560 KEPORT — 1888. 



At tbe time that Clerk Maxwell delivered his address no experiment was knowa 

 that could decide between the two hypotheses. Specihc inductive capacity, the 

 action of intervening matter, the delay in telegraphing, the time propagation of 

 electro-magnetic actions by means of conducting material — these were knovra, but 

 he knew that they could be explained by means of action at a distance, and had 

 been so explained. Waves in a conductor do noi 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 outside it, which is the special point at 

 issue. Clerk 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 electri- 

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

 e.vperimentum crucis 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 of light. A theorist may speak of pro- 

 pagation of actions in time without talking of a medium. This is all very well in 

 mathematical formulae, 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 ditticulties surrounding the complete interpretation of some of 

 llertzs e.xperiments. The conditions are compUcated, 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 

 niterference 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 frequency 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 

 ii 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, vibrating more than a hundred million times a 

 second. That is, there are as manj' 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 qiuck 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- 

 plaint; 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 

 resonant 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 combina- 

 tion 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 



