LAWS OF MAGNETO-ELECTRIC INDUCTION. 107 



the magnet". Finally, it was found that the earth 

 might supply the place of a magnet in this as in 

 other experiments 7 ; and the mere motion of a wire, 

 under proper circumstances, produced in it, it ap- 

 peared, a momentary electric current 8 . These facts 

 were curiously confirmed by the results in special 

 cases. They explained Arago's experiments ; for the 

 momentary effect became permanent by the revolu- 

 tion of the plate. And without using the magnet, a 

 revolving plate became an electrical machine 9 ; a 

 revolving globe exhibited electro-magnetic action 10 , 

 the circuit being complete in the globe itself with- 

 out the addition of any wire ; and a mere motion 

 of the wire of a galvanometer produced an electro- 

 dynamic effect upon its needle 11 . 



But the question occurs, What is the general 

 law which determines the direction of electric cur- 

 rents thus produced by the joint effects of motion 

 and magnetism ? Nothing but a peculiar steadiness 

 and clearness in his conceptions of space, could 

 have enabled Mr. Faraday to detect the law of this 

 phenomenon. For the question required that he 

 should determine the mutual relations in space 

 which connect the magnetic poles, the position of 

 the wire, the direction of the wire's motion, and the 

 electrical current produced in it. This was no easy 

 problem ; indeed, the mere relation of the magnetic 

 to the electric forces, the one set being perpen- 



6 Art. 53. 7 Second Series, Phil. Trans, p. 163. 



8 Art. 141. 9 Art. ISO. 10 Art, 164. " Art. 171. 



