564 THE POPULAR SCIENCE MONTHLY. 



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 magnet- 

 ism 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 magnetism 

 of the bar may be, as long as its condition remains permanent no cur- 

 rent 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 alteration of the magnetism of the bar will occur, and a 

 similar current will be induced in the coil. You have here the funda- 

 mental conception which led M. Gramme to the construction of his 

 beautiful machine.* 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 horseshoe magnet. The direction of the 

 current varied with the motion and with the character of the influenc- 

 ing pole. The result was that the currents in the two semicircles of 

 the coil surrounding the ring flowed in opposite directionss. But it was 

 easy, by the mechanical arrangement called a commutator, to gather 

 up the currents and cause them to flow in the same direction. The first 

 machines of Gramme, therefore, furnished direct currents, similar to 

 those yielded by the voltaic pile. M. Gramme subsequently so modified 

 his machine as to produce alternating currents. Such machines are 

 ■employed to produce the lights now exhibited on the Holborn Viaduct 

 and the Thames Embankment. 



Another machine of great alleged merit is that of M. Lontin. It 

 resembles in shape a toothed iron wheel, the teeth being used as cores, 

 round which are wound coils of copper wire. The wheel is caused to 

 rotate between the opposite poles of powerful electro-magnets. On 

 passing each pole the core or tooth is strongly magnetized, and instant- 

 ly evokes in the surrounding coil an induced current of corresponding 

 strength. The currents excited in approaching and retreating, and in 

 passing different poles, move in opposite directions, but by means of a 

 commutator these conflicting electric streams are gathered up and 

 caused to flow in a common bed. The bobbins in which the currents 

 are induced can be so increased in number as to augment indefinitely 

 the power of the machine. To excite his electro-magnets, M. Lontin 

 applies the principle of Mr. Wilde. A small machine furnishes a direct 



' " Comptes Rcndus," ISYl, p. ITG. See also Gaugain on the Gramme machine, 

 "Ann. do Chim. at de Phys.," vol. xxviii., p. 324. 



