32 



NATURE 



[May 14, 189? 



are placed in such a position that an end of one is near 

 that end of the other which will at any time be oppositely 

 electrified. For example, if the electric force in our 

 waves be in vertical lines, then if we place two elongated 

 conductors, one vertically above the other and separated 

 by a very small air space, the electric force alternating up 

 and down will cause currents to run up and down the 

 conductors simultaneously, and the upper ends of both 

 will be similarly electrified at any instant, while the lower 

 end of the upper one will always be oppositely electrified 

 to the upper end of the lower conductor, and if these two 

 points, or two short wires connected with them, be close 

 enough together, a spark will pass from one to the other 

 whenever the electric force sets up these electric oscilla- 

 tions in the conductor. Thus this apparatus is a detector 

 of the electric force. Whenever there is a spark we may 

 be sure that there is electric force, and whenever we can- 

 not get a spark we may be sure that there is either no 

 electric force or anyway too little to produce sparks. 

 The apparatus will be more sensitive for electric forces 

 that oscillate at the same rate as the natural vibration of 

 the electric charge on the conductor, because the effect 

 of each impulse will then add to that of the last ; resonance 

 will help to make the electrifications great, and so there 

 will be a better chance of our being able to produce a 

 spark. We may weaken the strength of this air gap by 

 reducing the pressure of the air in it. To do this the ends 

 of the conductors, or wires connected with them, must 

 lead into an exhausted air vessel, such as a Geissler's 

 tube. There is no doubt that much longer sparks may 

 thus be produced, but they are so dim and diffused that 

 when dealing with very minute quantities of electricity 

 those sparks in a vacuum are not more easily seen than 

 the smaller and intenser sparks in air at atmospheric 

 pressure. The additional complication and difficulty of 

 manipulation from having the terminals in a vacuum are not 

 compensated for by any advantages. This whole detect- 

 ing apparatus works on somewhat the same principle as a 

 resonator of definite size connected with one's ear when 

 used to detect a feeble note of the same pitch as the 

 resonator. Such a resonator might very well be used to 

 find out where this note existed and where it did not. It 

 would detect where there were compressions and rarefac- 

 tions of the air producing currents of air into and out of 

 your ear. In the same way the conductor sparking tells 

 where there are alternating electric forces making currents 

 alternately up and down the conductor, and ultimately 

 electrifying the end enough to make it spark. In the 

 sound resonator there is nothing exactly like this last 

 phenomenon. We have much more delicate ways 

 of detecting the currents of air than by making them 

 break anything. If anybody would allow the electric 

 currents from a Hertzian detector to be led directly into 

 the retina of his eye, it would probably be a very delicate 

 way of observing, though even in this direct application 

 of the current to an organ of sense it is possible that these 

 very rapidly alternating currents might fail to produce 

 any sensible effect, for they are not rapid enough to pro- 

 duce the photochemical effects by which we see. 



To recapitulate the arrangements proposed in order to 

 detect whether electric force is propagated with a finite 

 velocity, and if possible to measure it if finite. It is pro- 

 posed to create electric oscillations of very great rapidity, 

 oscillating some four or five hundred million times 

 per second, and it is expected thereby to produce waves 

 of electric force whose length will be less than a metre 

 if they are propagated with the velocity of light. It is 

 proposed to do this by causing an electric charge to 

 oscillate backwards and forwards between two con- 

 ductors, and across an air gap between them. This 

 oscillating charge is to be started by charging the 

 conductors, one positively and the other negatively, 

 until they discharge by a spark across this air gap. 

 By making the conductors small, and the distance the 

 NO. 1 1 24, VOL. 44] 



charge has to go from one to the other small, the 

 rate of oscillation of the charge can be made as great 

 as we require. If waves are produced by this arrange- 

 ment, we can reflect them at the surface of a large con- 

 ducting sheet, and then loops and nodes will be produced 

 where the incident and reflected waves co-exist. The 

 loops will be places where the alternating electric forces 

 are great, while at the nodes there will be no electric 

 forces at all. In order to detect where there are these 

 alternating electric forces and where there are none, it is 

 proposed to use either a single wire bent nearly into a 

 circle, with a very minute air gap between its ends, or 

 else two conductors placed end to end, with a minute 

 air gap between their ends. In either case, if the natural 

 period of vibration of a charge on the single conductor, 

 or on each of the conductors in the second arrangement, 

 is the same as the rate of alternation of the electric force 

 we wish to detect, there may be sufficient electrification 

 of the neighbouring ends to cause a spark across the 

 minute air gap. We are thus in possession of a complete 

 apparatus for determining whether electric waves are 

 produced, and what their wave-length is. 

 The experiment is conducted as follows : — 

 The two conductors which are to generate the waves 

 are placed, say, one above the other, so that the electric 

 charge will run up and down in a vertical line across the 

 spark gap between them. They might be placed hori- 

 zontally or in any other line, but for definiteness of 

 description it is well to suppose some definite position. 

 We may call them A and B. They are terminated in 

 polished knobs, between which the spark passes. A and 

 B are connected with the terminals of a Ruhmkorff coil, 

 or a Wimshurst or other apparatus by which a succession 

 of sparks may be conveniently made to pass from A to 

 B. Before the spark passes, A and B are being electri- 

 fied, and when the spark occurs the electricity on A 

 rushes over to B, and part of it charges B, while the elec- 

 tricity on B rushes across the spark, and partly charges A, 

 this taking place alternately up and down. Each time 

 there is less electricity, for some is neutralized during 

 each oscillation by the opposite charge ; for energy is 

 being spent, some in overcoming the resistance of the 

 spark gap, i.e. in producing the heat developed there, 

 and some in producing electric waves in the surrounding 

 medium. Thus the electric energy of the two oppositely 

 charged bodies A and B is gradually dissipated, and 

 one way of describing this is to say that the two opposite 

 electric charges combine and neutralize one another. 

 This whole language of talking of electric charges on 

 bodies, and electric currents from one to the other, of 

 electric charges neutralizing one another, and so forth, is 

 not in accordance with the most recent developments of 

 electro-magnetic theory. At the same time, those for 

 whom these articles are written are familiar with this 

 language, and with the view of the subject that it is 

 framed to suit, while they are unfamiliar with ether 

 electrically and magnetically strained and thereby the 

 seat of electric and magnetic energy, and consequently 

 it would have added very much to their difficulty in 

 grasping the details of a complicated question if it had 

 been described in unfamiliar terms, and from an un- 

 familiar point of view. 



The electric force in the neighbourhood of the vertical 

 generator will lie in vertical planes through it, and as 

 A and B are alternately positive and negative, the 

 electric force will alternately be from above downwards, 

 and from below upwards. If, then, this force is propa- 

 gated outwards in a series of waves, we may expect that 

 all round our generator waves of electric force will be 

 diverging ; waves in which the force will be alternately 

 down and up. The state of affairs might be roughly 

 illustrated by elastic strings stretched out in every direc- 

 tion from our generator. If their ends at the generator 

 be moved alternately down and up, waves will be propa- 



