1891.] The Faraday Centenary. 465 



the secondary current on the break of the current in the primary 

 circuit. It might perhaps be supposed by some that the model was 

 a kind of trick. Nothing could be further from the truth. The 

 analogy of the two things was absolutely essential. So far was this 

 the case that precisely the same argument and precisely the same 

 mathematical equations proved that the model and the electric 

 currents behaved in the way in which they had seen them behave in 

 the experiment. That might be considered to be a considerable 

 triumph of the modern dynamical method of including under the 

 same head phenomena the details of which might be so different as in 

 this case. If they had a current which alternately stopped and 

 started, and so on, for any length of time, they, as it were, produced 

 in a permanent manner some of the phenomena of electrical induc- 

 tion ; and if it were done with sufficient rapidity it would be evident 

 that something would be going on in the primary and in the 

 secondary circuit. The particular apparatus by which he proposed 

 to illustrate those effects of the alternating current was devised by a 

 skilful American electrician. Prof. Elihu Thomson, and he had no 

 doubt it would be new to many. The alternating current was led 

 into the electro-magnet by a suitable lead ; if another electric 

 circuit, to be called the secondary circuit, was held in the neigh- 

 bourhood of that, currents would be induced and might be made 

 manifest by suitable means. Such a secondary circuit he held in 

 his hand, and it was connected with a small electric glow-lamp. If 

 a current of sufficient intensity were induced in that secondary circuit 

 it would pass through the lamp, which would be rendered incan- 

 descent. [Illustrating,] It was perfectly clear there was no con- 

 juring there ; the incandescent lamp brightened up. One of the first 

 questions which presented itself was, what would be the effect of 

 putting something between ? Experimenting with a glass plate, he 

 showed there was no effect, but when they tried a copper plate the 

 lamp went completely out, showing that the copper plate was an 

 absolute screen to the effect, whatever it might be. Experiments of 

 that kind, of course in a much less developed and striking form, 

 were made by Faraday himself, and must be reckoned amongst some 

 of his greatest discoveries. 



Before going further, he might remark on what strong evidence 

 they got in that way of the fact that the propagation of the electric 

 energy which, having its source in the dynamo downstairs, eventually 

 illuminated that little lamp, was not merely along the wires, but was 

 capable of bridging over and passing across a space free from all 

 conducting material, and which might be air, glass, or, equally well, 

 vacuum. Another kindred effect of a striking nature, devised by Prof. 

 Elihu Thomson, consisted in the repulsive action which occurred 

 between the primary current circulating around a magnet and the 

 current induced in a single hoop of aluminium wire. Illustrating 

 this by experiment, he showed that the repulsion was so strong as to 

 throw the wire up a considerable height. Those effects were com- 



