460 FRAGMENTS OF SCIENCE 



we want to obtain in the same circuit several lights of 

 moderate intensity, machines of high internal resistance 

 and of correspondingly high electro -motive power must 

 be invoked. 



When a coil of covered wire surrounds a bar of iron, 

 the two ends of the coil being connected together, every 

 alteration of the magnetism of the bar is accompanied by 

 the development of an induced current in the coil. The 

 current is only excited during the period of magnetic 

 change. No matter how strong or how weak the magnet- 

 ism of the bar may be, as long as its condition remains 

 permanent no current is developed. Conceive, then, the 

 pole of a magnet placed near one end of the bar to be 

 moved along it toward the other end. During the time 

 of the pole's motion there will be an incessant change in 

 the magnetism of the bar, and accompanying this change 

 we shall have an induced current in the surrounding coil. 

 If, instead of moving the magnet, we move the bar and 

 its surrounding coil past the magnetic pole, a similar al- 

 teration of the magnetism of the bar will occur, and a 

 similar current will be induced in the coil. You have 

 here the fundamental conception which led M. Gramme 

 to the construction of his beautiful machine. 1 He aimed 

 at giving continuous motion to such a bar as we have 

 here described; and for this purpose he bent it into a 

 continuous ring, which, by a suitable mechanism, he 

 caused to rotate rapidly close to the poles of a horse- 

 shoe magnet. The direction of the current varied with 

 the motion and with the character of the influencing pole. 

 The result was that the currents in the two semicircles of 



1 "Comptes Rendus," 1871, p. 176. See also Gaugain on the Gramme 

 machine, "Ann. de Chem. et de Phys.," vol. xxviii. p. 324. 



