234 



SCIENTIFIC NEWS. 



[Sept. 7, iJ 



gation in time is explicable by action at a distance. It is the same 

 in the case of electro-magnetic actions. There were two hypotheses 

 as to the causes of electro-magnetic actions. One attributed electric 

 attraction to a property of a thing called electricity, to attract at a 

 distance, the other attributed it to a pull exerted by means of the 

 ether, somewhat in the way that air pushes balloons up. We do not 

 know what the structure of the ether is by means of which it can 

 pull, but neither do we know what the structure of a piece of india- 

 rubber is by means of which it can pull, and we might as well ignore 

 the indiarubber, though we know a lot about the laws of its action, 

 because we do not know i's structure, as to ignore the ether because 

 we do not know its structure. Anyway, what was wanted was an 

 f xperiment to decide between the hypothesis of direct action at a 

 distance and of action by means of a medium. At the time that 

 Clerk Maxwell delivered his address no experiment was known that 

 could decide between the two hypotheses. Specific inductive capa- 

 city, the action of intervening matter, the delay in telegraphing, the 

 time propagation of electro-magnetic actions by means of conducting 

 material — these were known, but he knew that they could be ex- 

 plained by means of action at a distance, and had been so explained. 

 Waves in a conductor do not necessarily postulate action through a 

 medium such as the ether. When we are dealing with a conductor 

 and a thing called electricity running over its surface, we are, of 

 course, postulating a medium on or in the conductor, but not out- 

 side it, which is the special point at issue. Cleik Maxwell believed 

 that just as the same air that transmits sound is able by differences 

 of pressure — i.e., by means of its energy per unit volume — to move 

 bodies immersed in it, so the same ether that transmits light causes 

 electrified bodies to move by means of its energy per unit volume. 

 He believed this, but there was no experiment known then to decide 

 between this hypothesis and that of direct action at a distance. As 

 I have endeavoured to impress upon you, no experimentum cruris 

 between the hypotheses is possible except an experiment proving 

 propagation in time either directly, or indirectly by an experiment 

 exhibiting phenomena like those of the interference cf light. A 

 theorist may speak of propagation of actions in time without talking 

 of a medium. This is all very well in mathematical formula;, but, 

 as in the case of light, we must consider what becomes of it after it 

 has left the sun and before it reaches the earth, so every hypothesis 

 assuming action in time really postulates a medium whether we talk 

 about it or not. There are some difficulties surrounding the com- 

 plete interpretation of some of Hertz's experiments. The conditions 

 are complicated, but I confidently expect that they will lead to a 

 decision on most of the outstanding questions on the theory of 

 electro-magnetic action. However, there is no doubt that he has 

 observed the interference of electro-magnetic waves quite analogous 

 to those of light, and that he has proved that electro-magnetic actions 

 are propagated in air with the velocity of light. By a beautiful 

 device Hertz has produced rapidly alternating currents of such fre- 

 quency that their wave length is only about two metres. I may 

 pause for a minute to call your attention to what that means. These 

 waves are propagated three hundred thousand kilometres in a second. 

 If they vibrated three hundred thousand times a second the waves 

 would be each a kilometre long. This rate of vibration is much 

 higher than the highest audible note, and yet the waves are much 

 tco long to be manageable. We want a vibration about a thousand 

 times as fast again with waves about a metre long. Hertz produced 

 such vibrations, vibrating more than a hundred million times a 

 a second. That is, there are as many vibrations in one second as 

 there are seconds — in a day? No, far more. In a week? No, 

 more even than that. The pendulum of a clock ticking seconds 

 would have to vibrate for four months before it would vibrate as 

 often as one of Hertz's vibrators vibrates in one second. And how did 

 he detect the vibrations and their interference ? He could not see 

 them ; they are much too slow for that ; they should go about a 

 million times as fast again to be visible. He could not hear them ; 

 they are much too quick for that. If they went a million times more 

 slowly they would be well heard. He made use of the principle of 

 resonance. You all understand how by a succession of well-timed 

 small impulses a large vibration may be set up. It explains many 

 things, from speech to spectrum analysis. It is related that a former 

 Marquess of Waterford used the principle to overturn lamp-posts ; 

 his ambition soared above knocker-wrenching. So that it is a 

 principle known to others besides scientific men. Hertz constructed 

 a circuit whose period of vibration for electric currents was the same 

 as that of his generating vibrator, and he was able to see sparks, due 

 to the induced vibration, leaping across a small air-space in this re- 

 sonant circuit. The well-timed electrical impulses broke down the 

 air-resistance just as those of my Lord of Waterford broke down the 

 lamp-post. The combination of a vibrating generating circuit 

 with a resonant-receiving circuit is one that I spoke of at the 

 meeting of the British Association at Southport as one by 

 which this very question might be studied, At the time 



I did not see any feasible way of detecting the induced re- 

 sonance ; I did not anticipate that it could produce sparks. By 

 its means, however, Hertz has-been able to observe the interference 

 between waves incident on a wall and the reflected waves. He placed 

 his generating vibrator several wave lengths away from a wall, and 

 placed the receiving resonant circuit between the generator and the 

 wall, and in this air-space he was able to observe that at some 

 points there were hardly any induced sparks, but at other and 

 greater distances from his generator they reappeared to disappear 

 again in regular succession at equal intervals between his generator 

 and the wall. It is exactly the same phenomenon as what are 

 known as Lloyd's bands in optics, which are due to the interference 

 between a direct and a reflected wave. It follows hence that, just 

 as Young's and Fresnell's researches on the Interference of Light 

 prove the undulatory theory of optics, so Hertz's experiment proves 

 the ethereal theory of electro-magnetism. Jt is a splendid result. 

 Henceforth I hope no learner will fail to be impressed with the 

 theory — hypothesis no longer — that electro-magnetic actions are 

 due to a medium pervading all known space, and that it is the 

 same medium as the one by which light is propagated, that non- 

 conductors can, and probably always do, as Professor Poynting has 

 taught us, transmit eleclro-magnetic energy. By means of variable 

 currents energy is propagated into space with the velocity of light. 

 The rotation of the earth is being slowly stopped by the diurnal 

 rotation of its magnetic poles. This seems a hopeful direction in 

 which to look for an explanation of the secular precession of 

 terrestrial magnetism. It is quite different from Edlund's curious 

 hypothesis that free space is a perfect conductor. If this were true, 

 there would be a pair of great antipoles outside the air, and ter- 

 restrial magnetism would not be much like what it is, and I think 

 the eanh would have stopped rotating long ago. With alternating 

 currents we do propagate energy through non-conductors. It seems 

 almost as if our future telegraph cables would be pipes. Just as 

 the long sound waves in speaking tubes go round corners, so these 

 electro-magnetic waves go round corners if they are not too sharp. 

 Professor Lodge will probably have something to tell us on this 

 point in connection with lightning conductors. The silvered glass- 

 bars used by surgeons to conduct light are exactly what I am 

 describing. They are a glas=, a non-conducting, and therefore 

 transparent, bar surrounded by a conducting, and therefore opaque, 

 silver theath, and they transmit the rapidly alternating currents we 

 call light. There would not be the same difficulty in utilising the 

 energy of these electro-magnetic waves as in utilising radiant heat. 

 Having all the vibrations of the same period we mijht utilise 

 Hertz's resoniting circuits, and in any c:se the second law of 

 thermodynamics would not trouble us when we could practically 

 attain to the abs.lute zero of thes-, as compared with heat, long 

 period vibrations. 



We seem to be approaching a theory as to the structure of the 

 ether. There are difficulties from diffusion in the simple theory 

 that it is a fluid full of motion, a sort of vortex sponge. There were 

 similar difficulties in the wave theory of l'ght owing to wave propa- 

 gation round corners, and there is as great a difficulty in the jelly 

 theory of the ether arising from the freedom of motion of matter 

 through it. It may be found that there is diffusion or it may be 

 found that there are polarised distributions of fluid kinetic energy 

 which are not unstable when the surfaces are fixed ; more than one 

 such is known. Osborne Reynolds has pointed out another, though 

 in my opinion less hopeful, direction in which to look for a theory 

 of the ether. Hard particles are abominations. Perhaps the im- 

 penetrability of a vortex would suffice. Olive Lodge speaks confi- 

 dently of a sort of chemical union of two opposite kinds of elements 

 forming the ether. The opposite sides of a vortex ring might per- 

 chance suit, or maybe the ether, after all, is but an atmosphere of 

 some infra-hydrogen element ; these two latter hypotheses may both 

 come to the same thing. Anyway we are learning daily what sort 

 of properties the ether must have. It must be the means of propa- 

 gation of light ; it must be the means by which electric and mag- 

 netic forces exist ; it should explain chemical actions, and, if possible, 

 gravity. 



On the vortex-sponge theory of the ether there is no real diffi- 

 culty by reason of complexity why it should not explain chemical 

 actions. In fact, there is every reason to expect that very much 

 more complex actions would take place at distances comparable with 

 the size of the vortices than at the distances at which we study the 

 simple phenomena of electro magnetism. Indeed, if vortices can 

 make a small piece of a strong elastic solid we can make watches 

 and build steam engines and any amount of complex machinery, so 

 that complexity can be no essential difficulty. Similarly the in- 

 stantaneous propagation of gravity, if it exists, is not an essential 

 difficulty, for vortices each oc-'upy all space, and they act on one 

 another simultaneously everywhere. The theory that material 

 atoms are simple vortex rings in a perfect liquid otherwise unmov- 



