DISCOVERY BY MEANS OF CONVERSE EXPERIMENTS. 523 



tion were discovered in this way. As electric currents pro- 

 duced magnetism, so magnetism was assumed to be able to 

 produce electric currents. ' In 1831 Faraday again sought 

 for electro-dynamic induction, and, after some futile trials, 

 at last found it in a form different from that in which he 

 had looked for it. It was then seen that, at the precise time 

 of making or breaking the contact which closed the gal- 

 vanic circuit, a momentary effect was induced in a neigh- 

 bouring wire, but disappeared instantly. Once in possession 

 of this fact, Mr. Faraday ran rapidly up the ladder of dis- 

 covery to the general point of view. Instead of suddenly 

 making or breaking the contact of the inducing circuit, a 

 similar effect was produced by removing the inducible 

 wire nearer to or farther from the circuit ; the effects 

 were increased by the proximity of soft iron ; when the 

 iron was affected by an ordinary magnet instead of the 

 voltaic wire, the same effect still recurred ; and thus it 

 appeared that, by making and breaking magnetic contact, 

 a momentary electric current was produced ; it was pro- 

 duced also by moving the magnet, or by moving the wire 

 with reference to the magnet. Finally, it was found that 

 the earth might supply the place of a magnet in this as 

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

 under proper circumstances, produced in it, it appeared, a 

 momentary electric current. These facts were curiously 

 confirmed by the results in special cases. They explained 

 Arago's experiments, for the momentary effect became per- 

 manent by the revolution of the plate ; and, without using 

 I the magnet, a revolving plate became an electrical machine ; 

 a revolving globe exhibited electro-magnetic action, the cur- 

 irent being complete in the globe itself without the addition 

 of any wire ; and the mere motion of the wire of a galvano- 

 meter produced an electro-dynamic effect upon its needle.' l 



1 "Whewell, History oftlie Inductive Sciences, vol. iii. 3rd ed. p. 85, 



