464 



The Faraday Centenary. 



[June 17, 



the time when Faraday first made it. With all his skill, Faraday- 

 did not light upon the truth without delay and difficulty. One of 

 Faraday's biographers thus wrote: — "In December 1824, he had 

 attempted to obtain an electric current by means of a magnet, and on 

 three occasions he had made elaborate and unsuccessful attempts to 

 produce a current in one wire by means of a current in another wire, 

 or by a magnet. He still persevered, and on August 29, 1831 — that 

 is to say, nearly seven years after his first attempts — he obtained the 

 first evidence that an electric current induced another in a different 

 circuit." On September 23rd, he writes to a friend, R. Phillips: "I 

 am busy just now again with electro-magnetism, and think I have 

 got hold of a good thing, but cannot say ; it may be a weed instead 

 of a fish that, after all my labour, I at last haul 

 uj)." We now know that it was a very big fish 

 indeed. Lord Rayleigh proceeded to say that he 

 now proposed to illustrate the mechanics of the 

 question of the induced current by means of a 

 model (see figure), the first idea of which was due 

 to Maxwell. The one actually employed was a 

 combination known as Huygens's gear, invented 

 by him in connection with the winding of clocks. 

 Two similar pulleys, A B, turn upon a piece of 

 round steel fixed horizontally. Over these is hung 

 an endless cord, and the two bights carry similar 

 pendant pulleys, C, D, from which again hang 

 weights, E, F. The weight of the cord being 

 negligible, the system is devoid of potential 

 energy ; that is, it will balance, whatever may be 

 the vertical distance between C and D. Since 

 either pulley, A, B, may turn independently of 

 the other, the system is capable of two inde- 

 pendent motions. If A, B turn in the same direc- 

 tion and with the same velocity one of the pendant 

 pulleys, C, D rises, and the other falls. If, on the 

 other hand, the motions of A, B are equal and 

 opposite, the axes of the pendant pulleys and the 

 attached weights remain at rest. In the elec- 

 trical analogue the rotatory velocity of A corre- 

 sponds to a current in a primary circuit, that 

 of B to a current in a secondary. If, when all is at rest, the rota- 

 tion of A be suddenly started, by force applied at the handle or 

 otherwise, the inertia of the masses E, F opposes their sudden move- 

 ' ment, and the consequence is that the pulley B turns backwards, i. e. 

 in the opposite direction to the rotation imposed upon A. This is 

 the current induced in a secondary circuit when an electromotive- 

 force begins to act in the primary. In like manner, if A, having 

 been for some time in uniform movement, suddenly stops, B enters 

 into motion in the direction of the former movement of A. This is 



