthed" systems, but arrangements for syntonic telegraphy have +been devised by Sir Oliver Lodge and Professor Braun which are +"non-earthed."</p> + +<p>Sir Oliver Lodge and Dr. Muirhead have devised also syntonic systems. +According to their last methods, <a name="tnd_77" id="tnd_77"></a><a href="#tn_77" class="tnlink" title="printer's error, systonic for syntonic">the systonic transmitting</a> and +receiving arrangements are as shown in Fig. 25.<a name="FNanchor_67_67" id="FNanchor_67_67"></a><a href="#Footnote_67_67" class="fnanchor">[67]</a> On examining the +<span class="pagenum"><a name="Page_78" id="Page_78">[Pg 78]</a></span> +diagrams it will be seen that the secondary terminals of the induction +coil are, as usual, connected to a pair of spark balls, and that these +spark balls are connected by a condenser and by a variable inductance. +One terminal of the condenser is earthed through another condenser of +large capacity, and the remaining terminal of the first condenser is +connected to an aerial. It should, therefore, be borne in mind in +dealing with electrical oscillations that a condenser of sufficient +capacity is practically a conductor, and an inductance coil of +sufficient inductance is practically a non-conductor. Hence the +insertion of a large capacity in the path of the aerial wire is no +advantage whatever and makes no essential difference in the +arrangement. In order to obtain any powerful radiation, the length of +the aerial, or sky wire, as they call it, must be so adjusted that its +length is one-quarter the wave-length corresponding to the oscillation +circuit, consisting of the condenser and variable inductance.</p> + +<p>The receiving arrangement consists of a similar sky wire or aerial +earthed through a condenser of large capacity and having in the +portion above this last condenser another condenser of similar +capacity. At the earthed side of this last condenser a connection is +made to a resonant circuit, consisting of a variable inductance, and +another condenser and a sensitive metallic filings tube of the Branly +type; also a portion of this resonant circuit is shunted by another +consisting of a battery and telegraphic relay, as shown in the +diagram. The circuit, including the coherer, is tuned to its own +aerial and also to that of the transmitting circuit, and under these +circumstances trains of waves thrown off at the transmitting aerial +will sympathetically affect the receiving aerial.</p> + +<p>There is nothing in the arrangement which specially calls for notice. +It is simply a variation of other known forms of syntonic transmitter +and receiver, and possesses all the advantages and disadvantages +attaching to such electrical syntonic methods.</p> + +<p>Professor Braun's syntonic system, the receiver and transmitter of +which have been described, is also in one form a non-earthed system. +Innumerable other patentees have taken out patents for devices which +are modifications in small degree of the above arrangements.</p> + +<p>It may be well to note at this point the disadvantages that are +possessed by any form of coherer as a telegraphic kumascope in +connection with proposed arrangements for the isolation of Hertzian +wave stations. All the detectors of the coherer type really depend for +their actuation upon electromotive force; that is to say, upon the +application to the terminals of the detector of a certain +electromotive force. Although there may be no sharp and defined +critical electromotive force, yet, nevertheless, as a matter of fact, +if the electromotive force applied exceeds a certain value, then the +detector passes suddenly from one state of conductivity to another. It +may be of great conductivity, as in the case of the Branly coherer, or +of lesser conductivity, as in the case of the so-called anti-coherers, +of which the Schäfer kumascope may be taken as a type. Accordingly, +when these instruments are subjected to a train of waves, each +individual group of which is damped, their operation is largely +governed by the fact that <span class="pagenum"><a name="Page_79" id="Page_79">[Pg 79]</a></span>if the first wave or oscillation set up in +the receiving circuit is powerful enough to break down the coherer, +then the receiving mechanism acts, no matter whether the first impulse +is followed by others or not.</p> + +<p>In comparison with so-called coherers, those depending upon the +changes in the magnetisation of iron by electrical oscillations +certainly have an advantage, because this is a process which requires +the application of alternating electric currents decreasing in +strength for a certain time; and it is found, therefore, that the +magnetic receivers do not require to be associated with such a stiff +or irresponsive resonant circuit to confine their indications to +oscillations or waves of one definite period, and that they lend +themselves much more perfectly to the work of "tuning" or syntonising +stations than do those kumascopes depending upon the contact or +coherer principle.</p> + +<p>We may then glance at the alternative solutions of the problem offered +by other investigators. M. Blondel has proposed to effect the +syntonisation of two stations, not by syntonising the receiver for the +exceedingly high-frequency oscillations of the individual electric +waves, but to syntonise it for the much lower frequency, corresponding +to that of the intervals between the groups of waves. Thus, for +instance, if an ordinary simple transmitting aerial is set up, the +production of sparks between the spark balls results in the emission +of short trains of waves, each of which may consist of half a dozen or +more individual waves, the time of production of the whole group being +very small compared with the interval between the groups. M. Blondel +proposes, however, to syntonise the receiver, not for the +high-frequency period of the waves themselves, which may be reckoned +in millions per second, but for the low-frequency period between the +groups of waves, which is reckoned in hundreds per second. Thus, for +instance, if sparks are made at the rate of fifty or a hundred per +second, they can be made to actuate the telephone receiver and so +produce in the telephone a sound corresponding to a frequency of 50 or +100; in other words, to make a low musical note or hum. This +continuous sound can be cut up, by means of a key placed in the +primary circuit of the transmitting arrangement, into long or short +periods, and hence the letters of the alphabet signal.</p> + +<p>M. Blondel's arrangements comprise a Mercadier's monotone telephone +and either a coherer or a particular form of vacuum tube as a +kumascope. On August 16, 1898, M. Blondel deposited with the Academy +of Sciences in Paris a sealed envelope containing a description of his +improvements in syntonic wireless telegraphy, which was opened on May +19, 1900.<a name="FNanchor_68_68" id="FNanchor_68_68"></a><a href="#Footnote_68_68" class="fnanchor">[68]</a> The arrangement of the receiving apparatus was as +follows:—A single-battery cell keeps a condenser charged until the +kumascope is rendered conductive by the oscillations coming down the +aerial; and under these circumstances the condenser discharges through +the telephone and causes a tick to be heard in it. If the trains of +waves are at the rate of 50 or 100 per second, these small sounds run +together into a musical note, and this continuous hum can <span class="pagenum"><a name="Page_80" id="Page_80">[Pg 80]</a></span>be cut up +into long and short spaces, in accordance with the Morse alphabet +signals. The telephone must not be an ordinary telephone, capable of +being influenced by any frequency, but be one which responds only to a +particular note, and under these conditions the receiving arrangement +is receptive only when the trains of waves arrive at certain regular +predetermined intervals, corresponding with the tone to which the +telephone is sensitive.</p> + +<hr style="width: 45%;" /> + +<p>A number of more or less imperfect arrangements, having the isolation +of communications for their object, have been devised or patented, +which are dependent upon the use of several aerials, each supposed to +be responsive only to a particular frequency; and attempts have been +made to solve the problem of isolation by MM. Tommasi, Tesla, Jegon, +Tissot, Ducretet and others.</p> + +<p>We may then pass on to notice the attempts that have been made to +secure isolation by a plan which is not dependent on electrical +syntony. One of these, which has the appearance of developing into a +practical solution of the problem, is that due to Anders Bull.<a name="FNanchor_69_69" id="FNanchor_69_69"></a><a href="#Footnote_69_69" class="fnanchor">[69]</a> In +the first arrangements proposed by this inventor, a receiver is +constructed which is not capable of being acted upon merely by a +single wave or train of waves or even a regularly-spaced train of +electric waves, but only by a group of wave trains which are separated +from one another by certain unequal, predetermined intervals of time. +Thus, for instance, to take a simple instance, the transmitting +arrangements are so devised as to send out groups of electric waves, +these wave trains following one another at time intervals which may be +represented by the numbers 1, 3 and 5; that is to say, the interval +which elapses between the second and third is three times that between +the first two, and the interval between the fourth and fifth is five +times that between the first two. This is achieved by making five +electric oscillatory sparks with a transmitter of the ordinary kind, +the intervals between which are settled by the intervals between holes +punched upon strips of paper, like that used in a Wheatstone automatic +telegraphic instrument. It will easily be understood that by a device +of this kind, groups of sparks can be made, say, five sparks rapidly +succeeding each other, but not at equal intervals of time. One such +group constitutes the Morse dot, and two or three such groups +succeeding one another very quickly constitute the Morse dash. These +waves, on arriving at the receiving station, are caused to actuate a +punching arrangement by the intermediation of a coherer or other +kumascope, and to punch upon a uniformly moving strip of paper holes, +which are at intervals of time corresponding to the intervals between +the sparks at the transmitting station. This strip of paper then +passes through another telegraphic instrument, which is so constructed +that it prints upon another strip a dot or a dash, according to the +disposition of the holes on the first strip. Accordingly, taken as a +whole, the receiving arrangement is not capable of being influenced so +as to print a telegraphic sign except by the operation of a series of +wave trains succeeding one another at certain assigned intervals of +time.</p> + +<p><span class="pagenum"><a name="Page_81" id="Page_81">[Pg 81]</a></span></p> +<p>An improvement has been lately described by the same inventor,<a name="FNanchor_70_70" id="FNanchor_70_70"></a><a href="#Footnote_70_70" class="fnanchor">[70]</a> in +which the apparatus used, although more complicated, performs the same +functions. At each station two instruments have to be employed; at the +transmitting station one to effect the conversion of Morse signals +into the properly arranged series of wave trains, and at the receiving +station an instrument to effect the re-conversion of the series of +wave trains into the Morse signals. These are called respectively the +dispenser and the collector. The details of the arrangements are +somewhat complicated, and can only be described by the aid of numerous +detailed drawings, but the inventor states that he has been able to +carry on Hertzian wave telegraphy by means of these arrangements for +short distances. Moreover, the method lends itself to an arrangement +of multiplex telegraphy, by sending out from different transmitters +signals which are based upon different arrangements of time intervals +between the electric wave trains. Although this method may succeed in +preventing a receiving arrangement from being influenced by vagrant +waves or waves not intended for it, yet an objection which arises is +that there is nothing to prevent any one from intercepting these wave +trains, and with a little skill interpreting their meaning. Thus, if +the record were received in the ordinary way on a simple receiver, +corresponding to a Morse dot would be printed five dots at unequal +intervals, and corresponding to a Morse dash would be printed two such +sets of five dots. A little skill would then enable an operator to +interpret these arbitrary signals. On the other hand, the inventor +asserts that he can overcome this difficulty by making intervals of +time between the impulses in the series so long that the latter become +longer than the intervals between each of the series of waves which +are despatched in continuous succession when the key is pressed for a +dash. In this case, when telegraphing, the series of dots would +overlap and intermingle with each other in a way which would make the +record unintelligible if received in the usual manner, but would be +perfectly legible if received and interpreted by a receiver adapted +for the purpose.</p> + +<p>Another way of obliterating the record, as far as outsiders are +concerned, is to interpolate between the groups of signals an +irregular series of dots—<i>i.e.</i>, of wave trains—which would affect +an ordinary coherer, and so make an unintelligible record on an +ordinary receiver, but these dots are not received or picked up by the +appropriate selecting instrument used in the Anders Bull system.</p> + +<p>The matter most interesting to the public at the present time is the +long-distance telegraphy by Hertzian waves to the accomplishment of +which Mr. Marconi has devoted himself with so much energy of late +years. Everyone, except perhaps those whose interests may be +threatened by his achievements, must accord their hearty admiration of +the indomitable perseverance and courage which he has shown in +overcoming the immense difficulties which have presented themselves. +Five years ago he was engaged in sending signals from Alum Bay, in the +Isle of Wight, to Bournemouth, a distance of twelve or fourteen miles; +and to-day he has conquered twice that number of <span class="pagenum"><a name="Page_82" id="Page_82">[Pg 82]</a></span>hundred miles and +succeeded in sending, not merely signals, but long messages of all +descriptions over three thousand miles across the Atlantic. Critics +there are in abundance, who declare that the process can never become +a commercial one, that it will destroy short-distance Hertzian +telegraphy, or that the multiplication of long-distance stations will +end in the annihilation of all Hertzian wave telegraphy. No one, +however, can contemplate the history of any development of applied +science without seriously taking to heart the lesson that the +obstacles which arise and which prove serious in any engineering +undertaking are never those which occur to armchair critics. Sometimes +the seemingly impossible proves the most easy to accomplish, whilst +difficulties of a formidable nature often spring up where least +expected.</p> + +<p>The long-distance transmission is a matter of peculiar interest to the +author of these articles, because he was at an early stage in +connection with it invited to render Mr. Marconi assistance in the +matter.<a name="FNanchor_71_71" id="FNanchor_71_71"></a><a href="#Footnote_71_71" class="fnanchor">[71]</a> The particular work entrusted to him was that of planning +the electrical engineering arrangements of the first power station +erected for the production of electric waves for long-distance +Hertzian wave telegraphy at Poldhu, in Cornwall. When Mr. Marconi +returned from the United States in the early part of 1900, he had +arrived at the conclusion that the time had come for a serious attempt +to accomplish wireless telegraphy across the Atlantic. Up to that date +the project had been an inventor's dream, much discussed, long +predicted, but never before practically taken in hand. The only +appliances, moreover, which had been used for creating Hertzian waves +were induction coils or small transformers, and the greatest distance +covered, even by Mr. Marconi himself, had been something like 150 +miles over sea. Accordingly, to grapple with the difficulty of +creating an electric wave capable of making itself felt at a distance +of 3,000 miles, even with the delicate receiving appliances invented +by Mr. Marconi, seemed to require the means of producing at least four +hundred times the wave-energy that had been previously employed. The +author was, therefore, requested to prepare plans and specifications +for an electric generating plant for this purpose, which would enable +electrical oscillations to be set up in an aerial on a scale never +before accomplished.</p> + +<p>This work involved, not merely the ordinary experience of an +electrical engineer, but also the careful consideration of many new +problems and the construction of devices not before used. Every step +had to be made secure by laboratory experiments before the +responsibility could be incurred of advising on the nature of the +machinery and appliances to be ordered. Many months in the year 1901 +were thus occupied by the author in making small-scale experiments in +London and in superintendence of large-scale experiments at the site +of the first power station at Poldhu, near Mullion, in Cornwall, +before the plant was erected and any attempt was made by Mr. Marconi +to commence actual telegraphic experiments. As this work was of a +highly confidential nature, it is obviously impossible to enter into +the details of the arrangements, either as made by the writer in the +first instance, <span class="pagenum"><a name="Page_83" id="Page_83">[Pg 83]</a></span>or as they have been subsequently modified by Mr. +Marconi. The design of the aerial and of the oscillation transformers +and many of the details in the working appliances are entirely due to +Mr. Marconi, but as a final result, a power plant was erected for the +production of Hertzian waves on a scale never before attempted. The +utilisation of 50 <span class="smcap">H.P.</span> or 100 <span class="smcap">H.P.</span> for electric wave production has +involved dealing with many difficult problems in electrical +engineering, not so much in novelty of general arrangement as in +details. It will easily be understood that Leyden jars, spark balls +and oscillators, which are quite suitable for use with an induction +coil, would be destroyed immediately if employed with a large +alternating-current plant and immensely powerful transformers.</p> + +<div class="figcenter" style="width: 583px;"> +<img src="images/fig26.jpg" width="583" height="460" alt="FIG. 26.--WOODEN TOWERS SUPPORTING THE MARCONI AERIAL +AT POLDHU POWER STATION, CORNWALL, ENGLAND." title="" /> +<span class="caption smcap">Fig. 26.—Wooden Towers supporting the Marconi Aerial +at Poldhu Power Station, Cornwall, England.</span> +</div> + +<p>In the initial experiments with this machinery and in its first +working there was very considerable risk, owing to its novel and +dangerous nature; but throughout the whole of the work from the very +beginning, no accident of any kind has taken place, so great have been +the precautions taken. The only thing in the nature of a mishap was +the collapse of a ring of tall masts, erected in the first place to +sustain the aerial wires, but which now have been replaced by four +substantial timber towers, 215 feet in height, placed at the corners +of a square, 200 feet in length. These four towers sustain a conical +arrangement of insulated wires (see Fig. 26) which can be used in +sections and which constitute the transmitting radiator or receiver, +as the case may be. Each of these wires is 200 feet in length and +formed of bare stranded wire.</p> + +<p><span class="pagenum"><a name="Page_84" id="Page_84">[Pg 84]</a></span></p> +<p>At the outset, there was much uncertainty as to the effect of the +curvature of the earth on the propagation of a Hertzian wave over a +distance of many hundreds of miles. In the case of the Atlantic +transmission between the station at Poldhu in Cornwall and that at +Cape Cod in Massachusetts, U.S.A., we have two stations separated by +about 45 degrees of longitude on a great circle, or one-eighth part of +the circumference of the world. In this case, the versine of the arc +or height of the sea at the half-way point above the straight line or +chord joining the two places is 300 miles.</p> + +<p>The question has recently attracted the attention of several eminent +mathematical physicists. The extent to which a free wave propagated in +a medium bends round any object or is diffracted depends on the +relation between the length of the wave and the size of the object. +Thus, for instance, an object the size of an orange held just in front +of the mouth does not perceptibly interfere with the propagation of +the waves produced by the speaking or singing voice, because these are +from two to six feet in length: but if arrangements are made by means +of a Galton whistle to produce air waves half an inch in length, then +an obstacle the size of an orange causes a very distinct acoustic +shadow. The same thing is true of waves in the ether. The amount of +bending of light waves round material objects is exceedingly small, +because the average length of light waves is about +one-fifty-thousandth part of an inch. In the case of Hertzian wave +telegraphy, we are, however, dealing with ether waves many hundreds of +feet in length, and the waves sent out from Poldhu have a wave-length +of a thousand feet or more, say, one-fifth to one-quarter of a mile. +The distance, therefore, between Poldhu and Cape Cod is only at most +about twelve thousand wave-lengths, and stands in the same relation to +the length of the Hertzian wave used as does a body the diameter of a +pea to the wave-length of yellow light. There is unquestionably a +large amount of diffraction or bending of the electric wave round the +earth, and, proportionately speaking, it is larger than in the case of +light waves incident on objects of the same relative size.</p> + +<p>Quite recently Mr. H. M. Macdonald (see <i>Proc.</i> Roy. Soc., London, +Vol. LXXI., p. 251) has submitted the problem to calculation, and has +shown that the power required to send given electric waves 3,000 miles +along a meridian of the earth is greater than would be required to +send them over the same distance if the sea surface were flat in the +ratio of 10 to 3. Hence the rotundity of the earth does introduce a +very important reduction factor, although it does not inhibit the +transmission. Mr. Macdonald's mathematical argument has, however, been +criticised by Lord Rayleigh and by M. H. Poincaré (see <i>Proc.</i> Roy. +Soc., Vol. LXXII., p. 40, 1903).</p> + +<p>The accomplishment of very long distances by Hertzian wave telegraphy +is, however, not merely a question of power, it is also a question of +wave-length. Having regard, however, to the possibility that the +propagation which takes place in Hertzian wave telegraphy is not that +simply of a free wave in space, but the transmission of a semi-loop of +electric strain with its feet tethered to the earth, it is quite +possible that if it were worth while to make the attempt, an ether +<span class="pagenum"><a name="Page_85" id="Page_85">[Pg 85]</a></span> +disturbance could be made in England sufficiently powerful to be felt +in New Zealand.</p> + +<p>Leaving, however, these hypothetical questions and matters of pure +conjecture, we may consider some of the facts which have resulted from +Mr. Marconi's long-distance experiments. One of the most interesting +of these is the effect of daylight upon the wave propagation. In one +of his voyages across the Atlantic, when receiving signals from Poldhu +on board the <span class="smcap">S.S.</span> <i>Philadelphia</i>, he noticed that the signals were +received by night when they could not be detected by day.<a name="FNanchor_72_72" id="FNanchor_72_72"></a><a href="#Footnote_72_72" class="fnanchor">[72]</a> In these +experiments Mr. Marconi instructed his assistants at Poldhu to send +signals at a certain rate from 12 to 1 a.m., from 6 to 7 a.m., from 12 +to 1 p.m., and from 6 to 7 p.m., Greenwich mean time, every day for a +week. He has stated that on board the <i>Philadelphia</i> he did not notice +any apparent difference between the signals received in the day and +those received at night until after the vessel had reached a distance +of 500 statute miles from Poldhu. At distances of over 700 miles, the +signals transmitted during the day failed entirely, while those sent +at night remained quite strong up to 1,551 miles, and were clearly +decipherable up to a distance of 2,099 miles from Poldhu. Mr. Marconi +also noted that at distances of over 700 miles, the signals at 6 a.m., +in the week between February 23 and March 1, were quite clear and +distinct, whereas by 7 a.m. they had become weak almost to total +disappearance. This fact led him at first to conclude that the cause +of the weakening was due to the action of the daylight upon the +transmitting aerial, and that as the sun rose over Poldhu, so the wave +energy radiated, diminished, and he suggested as an explanation the +known fact of the dissipating action of light upon a negative charge.</p> + +<p>Although the facts seem to support this view, another explanation may +be suggested. It has been shown by Professor J. J. Thomson that +gaseous ions or electrons can absorb the energy of an electric wave, +if present in a space through which waves are being transmitted.<a name="FNanchor_73_73" id="FNanchor_73_73"></a><a href="#Footnote_73_73" class="fnanchor">[73]</a> +If it be a fact, as suggested by Professor J. J. Thomson, that the sun +is projecting into space streams of electrons, and if these are +continually falling in a shower upon the earth, in accordance with the +fascinating hypothesis of Professor Arrhenius, then that portion of +the earth's atmosphere which is facing the sun will have present in it +more electrons or gaseous ions than that portion which is turned +towards the dark space, and it will therefore be less transparent to +long Hertzian waves.<a name="FNanchor_74_74" id="FNanchor_74_74"></a><a href="#Footnote_74_74" class="fnanchor">[74]</a> In other words, clear sunlit air, though +extremely transparent to light waves, acts as if it were a slightly +turbid medium for long Hertzian waves. The dividing line between that +portion of the earth's atmosphere which is impregnated with gaseous +ions or <span class="pagenum"><a name="Page_86" id="Page_86">[Pg 86]</a></span>electrons is not sharply delimited from the part not so +illuminated, and there may be, therefore, a considerable penetration +of these ions into the regions which I may call the twilight areas. +Accordingly, as the earth rotates, a district in which Hertzian waves +are being propagated is brought, towards the time of sunrise, into a +position in which the atmosphere begins to be ionised, although far +from as freely as is the case during the hours of bright sunshine.</p> + +<p>Mr. Marconi states that he has found a similar effect between inland +stations, signals having been received by him during the night between +Poldhu and Poole with an aerial the height of which was not sufficient +to receive them by day. It has been found, however, that the effect +simply amounts to this, that rather more power is required by day than +by night to send signals by Hertzian waves over long distances.</p> + +<p>Some interesting observations have also been made by Captain H. B. +Jackson, R.N.,<a name="FNanchor_75_75" id="FNanchor_75_75"></a><a href="#Footnote_75_75" class="fnanchor">[75]</a> on the influence of various states of the +atmosphere upon Hertzian wave telegraphy. These experiments were all +made between ships of the British Royal Navy, furnished with Hertzian +wave telegraphy apparatus on the Marconi system. Some of his +observations concerned the <a name="tnd_86" id="tnd_86"></a><a href="#tn_86" class="tnlink" title="printer's error, interpositon for interposition">effect of the interpositon of land</a> between +two ships. He found that the interposition of land containing iron +ores reduced the signalling distances, compared with the maximum +distance at open sea, to about 30 per cent. of the latter; whilst hard +limestone reduced it to nearly 60 per cent. and soft sandstone or +shale to 70 per cent. These results show that there is a considerable +absorption effect when waves of certain wave-length pass through or +over hard rocks containing iron ores. It would be interesting to know, +however, whether this reduction was in any degree proportional to the +dryness or moisture of the soil. Earth conductivity is far more +dependent upon the presence or absence of moisture than upon the +particular nature of the material which composes it other than water.</p> + +<p>The observations of Captain Jackson, however, only confirm the already +well-known fact that Hertzian waves, as employed in the Marconi system +of wireless telegraphy, within a certain range of wave-length, are +considerably weakened by their passage through land, over land or +round land. In some cases he noticed that quite sharp electric shadows +were produced by rocky promontories projecting into the line of +transmission. His attention was also directed (<i>loc. cit.</i>) to the +more important matter of the effect of atmospheric electrical +conditions upon the transmission. The effect of all lightning +discharges, whether visible or invisible, is to make a record on the +telegraphic receiver. On the approach of an atmospheric electrical +disturbance towards the receiving station on a ship, the first visible +indications generally are the recording of dots at intervals from a +few minutes to a few seconds on the telegraphic tape. Captain Jackson +states that the most frequent record is that of three dots, the first +being separated from the other two by a slight interval like the +letters <span class="pagenum"><a name="Page_87" id="Page_87">[Pg 87]</a></span>E I on the Morse code, and this is the sign most frequently +recorded by distant lightning. But in addition to this, dashes are +recorded and irregular signs, which, however, sometimes spell out +words in the Morse code. He noted that these disturbances are more +frequent in summer and autumn than in winter and spring, and in the +neighbourhood of high mountains more than in the open sea. In settled +weather, if present, they reach their maximum between 8 p.m. and 10 +p.m., and frequently last during the whole of the night, with a +minimum of disturbance between 9 a.m. and 1 p.m. Another important +matter noted by Captain Jackson is the shorter distance at which +signals can usually be received when any electrical disturbances are +present in the atmosphere, compared with the distance at which they +can be received when none are present. This reduction in signalling +distance may vary from 20 to 70 per cent, of that obtainable in fine +weather. It does not in any way decrease with the number of lightning +flashes, but rather the reverse, the loss in signalling distance +generally preceding the first indications on the instrument of the +approaching electrical disturbance. It is clear that these +observations fit in very well with the theory outlined above, viz., +that the atmosphere when impregnated with free electrons or +negatively-charged gaseous ions is more opaque to Hertzian waves than +when they are absent. Captain Jackson gives an instance of ships whose +normal signalling distance was 65 miles, failing to communicate at 22 +miles when in the neighbourhood of a region of electrical disturbance. +These effects in the case of wireless telegraphy have their parallel +in the disturbances caused to telegraphy with wires by earth currents +and magnetic storms.</p> + +<p>Another effect which he states reduces <a name="tnd_87" id="tnd_87"></a><a href="#tn_87" class="tnlink" title="printer's error, signaling for signalling">the usual maximum signaling</a> +distance is the presence of material particles held in suspension by +the water spherules in moist atmosphere. The effect has been noticed +in the Mediterranean Sea when the sirocco wind is blowing. This is a +moist wind conveying dust and salt particles from the African coast. A +considerable reduction in signalling distance is produced by its +advent.</p> + +<p>Another interesting observation due to Captain Jackson is the +existence of certain zones of weak signals. Thus, for instance, two +ships at a certain distance may be communicating well; if their +distance increases, the signalling falls off, but is improved again at +a still greater distance. He advances an ingenious theory to show that +this fact may be due to the interference between two sets of waves +sent out by the transmitter having different wave-lengths.</p> + +<p>Finally, in the Paper referred to, he emphasises the well-known fact +that long-distance signalling can only be accomplished by the aid of +an aerial wire and a "good earth." Summing up his results, he +concludes: (1) That intervening land of any kind reduces the practical +signalling distance between two ships or stations, compared with that +which would be obtainable over the open sea, and that this loss in +distance varies with the height, thickness, contour, and nature of the +land; (2) material particles, such as dust and salt, held in +suspension in a moist atmosphere also reduce the signalling distance, +probably by <span class="pagenum"><a name="Page_88" id="Page_88">[Pg 88]</a></span>dissipating and absorbing the waves; (3) that electrical +disturbances in the atmosphere also act most adversely in addition to +affecting the receiving instrument and making false signals or +<i>strays</i>, as they are called; (4) that with certain forms of +transmitting arrangement, interference effects may take place which +have the result of creating certain areas of silence very similar to +those which are observed in connection with sound signals from a +siren.</p> + +<p>It is clear, therefore, from all the above observations, that +Hertzian-wave telegraphy taking place through the terrestrial +atmosphere is not by any means equivalent to the propagation of a wave +in free or empty space; and that just as the atmosphere varies in its +opacity to rays of light, sometimes being clear and sometimes clouded, +so it varies from time to time in transparency to Hertzian waves, the +cause of this variation in transparency probably being the presence in +the atmosphere of negatively-charged corpuscles or electrons. If there +are present in the atmosphere at certain times "clouds of electrons" +or "electronic fogs," these may have the effect of producing a certain +opacity, or rather diminution in transparency to Hertzian waves, just +as water particles do in the case of sunlight.</p> + +<p>We may, therefore, in conclusion, review a few of the outstanding +problems awaiting solution in connection with Hertzian wave wireless +telegraphy. In spite of the fact that this new telegraphy has not been +accorded a very hearty welcome by the representatives of official or +established telegraphy in Great Britain, it has reached a point, +unquestionably owing to Mr. Marconi's energy and inventive power, at +which it is bound to continue its progress. But that progress will not +be assisted by shutting our eyes to facts. Many problems of great +importance remain to be solved. We have not yet reached a complete +solution of all the difficulties connected with isolation of stations. +In the next place, the question of localising the source of the +signals and waves is most important. Our kumascopes and receiving +appliances at present are like the rudimentary eyes of the lower +organisms, which are probably sensitive to mere differences in light +and darkness, but which are not able to <i>see</i> or <i>visualise</i>, in the +sense of locating the direction and distance of a radiating or +luminous body. Just as we have, as little children, to learn to see, +so a similar process has to be accomplished in connection with +Hertzian telegraphy, and the accomplishment of this does not seem by +any means impossible or even distant. We are dealing with +hemispherical waves of electric and magnetic force, which are sent out +from a certain radiating centre, and in order to localise that centre +we have to determine the position of the plane of the wave and also +the curvature of the surface at the receiving point. Something, +therefore, equivalent to a range finder in connection with light is +necessary to enable us to locate the distance and the direction of the +radiant point.</p> + +<p>Lastly, there are important improvements possible in connection with +the generation of the waves themselves. At the present moment, our +mode of generating Hertzian waves involves a dissipation of energy in +the form of the light and heat of the spark. Just as in the case of +ordinary artificial illuminants, such as lamps of various kinds, we +have to manufacture a large amount of ether radiation of long wave +length, which is of no use to us for visual purposes—in fact, +creating ninety-five per cent, of dark and useless waves for every +five per cent. of luminous or useful waves—so in connection with +present methods of generating Hertzian waves, we are bound <span class="pagenum"><a name="Page_89" id="Page_89">[Pg 89]</a></span>to +manufacture by the discharge spark a large amount of light and heat +rays which are not wanted, in order to create the Hertzian waves we +desire. It is impossible yet to state precisely what is the +efficiency, in the ordinary sense of the word, of a Hertzian wave +radiator; how much of the energy imparted to the aerial falls back +upon it and contributes to the production of the spark, and how much +is discharged into the ether in the form of a wave.</p> + +<p>Nothing is more remarkable, however, than the small amount of energy +which, if properly utilised in electric wave making, will suffice to +influence a sensitive receiver at a distance of even one or two +hundred miles. Suppose, for instance, that we charge a condenser +consisting of a battery of Leyden jars, having a capacity of one +seventy-fifth of a microfarad, to a potential of 15,000 volts; the +energy stored up in this condenser is then equal to 1·5 joules, or a +little more than one foot-pound. If this energy is discharged in the +form of a spark five millimetres in length through the primary coil of +an oscillation transformer, associated with an aerial 150 feet in +height, the circuits being properly tuned by Mr. Marconi's method, +then such an aerial will affect, as he has shown, one of Mr. Marconi's +receivers, including a nickel silver filings coherer tube, at a +distance of over two hundred miles over sea. Consider what this means. +The energy stored up in the Leyden jars cannot all be radiated as wave +energy by the aerial, probably only half of it is thus radiated. Hence +the impartation to the ether at any one locality of about half a +foot-pound of energy in the form of a long Hertzian wave is sufficient +to affect sensitive receivers situated at any point on the +circumference of a circle of 200 miles radius described on the open +sea. Hertzian wave telegraphy is sometimes described as being +extravagant in power, but, as a matter of fact, the most remarkable +thing about it is the small amount of power really involved in +conducting it. On the other hand, Hertzian wave manufacture is not +altogether a matter of power. It is much more dependent upon the +manner in which the ether is struck. Just as half an ounce of dynamite +in exploding may make more noise than a ton of gunpowder, because it +hits the air more suddenly, so the formation of an effective wave in +the ether is better achieved by the right application of a small +energy than by the wrong mode of application of a much larger amount. +If we translate this fact into the language of electronic theory, it +amounts simply to this. It is the electron alone which has a grip of +the ether. To create an ether wave, we have to start or stop crowds of +electrons very suddenly. If in motion, their motion implies energy, +but it is not only their energy which is concerned in the wave making, +but the acceleration, positive or negative—<i>i.e.</i>, the quickness with +which they are started or stopped. It is possible we may discover in +time a way of manufacturing long ether waves without the use of an +electric spark, but at present we <span class="pagenum"><a name="Page_90" id="Page_90">[Pg 90]</a></span>know only one way of doing +this—viz., by the discharge of a condenser, and in the discharge of +large condensers of very high potentials it is difficult to secure +that extreme suddenness of starting the discharge which we can do in +the case of smaller capacities and voltages.</p> + +<p>How strange it is that the discharge of a Leyden jar studied so +profoundly by Franklin, Henry, Faraday, Maxwell, Kelvin and Lodge +should have become an electrical engineering appliance of great +importance!</p> + +<p>Whilst there are many matters connected with the commercial aspect of +Hertzian wave telegraphy with which we are not here concerned, there +is one on which a word may properly be said. The ability to +communicate over long distances by Hertzian waves is now demonstrated +beyond question, and even if all difficulties are not overcome at +once, it has a field of very practical utility, and may even become of +national importance. Under these circumstances, we may consider +whether it is absolutely necessary to place the signalling stations so +near the coast. The greater facility of transmission over sea has +already been discussed and explained, but in time of war, the masts +and towers which are essential at present in connection with +transmitting stations could be wrecked by shot or shell from an +enemy's battleship at a distance of five or six miles out at sea, and +would certainly be done within territorial waters. Should not this +question receive attention in choosing the location of important +signalling stations? For if they can, without prejudice to their use, +be placed inland by a distance sufficient to conceal them from sight, +their value as a national asset in time of war might be greatly +increased.</p> + +<p>It has been often contended that whilst cables could be cut in time of +war no one can cut the ether; but wireless telegraph stations in +exposed situations on high promontories, where they are visible for +ten to fifteen miles out at sea and undefended by any forts, could +easily be destroyed. The great towers which are essential to carry +large aerials are a conspicuous object for ten miles out at sea; and a +single well-placed shell from a six-inch gun would wreck the place and +put the station completely out of use for many months. Hence if +oceanic telegraphy is ever to be conducted in a manner in which the +communication will be inviolable or, at any rate, not be capable of +interruption by acts of war, the careful selection of the sites for +stations is a matter of importance. A small station consisting of a +single 150-foot mast and a wooden hut can easily be removed or +replaced, but an expensive power station, the mere aerial of which may +cost several thousand pounds, is not to be put up in a short time.<a name="FNanchor_76_76" id="FNanchor_76_76"></a><a href="#Footnote_76_76" class="fnanchor">[76]</a></p> + +<p>Meanwhile, whatever may be the future achievements of this new +<i>supermarine</i> wireless telegraphy conducted over long distances, there +can be no question as to its enormous utility and present value for +intercommunication between ships on the ocean and ships and the shore. +At the present time, there are some forty or more of the transatlantic +ocean liners and many other ships equipped with this <span class="pagenum"><a name="Page_91" id="Page_91">[Pg 91]</a></span>Hertzian wave +wireless telegraph apparatus on the Marconi system. Provided with this +latest weapon of applied science, they are able to chat with one +another, though a hundred miles apart on the ocean, with the ease of +guests round a dinner table, to exchange news or make demands for +assistance.</p> + +<div class="poem"><div class="stanza"> +<span class="i0">Ships that pass in the night, and speak each other in passing—<br /></span> +<span class="i0">Only a signal shown, and a distant voice in the darkness;<br /></span> +<span class="i0">So, on the ocean of life, we pass and speak one another,<br /></span> +<span class="i0">Only a look and a voice, then darkness again, and a silence.<br /></span> +</div></div> + +<p>Abundant experience has been gathered to show the inexpressible value +of this means of communication in case of accident, and it can hardly +be doubted that before long the possession of this apparatus on board +every passenger vessel will be demanded by the public, even if not +made compulsory. Although the privacy of an ocean voyage may have been +somewhat diminished by this utilisation of ether waves, there is a +vast compensation in the security that is thereby gained to human life +and property by this latest application of the great energies of +nature for the use and benefit of mankind.</p> + +<p> </p> +<p> </p> + +<hr style="width: 15%;" /> + +<p> </p> +<p> </p> + +<hr style="width: 100%;" /> +<h5>GEO. TUEKER, PRINTER, SALISBURY COURT, FLEET STREET, LONDON.</h5> + +<p> </p> + +<div class="footnotes"> +<div class="footnote_title"><p>Footnotes:</p></div> + +<div class="footnote"><p><a name="Footnote_1_1" id="Footnote_1_1"></a><a href="#FNanchor_1_1"><span class="label">[1]</span></a> This series of articles is based on the Cantor Lectures +delivered before the Society of Arts, London, in March, 1903. The +lectures were attended by many of the leading British scientific men +and electrical engineers, and attracted wide attention as the most +complete and authoritative statement hitherto made of wireless +telegraphy. In writing the articles for the "Popular Science Monthly," +the author has omitted advanced technicalities in order that the +substance may be suitable for the general reader.—<span class="smcap">Editor</span>.</p></div> + +<div class="footnote"><p><a name="Footnote_2_2" id="Footnote_2_2"></a><a href="#FNanchor_2_2"><span class="label">[2]</span></a> For a more detailed account of this hypothesis, the +reader is referred to an article by the present writer, entitled "The +Electronic Theory of Electricity," published in the "Popular Science +Monthly" for May, 1902.</p></div> + +<div class="footnote"><p><a name="Footnote_3_3" id="Footnote_3_3"></a><a href="#FNanchor_3_3"><span class="label">[3]</span></a> See J. J. Thomson, "Recent Researches in Electricity and +Magnetism," chap. I., p. 16.</p></div> + +<div class="footnote"><p><a name="Footnote_4_4" id="Footnote_4_4"></a><a href="#FNanchor_4_4"><span class="label">[4]</span></a> See O. Heaviside, "Electromagnetic Theory," Vol. I., p. +54.</p></div> + +<div class="footnote"><p><a name="Footnote_5_5" id="Footnote_5_5"></a><a href="#FNanchor_5_5"><span class="label">[5]</span></a> Wiedemann's <i>Annalen</i>, 36, p. 1, 1889; or in his +republished Papers, "Electric Waves," p. 137, English translation by +D. E. Jones.</p></div> + +<div class="footnote"><p><a name="Footnote_6_6" id="Footnote_6_6"></a><a href="#FNanchor_6_6"><span class="label">[6]</span></a> The fraction 7/22 here denotes a stranded wire formed of +seven strands, each single wire having a diameter expressed by the +number 22 on the British standard wire gauge.</p></div> + +<div class="footnote"><p><a name="Footnote_7_7" id="Footnote_7_7"></a><a href="#FNanchor_7_7"><span class="label">[7]</span></a> G. Marconi, "Syntonic Wireless Telegraphy," <i>Journal</i> of +the Society of Arts, Vol. XLIX., p. 501, 1901.</p></div> + +<div class="footnote"><p><a name="Footnote_8_8" id="Footnote_8_8"></a><a href="#FNanchor_8_8"><span class="label">[8]</span></a> Instruction for the manufacture of large induction coils +may be obtained from a "Treatise on the Construction of Large +Induction Coils," by A. T. Hare. (Methuen & Co., London.) +</p><p> +Also see Vol. II. of "The Alternate-Current Transformer," by J. A. +Fleming, chap. I. ("The Electrician" Printing and Publishing Co., 1, 2 +and 3, Salisbury-court, Fleet-street, London, E.C.)</p></div> + +<div class="footnote"><p><a name="Footnote_9_9" id="Footnote_9_9"></a><a href="#FNanchor_9_9"><span class="label">[9]</span></a> See "The Alternate-Current Transformer," by J. A. +Fleming. Vol. I., p. 184.</p></div> + +<div class="footnote"><p><a name="Footnote_10_10" id="Footnote_10_10"></a><a href="#FNanchor_10_10"><span class="label">[10]</span></a> Du Moncel states that MacGauley of Dublin independently +invented the form of hammer break as now used. See "The +Alternate-Current Transformer," Vol. II. chap. I. J. A. Fleming.</p></div> + +<div class="footnote"><p><a name="Footnote_11_11" id="Footnote_11_11"></a><a href="#FNanchor_11_11"><span class="label">[11]</span></a> See Professor J. Trowbridge, "On the Induction Coil" +<i>Phil. Mag.</i>, April, 1902 Vol. III., Series 6, p. 393.</p></div> + +<div class="footnote"><p><a name="Footnote_12_12" id="Footnote_12_12"></a><a href="#FNanchor_12_12"><span class="label">[12]</span></a> See Dr. Wehnelt's article in the <i>Elektrotechnische +Zeitschrift</i>, January, 1899.</p></div> + +<div class="footnote"><p><a name="Footnote_13_13" id="Footnote_13_13"></a><a href="#FNanchor_13_13"><span class="label">[13]</span></a> See <i>The Electrician</i>, Vol. XLII., 1899, pp. 721, 728, +731, 732 and 841; communications from Mr. Campbell Swinton, Professor +S. P. Thompson, Dr. Marchant, the author and others; also p. 864, same +volume, for a leader on the subject; also p. 870, letters by M. +Blondel and Professor E. Thomson. See also <i>The Electrician</i>, Vol. +XLIII., p. 5, 1899, extracts from a Paper by P. Barry; <i>Comptes +Rendus</i>, April, 1899. See also the <i>Electrical Review</i>, Vol. XLIV., p. +235, 1899, February 17.</p></div> + +<div class="footnote"><p><a name="Footnote_14_14" id="Footnote_14_14"></a><a href="#FNanchor_14_14"><span class="label">[14]</span></a> See <i>The Electrician</i>, Vol. XLII., 1899.</p></div> + +<div class="footnote"><p><a name="Footnote_15_15" id="Footnote_15_15"></a><a href="#FNanchor_15_15"><span class="label">[15]</span></a> For a discussion of the function of the condenser in an +ordinary induction coil, see "The Alternate-Current Transformer," by +J. A. Fleming. Vol. II., p. 51.</p></div> + +<div class="footnote"><p><a name="Footnote_16_16" id="Footnote_16_16"></a><a href="#FNanchor_16_16"><span class="label">[16]</span></a> See Lord Rayleigh, <i>Phil. Mag.</i>, December, 1901.</p></div> + +<div class="footnote"><p><a name="Footnote_17_17" id="Footnote_17_17"></a><a href="#FNanchor_17_17"><span class="label">[17]</span></a> It has sometimes been stated that the spark balls <a name="tnd_fn17" id="tnd_fn17"></a><a href="#tn_fn17" class="tnlink" title="printer's error, missing letter t">must +be <i>solid</i> metal and no hollow,</a> but this is a fallacy, and has been +disproved by Mr. C. A. Chant. See "An Experimental Investigation into +the Skin Effect in Electrical Oscillators," <i>Phil. Mag.</i>, Vol. III., +Sec. 6, p. 425, 1902.</p></div> + +<div class="footnote"><p><a name="Footnote_18_18" id="Footnote_18_18"></a><a href="#FNanchor_18_18"><span class="label">[18]</span></a> See <i>Proc.</i> Roy. Soc., London, February 23 and April 12, +1860; or reprint of Papers on electrostatics and magnetism, p. 247.</p></div> + +<div class="footnote"><p><a name="Footnote_19_19" id="Footnote_19_19"></a><a href="#FNanchor_19_19"><span class="label">[19]</span></a> See <i>Phil. Mag.</i>, August, 1902, Vol. IV., p. 224, 6th +Series. Mr. Jervis-Smith has also described an experiment to show how +much the use of compressed air round a spark gap is of advantage in +working an ordinary Tesla coil. In his British specification, No. +12,039 of 1896, Mr. Marconi had long previously mentioned the use of +compressed air round the spark gap.</p></div> + +<div class="footnote"><p><a name="Footnote_20_20" id="Footnote_20_20"></a><a href="#FNanchor_20_20"><span class="label">[20]</span></a> This energy storage is at the rate of 44 foot-pounds per +cubic foot of glass. This figure shows what a relatively small amount +of energy is capable of being stored up in the form of electric strain +in glass. In the case of an air condenser, it is only stored at the +rate of 1 foot-pound per cubic foot.</p></div> + +<div class="footnote"><p><a name="Footnote_21_21" id="Footnote_21_21"></a><a href="#FNanchor_21_21"><span class="label">[21]</span></a> See British specification No. 7,777 of 1900.—G. +Marconi. "Improvements in Apparatus for Wireless Telegraphy."</p></div> + +<div class="footnote"><p><a name="Footnote_22_22" id="Footnote_22_22"></a><a href="#FNanchor_22_22"><span class="label">[22]</span></a> That this number really does represent the order of this +oscillation frequency in an aerial has been shown by C. Tissot, +<i>Comptes Rendus</i>, 132, p. 763, March 25, 1901, by photographs taken of +the oscillatory spark of a Hertzian wave telegraphic transmitter. (See +<i>Science Abstracts</i>, Vol. IV., Abs. 1,518.) He found frequencies from +0·5 million to 1·6 million.</p></div> + +<div class="footnote"><p><a name="Footnote_23_23" id="Footnote_23_23"></a><a href="#FNanchor_23_23"><span class="label">[23]</span></a> The term "jigger" is one of those slang terms which +contrive to effect a permanent attachment to various arts and crafts. +Similarly, the word "booster" is now used for a step-up or +voltage-raising transformer or dynamo, inserted in series with an +electric supply main. The word "boost" is a slang term signifying to +raise or lift up. "To give a real good boost" is an expression for +lending a helping hand. The term "jigger," in the same manner, is an +adaptation of a seaman's term for hoisting tackle or lift.</p></div> + +<div class="footnote"><p><a name="Footnote_24_24" id="Footnote_24_24"></a><a href="#FNanchor_24_24"><span class="label">[24]</span></a> The "earth" itself probably only conducts +electrolytically. All such materials as sand, clay, chalk, etc., and +most surface soils are fairly good insulators when very dry, but +conduct in virtue of moisture present in them.</p></div> + +<div class="footnote"><p><a name="Footnote_25_25" id="Footnote_25_25"></a><a href="#FNanchor_25_25"><span class="label">[25]</span></a> <i>The Electrician</i>, Vol. XL., p. 86 (leader).</p></div> + +<div class="footnote"><p><a name="Footnote_26_26" id="Footnote_26_26"></a><a href="#FNanchor_26_26"><span class="label">[26]</span></a> British Patent Specification, C. and S. A. Varley, No. +165, 1866.</p></div> + +<div class="footnote"><p><a name="Footnote_27_27" id="Footnote_27_27"></a><a href="#FNanchor_27_27"><span class="label">[27]</span></a> See also <i>Journal de Physique</i>, Vol. V., p. 573, 1886.</p></div> + +<div class="footnote"><p><a name="Footnote_28_28" id="Footnote_28_28"></a><a href="#FNanchor_28_28"><span class="label">[28]</span></a> See <i>Comptes Rendus</i>, Vol. CXI., p. 785; Vol. CXII., p. +112, 1891; or <i>La Lumière Electrique</i>, Vol. XL., pp. 301, 506, 1891; +or <i>The Electrician</i>, Vol. XXVII., 1891, pp. 221, 448.</p></div> + +<div class="footnote"><p><a name="Footnote_29_29" id="Footnote_29_29"></a><a href="#FNanchor_29_29"><span class="label">[29]</span></a> See <i>The Electrician</i>, Vol. XXIX., 1892, pp. 397 and +432.</p></div> + +<div class="footnote"><p><a name="Footnote_30_30" id="Footnote_30_30"></a><a href="#FNanchor_30_30"><span class="label">[30]</span></a> Mr. W. B. Croft, <i>Proc.</i> Phys. Soc., Vol. XII., p. 421. +Report of meeting on October 27, 1893.</p></div> + +<div class="footnote"><p><a name="Footnote_31_31" id="Footnote_31_31"></a><a href="#FNanchor_31_31"><span class="label">[31]</span></a> See Professor Minchin, <i>Proc.</i> Phys. Soc., November 24, +1893; or <i>The Electrician</i>, Vol. XXXII., 1893, p. 123. See also +Professor Minchin, <a name="tnd_fn31" id="tnd_fn31"></a><a href="#tn_fn31" class="tnlink" title="printer's error, missing full stop after abbreviation"><i>Phil Mag.</i></a>, January, 1894, Vol. XXXVII., p. 90, +"On the Action of Electromagnetic Radiation on Films containing +Metallic Powders."</p></div> + +<div class="footnote"><p><a name="Footnote_32_32" id="Footnote_32_32"></a><a href="#FNanchor_32_32"><span class="label">[32]</span></a> This lecture was afterwards published as a book, the +first edition bearing the same title as the lecture—viz., "The Work +of Hertz and Some of His Successors." In the second edition, published +in 1898, an appendix was added (p. 59) containing "The History of the +Coherer Principle," and the original title of the work had prefixed to +it "Signalling Without Wires."</p></div> + +<div class="footnote"><p><a name="Footnote_33_33" id="Footnote_33_33"></a><a href="#FNanchor_33_33"><span class="label">[33]</span></a> See <i>The Electrician</i>, Vol. XXVII., p. 222, 1891. E. +Branly, "Variations of Conductivity under Electrical Influence."</p></div> + +<div class="footnote"><p><a name="Footnote_34_34" id="Footnote_34_34"></a><a href="#FNanchor_34_34"><span class="label">[34]</span></a> See <i>The Electrician</i>, Vol. XL., p. 90. Sir Oliver +Lodge, "The History of the Coherer Principle."</p></div> + +<div class="footnote"><p><a name="Footnote_35_35" id="Footnote_35_35"></a><a href="#FNanchor_35_35"><span class="label">[35]</span></a> See Professor E. Branly, "A Sensitive Coherer," <i>Comptes +Rendus</i>, Vol. CXXXIV., p. 1,187, 1902; or <i>Science Abstracts</i>, Vol. +V., p. 852, 1902.</p></div> + +<div class="footnote"><p><a name="Footnote_36_36" id="Footnote_36_36"></a><a href="#FNanchor_36_36"><span class="label">[36]</span></a> This device of making the inter-electrode gap in a +tubular filings coherer wedge-shaped has been patented again and again +by various inventors. See German patent No. 116,113, Class 21a, 1900. +It has also been claimed by M. Tissot.</p></div> + +<div class="footnote"><p><a name="Footnote_37_37" id="Footnote_37_37"></a><a href="#FNanchor_37_37"><span class="label">[37]</span></a> See <i>The Electrician</i>, Vol. XXVII., 1891, p. 448.</p></div> + +<div class="footnote"><p><a name="Footnote_38_38" id="Footnote_38_38"></a><a href="#FNanchor_38_38"><span class="label">[38]</span></a> <i>Journal</i> of the Russian Physical and Chemical Society, +Vol. XXVIII., Division of Physics, Part I., January, 1896.</p></div> + +<div class="footnote"><p><a name="Footnote_39_39" id="Footnote_39_39"></a><a href="#FNanchor_39_39"><span class="label">[39]</span></a> See British Patent Specification No. 12,039, June 2, +1896.</p></div> + +<div class="footnote"><p><a name="Footnote_40_40" id="Footnote_40_40"></a><a href="#FNanchor_40_40"><span class="label">[40]</span></a> British Patent Specification No. 19,710 of 1899.</p></div> + +<div class="footnote"><p><a name="Footnote_41_41" id="Footnote_41_41"></a><a href="#FNanchor_41_41"><span class="label">[41]</span></a> <a name="tnd_fn41" id="tnd_fn41"></a><a href="#tn_fn41" class="tnlink" title="printer's error, extra full stop after reference"><i>Comptes Rendus.</i></a>, Vol. CXXVIII., p. 1,225, 1889; +<i>Science Abstracts</i>, Vol. II., p. 521.</p></div> + +<div class="footnote"><p><a name="Footnote_42_42" id="Footnote_42_42"></a><a href="#FNanchor_42_42"><span class="label">[42]</span></a> <i>Il Nuovo Cimento</i>, Vol. X., p. 279, 1899.</p></div> + +<div class="footnote"><p><a name="Footnote_43_43" id="Footnote_43_43"></a><a href="#FNanchor_43_43"><span class="label">[43]</span></a> <i>Wied Ann.</i>, Vol. LXVIII., p. 594, 1899; <i>Science +Abstracts</i>, Vol. II., p. 757.</p></div> + +<div class="footnote"><p><a name="Footnote_44_44" id="Footnote_44_44"></a><a href="#FNanchor_44_44"><span class="label">[44]</span></a> <i>Comptes Rendus</i>, Vol. CXXX., p. 902, 1900; <i>Science +Abstracts</i>, Vol. III., p. 615.</p></div> + +<div class="footnote"><p><a name="Footnote_45_45" id="Footnote_45_45"></a><a href="#FNanchor_45_45"><span class="label">[45]</span></a> See <i>Proc.</i> Roy. Soc., London, Vol. LXXI., p. 402.</p></div> + +<div class="footnote"><p><a name="Footnote_46_46" id="Footnote_46_46"></a><a href="#FNanchor_46_46"><span class="label">[46]</span></a> See Report by Capt. Quintino Bonomo, "Telegrafia Senza +Fili," Rome, 1902; <i>L'Elettricista</i>, Ser. II., Vol. I., pp. 118, 173.</p></div> + +<div class="footnote"><p><a name="Footnote_47_47" id="Footnote_47_47"></a><a href="#FNanchor_47_47"><span class="label">[47]</span></a> See Royal Institution, Friday evening discourse, by Mr. +Marconi, June 13, 1902; also <i>The Electrician</i>, Vol. XLIX., p. 490; +also a letter to <i>The Times</i> of July 3, 1902, by the Marchese Luigi +Solari.</p></div> + +<div class="footnote"><p><a name="Footnote_48_48" id="Footnote_48_48"></a><a href="#FNanchor_48_48"><span class="label">[48]</span></a> See U.S.A. Patent Specification No. 700,161, May 24, +1900.</p></div> + +<div class="footnote"><p><a name="Footnote_49_49" id="Footnote_49_49"></a><a href="#FNanchor_49_49"><span class="label">[49]</span></a> See E. Marx, <i>Phys. Zeitschrift</i>, Vol. II., p. 249; +<i>Science Abstracts</i>, Vol. IV., p. 471. See also German Patent +Specification No. 121,663, Class 21a.</p></div> + +<div class="footnote"><p><a name="Footnote_50_50" id="Footnote_50_50"></a><a href="#FNanchor_50_50"><span class="label">[50]</span></a> See "The Scientific Writings of Professor Joseph +Henry."</p></div> + +<div class="footnote"><p><a name="Footnote_51_51" id="Footnote_51_51"></a><a href="#FNanchor_51_51"><span class="label">[51]</span></a> <i>Phil. Trans.</i> Roy. Soc., London, 1897, Vol. <span class="smcap">CLXXXIX.a</span>, +p. 1.</p></div> + +<div class="footnote"><p><a name="Footnote_52_52" id="Footnote_52_52"></a><a href="#FNanchor_52_52"><span class="label">[52]</span></a> See <i>Proc.</i> Roy. Soc., London, June 12, 1902. "Note on a +Magnetic Detector for Electric Waves which can be employed as a +Receiver for Space Telegraphy," by G. Marconi.</p></div> + +<div class="footnote"><p><a name="Footnote_53_53" id="Footnote_53_53"></a><a href="#FNanchor_53_53"><span class="label">[53]</span></a> See U.S.A. Patent Specification No. 716,000, Application +of July 5, 1901.</p></div> + +<div class="footnote"><p><a name="Footnote_54_54" id="Footnote_54_54"></a><a href="#FNanchor_54_54"><span class="label">[54]</span></a> See the <i>Electrical Review</i>, Vol. XLIV., 1899, May 26; +<i>Wied Ann.</i>, Vol. LXVIII., p. 92; or German Patent Specification No. +107,843.</p></div> + +<div class="footnote"><p><a name="Footnote_55_55" id="Footnote_55_55"></a><a href="#FNanchor_55_55"><span class="label">[55]</span></a> U.S.A. Patent Specification No. 706,742, 1902.</p></div> + +<div class="footnote"><p><a name="Footnote_56_56" id="Footnote_56_56"></a><a href="#FNanchor_56_56"><span class="label">[56]</span></a> See British Patent Specification, G. Marconi, No. +12,039, June 2, 1896.</p></div> + +<div class="footnote"><p><a name="Footnote_57_57" id="Footnote_57_57"></a><a href="#FNanchor_57_57"><span class="label">[57]</span></a> See G. Marconi, British Patent Specification No. 12,326, +of June 1, 1898.</p></div> + +<div class="footnote"><p><a name="Footnote_58_58" id="Footnote_58_58"></a><a href="#FNanchor_58_58"><span class="label">[58]</span></a> See the <i>Electrical Review</i>, September 26, 1902, Vol. +LI., p. 543.</p></div> + +<div class="footnote"><p><a name="Footnote_59_59" id="Footnote_59_59"></a><a href="#FNanchor_59_59"><span class="label">[59]</span></a> There is a good deal of contradiction between various +inventors on this point, some saying that "earthed" aerials obviate +atmospheric electrical disturbances, and others that insulated aerials +are in this respect superior. The truth appears to be that, neither +form is absolutely free from risk of disturbance by this cause.</p></div> + +<div class="footnote"><p><a name="Footnote_60_60" id="Footnote_60_60"></a><a href="#FNanchor_60_60"><span class="label">[60]</span></a> The capacity of an electrical circuit corresponds to the +elastic pliability, or what is commonly called the elasticity, of a +material substance, and the inductance to mass or inertia. Hence +capacity and inductance are qualities of an electric circuit which are +analogous to the elasticity and inertia of such a body as a heavy +spring.</p></div> + +<div class="footnote"><p><a name="Footnote_61_61" id="Footnote_61_61"></a><a href="#FNanchor_61_61"><span class="label">[61]</span></a> See Cantor Lectures, on "Electrical Oscillations and +Electric Waves," delivered before the Society of Arts, London, +November 26, December 4, 10, 17, 1900. Lecture I., p. 12, of reprint.</p></div> + +<div class="footnote"><p><a name="Footnote_62_62" id="Footnote_62_62"></a><a href="#FNanchor_62_62"><span class="label">[62]</span></a> A fuller account of these experiments was given by the +author in a letter to the London <i>Times</i> published on April 14, 1903.</p></div> + +<div class="footnote"><p><a name="Footnote_63_63" id="Footnote_63_63"></a><a href="#FNanchor_63_63"><span class="label">[63]</span></a> See <i>Journal</i> of the Society of Arts, Vol. XLIX., p. +505. "Syntonic Wireless Telegraphy," by G. Marconi.</p></div> + +<div class="footnote"><p><a name="Footnote_64_64" id="Footnote_64_64"></a><a href="#FNanchor_64_64"><span class="label">[64]</span></a> See German Patent Specifications, Class 21a, No. 7,452 +of 1900, and also No. 8,087 of 1901.</p></div> + +<div class="footnote"><p><a name="Footnote_65_65" id="Footnote_65_65"></a><a href="#FNanchor_65_65"><span class="label">[65]</span></a> See German Patent Specification, Class 21a, No. 7,498 of +1900, applied for November 9, 1900. The above-mentioned patent is +subsequent in date to Mr. Marconi's experiments on the same subject.</p></div> + +<div class="footnote"><p><a name="Footnote_66_66" id="Footnote_66_66"></a><a href="#FNanchor_66_66"><span class="label">[66]</span></a> See <i>The Electrician</i>, January 18, 1900, Vol. XLVI., p. +475. Also reprint of a Paper of Professor A. Slaby, "Abgestimmte und +mehrfache Funkentelegraphie."</p></div> + +<div class="footnote"><p><a name="Footnote_67_67" id="Footnote_67_67"></a><a href="#FNanchor_67_67"><span class="label">[67]</span></a> See British Specification No. 11,348 of 1901.</p></div> + +<div class="footnote"><p><a name="Footnote_68_68" id="Footnote_68_68"></a><a href="#FNanchor_68_68"><span class="label">[68]</span></a> See <i>Comptes Rendus</i>, May 21, 1900; Rapports du Congrès +International d'Electricité, Paris, 1900, p. 341.</p></div> + +<div class="footnote"><p><a name="Footnote_69_69" id="Footnote_69_69"></a><a href="#FNanchor_69_69"><span class="label">[69]</span></a> See <i>The Electrician</i>, Vol. XLVI., p. 573, February 8, +1901.</p></div> + +<div class="footnote"><p><a name="Footnote_70_70" id="Footnote_70_70"></a><a href="#FNanchor_70_70"><span class="label">[70]</span></a> See <i>The Electrician</i>, Vol. L., p. 418, January 2, +1903.</p></div> + +<div class="footnote"><p><a name="Footnote_71_71" id="Footnote_71_71"></a><a href="#FNanchor_71_71"><span class="label">[71]</span></a> See Mr. Marconi's Friday evening discourse at the Royal +Institution, June 13, 1902; also <i>The Electrician</i>, Vol. XLIX., p. +390.</p></div> + +<div class="footnote"><p><a name="Footnote_72_72" id="Footnote_72_72"></a><a href="#FNanchor_72_72"><span class="label">[72]</span></a> See <i>Proc.</i> Roy. Soc., June 12, 1902. "A Note on the +Effect of Daylight upon the Propagation of Electromagnetic Impulses +over Long Distances," by G. Marconi.</p></div> + +<div class="footnote"><p><a name="Footnote_73_73" id="Footnote_73_73"></a><a href="#FNanchor_73_73"><span class="label">[73]</span></a> See <i>Phil. Mag.</i>, Vol. IV., p. 253, Series 6, August, +1902. J. J. Thomson, "On Some Consequences of the Emission of +Negatively-electrified Corpuscles by Hot Bodies."</p></div> + +<div class="footnote"><p><a name="Footnote_74_74" id="Footnote_74_74"></a><a href="#FNanchor_74_74"><span class="label">[74]</span></a> The opinion that ionisation of the air by sunlight is a +cause of obstruction to Hertzian waves propagated over long distances +has also been expressed by Mr. J. E. Taylor. See <i>Proc.</i> Roy. Soc., +Vol. LXXI., p. 225, 1903. "Characteristics of Earth Current +Disturbances and their Origin."</p></div> + +<div class="footnote"><p><a name="Footnote_75_75" id="Footnote_75_75"></a><a href="#FNanchor_75_75"><span class="label">[75]</span></a> See <i>Proc.</i> Roy. Soc., May 15, 1902. "On Some Phenomena +affecting the Transmission of Electric Waves over the Surface of the +Sea and Earth," by Captain H. B. Jackson, R.N., F.R.S.</p></div> + +<div class="footnote"><p><a name="Footnote_76_76" id="Footnote_76_76"></a><a href="#FNanchor_76_76"><span class="label">[76]</span></a> Mr. Marconi has informed the writer that these strategic +questions have received attention in selecting the sites for large +Marconi power stations in Italy.</p></div> + +</div> + +<hr style="width: 65%;" /> + +<div class="bbox"> + <p><b>Detailed Transcriber's Notes</b></p> + + <p>The text has been made to match the original text as much as possible + retaining all apparent printer's errors and inconsistencies. The + following, detail the apparent printer's errors etc. identified in + the original text.</p> + + <p>Variation in spelling, Strasburg and Strassburg for Strasbourg.</p> + + <p>There are a number of inconsistencies in hyphenation present in the + original text. Those concerned with the variation between one word or + a hyphenated word are detailed below. Those concerned with the + variation between multiple words and hyphenated words are too + numerous to detail individually.</p> + + <p>Inconsistent hyphenation of word, 'anti-node' and 'antinode' both + present in original text.</p> + + <p>Inconsistent hyphenation of word, 'electro-dynamic' and + 'electrodynamic' both present in original text.</p> + + <p>Inconsistent hyphenation of word, 'horse-shoe' and 'horseshoe' both + present in original text.</p> + + <p>Inconsistent hyphenation of word, 'over-blowing' and 'overblowing' + both present in original text.</p> + + <p>Page <a href="#Page_5">5</a><a name="tn_5" id="tn_5"></a>, possible printer's error, a for at, <a href="#tnd_5">'consisting when a rest'</a>.</p> + + <p>Page <a href="#Page_6">6</a><a name="tn_6" id="tn_6"></a>, printer's error, comma rather than full stop at end of + sentence, <a href="#tnd_6">'ether constituting electric radiation,'</a>.</p> + + <p>Page <a href="#Page_10">10</a><a name="tn_10" id="tn_10"></a>, printer's error, millmetre for millimetre, <a href="#tnd_10">'three thousand + volts per millmetre,'</a>.</p> + + <p>Page <a href="#Page_13">13</a><a name="tn_13a" id="tn_13a"></a>, possible printer's error, set for sets, <a href="#tnd_13a">'there are three set + of phenomena'</a>.</p> + + <p>Page <a href="#Page_13">13</a><a name="tn_13b" id="tn_13b"></a>, printer's error, duplicate word, <a href="#tnd_13b">'detached and and travel + away.'</a>.</p> + + <p>Page <a href="#Page_22">22</a><a name="tn_22a" id="tn_22a"></a>, printer's error, correponding for corresponding, + <a href="#tnd_22a">'correponding to this frequency'</a>.</p> + + <p>Page <a href="#Page_22">22</a><a name="tn_22b" id="tn_22b"></a>, printer's error, consist for consists, <a href="#tnd_22b">'due to Braun, consist + of attaching'</a>.</p> + + <p>Page <a href="#Page_24">24</a><a name="tn_24" id="tn_24"></a>, printer's error, one-hundreth for one-hundredth, <a href="#tnd_24">'capacity of + one-hundreth of a microfarad,'</a>.</p> + + <p>Page <a href="#Page_28">28</a><a name="tn_28" id="tn_28"></a>, printer's error, missing full stop at end of sentence added, + <a href="#tnd_28">'in the case of the hammer break.'</a>.</p> + + <p>Page <a href="#Page_33">33</a><a name="tn_33" id="tn_33"></a>, printer's error, supppse for suppose, <a href="#tnd_33">'Let us supppse'</a>.</p> + + <p>Page <a href="#Page_46">46</a><a name="tn_46" id="tn_46"></a>, printer's error, comma rather than full stop at end of + sentence, <a href="#tnd_46">'to the transmitting aerial,'</a>.</p> + + <p>Page <a href="#Page_48">48</a><a name="tn_48" id="tn_48"></a>, possible printer's error, alterations for alternations, + <a href="#tnd_48">'alterations of electric strain'</a>.</p> + + <p>Page <a href="#Page_54">54</a><a name="tn_54" id="tn_54"></a>, printer's error, Banly for Branly, <a href="#tnd_54">'proved that in a Banly + tube,'</a>.</p> + + <p>Page <a href="#Page_56">56</a><a name="tn_56" id="tn_56"></a>, variation in spelling, unsensitive for insensitive, <a href="#tnd_56">'wounded + and unsensitive.'</a>.</p> + + <p>Page <a href="#Page_59">59</a><a name="tn_59a" id="tn_59a"></a>, possible printer's error, sensive for sensitive <a href="#tnd_59a">'to work a + sensive recording apparatus'</a>.</p> + + <p>Page <a href="#Page_59">59</a><a name="tn_59b" id="tn_59b"></a>, possible printer's error, arragement for arrangement, <a href="#tnd_59b">'most + interesting arragement'</a>.</p> + + <p>Page <a href="#Page_61">61</a><a name="tn_61" id="tn_61"></a>, printer's error, missing letter i, <a href="#tnd_61">'as shown n Fig. 18,'</a>.</p> + + <p>Page <a href="#Page_71">71</a><a name="tn_71a" id="tn_71a"></a>, printer's error, osciilating for oscillating, <a href="#tnd_71a">'to that of the + osciilating circuit'</a>.</p> + + <p>Page <a href="#Page_71">71</a><a name="tn_71b" id="tn_71b"></a>, printer's error, impluse for impulse, <a href="#tnd_71b">'the period of that + impluse'</a>.</p> + + <p>Page <a href="#Page_74">74</a><a name="tn_74" id="tn_74"></a>, possible printer's error, extra comma in date, <a href="#tnd_74">'on May, 17, + 1901.'</a>.</p> + + <p>Page <a href="#Page_76">76</a><a name="tn_76" id="tn_76"></a>, printer's error, arangements for arrangements, <a href="#tnd_76">'variation of + the above arangements'</a>.</p> + + <p>Page <a href="#Page_77">77</a><a name="tn_77" id="tn_77"></a>, printer's error, systonic for syntonic, <a href="#tnd_77">'the systonic + transmitting'</a>.</p> + + <p>Page <a href="#Page_86">86</a><a name="tn_86" id="tn_86"></a>, printer's error, interpositon for interposition, <a href="#tnd_86">'effect of + the interpositon of land'</a>.</p> + + <p>Page <a href="#Page_87">87</a><a name="tn_87" id="tn_87"></a>, printer's error, signaling for signalling, <a href="#tnd_87">'the usual + maximum signaling'</a>.</p> + + <p>Footnote <a href="#Footnote_17_17">17</a><a name="tn_fn17" id="tn_fn17"></a>, printer's error, missing letter t, <a href="#tnd_fn17">'must be <i>solid</i> + metal and no hollow,'</a>.</p> + + <p>Footnote <a href="#Footnote_31_31">31</a><a name="tn_fn31" id="tn_fn31"></a>, printer's error, missing full stop after abbreviation, + <a href="#tnd_fn31">'<i>Phil Mag.</i>'</a>.</p> + + <p>Footnote <a href="#Footnote_41_41">41</a><a name="tn_fn41" id="tn_fn41"></a>, printer's error, extra full stop after reference, + <a href="#tnd_fn41">'<i>Comptes Rendus.</i>'</a>.</p> + + +</div> + + + + + + + + +<pre> + + + + + +End of the Project Gutenberg EBook of Hertzian Wave Wireless Telegraphy, by +John Ambrose Fleming + +*** END OF THIS PROJECT GUTENBERG EBOOK HERTZIAN WAVE WIRELESS TELEGRAPHY *** + +***** This file should be named 38526-h.htm or 38526-h.zip ***** +This and all associated files of various formats will be found in: + https://www.gutenberg.org/3/8/5/2/38526/ + +Produced by Robert Cicconetti, Tim Madden and the Online +Distributed Proofreading Team at https://www.pgdp.net (This +file was produced from images generously made available +by 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