MAGNETO-ELECTRIC GENERATORS. 



47 



ceases to circulate in the metallic spiral acting as its con- 

 ductor. This phenomenon, first observed by Ampere and 

 Arago a short time after the discovery by CErsted of the 

 effects of currents on the magnetic needle, was the starting- 

 point of electro-magnetism and of the majority of the me- 

 chanical applications of electricity. 



By reasoning according to the principle of reciprocal 

 actions, it might have been deduced a priori from this phe- 

 nomenon that a permanent magnet by reacting on a closed 

 circuit would cause an electric current in that circuit. Yet it 



FIG. IT. 



was ten years afterwards, that is to say, about 1830, that the 

 illustrious English physicist, Faraday, first proved the existence 

 of this phenomenon and determined its various character- 

 istics. Numerous experiments instituted by Faraday on this 

 subject showed that if a magnetic bar be thrust within a 

 coil covered with insulated wire, or if a second coil traversed 

 by a current be so thrust, as shown in Figs. 10 and n, an 

 energetic current capable of affecting a galvanometer is, in 

 fact, induced in the first coil. But there was a somewhat 

 peculiar circumstance which theory could not have enabled 



