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NATURE 



{Sept. 6, 1888 



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 frequency that their wave- 

 length is only about 2 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 too 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, vibiating more than a hundred 

 million times a second. That is, there are as many vibra- 

 tions in one second as there are seconds — in a day? No, 

 far more. In a week ? No, more even than that. The pen- 

 dulum 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 ex- 

 plains 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 resonance : 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 dis- 

 appear 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 Fresnel's researches on the inter- 

 ference of light prove the undulatory theory of optics, so Hertz's 

 experiment proves the ethereal theory of electro-magnetism. It 

 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 Prof. Poynting has taught us, transmit electro-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 terrestrial magnetism would not be 

 much like what it is, and I think the earth would have stopped 

 rotating long ago. With alternating currents we do propagate 

 energy through nonconductors. 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. Prof. Lodge 

 will probably have something to tell us on this point in connec- 

 tion with lightning-conductors. The silvered glass-bars used by 

 surgeons to conduct light are exactly what I am describing. 

 They are a glass, a non-conducting, and therefore transparent, 



bar surrounded by a conducting, and therefore opaque, silver 

 sheath, and they transmit the rapidly alternating currents we call 

 light. There would not be the same difficulty in utilizing the 

 energy of these electro-magnetic waves as in utilizing radiant 

 heat. Having all the vibrations of the same period we might 

 utilize Hertz's resonating circuits, and in any case the second law 

 of thermodynamics would not trouble us when we could 

 practically attain to the absolute zero of these, 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 light owing to wave 

 propagation 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 polarized 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 impenetrability of a vortex would 

 suffice. Oliver Lodge speaks confidently of a sort of chemical 

 union of two opposite kinds of elements forming the ether. The 

 opposite sides of a vortex- ring might perchance 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 propagation of light ; 

 it must be the means by which electric and magnetic forces 

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

 gravity. 



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

 difficulty 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 instantaneous propagation of 

 gravity, if it exists, is not an essential difficulty, for vortices each 

 occupy all space, and they act on one another simultaneously 

 everywhere. The theory that material atoms are simple vortex- 

 rings in a perfect liquid otherwise unmoving is insufficient, but 

 with the innumerable possibilities of fluid motion it seems almost 

 impossible but that an explanation of the properties of the uni- 

 verse will be found in this conception. Anything purporting to 

 be an explanation founded on such ideas as "an inherent 

 property of matter to attract," or building up big elastic solids 

 out of little ones, is not of the nature of an ultimate explanation 

 at all ; it can only be a temporary stopping-place. There are 

 metaphysical grounds, too, for reducing matter to motion and 

 potential to kinetic energy. 



These ideas are not new, but it is well to enunciate them from 

 time to time, and a Presidential address in Section A is a fitting 

 time. Besides all this, it has become the fashion to indulge in 

 quaint cosmical theories and to dilate upon them before learned 

 Societies and in learned journals. I would suggest, as one who 

 has been bogged in this quagmire, that a successor in this chair 

 might well devote himself to a review of the cosmical theories 

 propounded within the last few years. The opportunities for 

 piquant critiefsm would be splendid. 



Returning to the sure ground of experimental research, let us 

 for a moment contemplate what is betokened by this theory that 

 in electro-magnetic engines we are using as our mechanism the 

 ether, the medium that fills all known space. It was a great 

 step in human progress when man learnt to make material 

 machines, when he used the elasticity of his bow and the rigidity 

 of his arrow to provide food and defeat his enemies. It was a 

 great advance when he learnt to use the chemical action of fire , 

 when he learnt to use water to float his boats and air to drive 

 them ; when he used artificial selection to provide himself with 

 food and domestic animals. For two hundred years he has made 

 heat his slave to drive his machinery. Fire, water, earth, and 

 air have long been his slaves, but it is only within the last few 

 years that man has won the battle lost by the giants of old, has 

 s'iatched the thunderbolt from Jove himself, and enslaved the 

 all-pervading ether. 



