676 FRAGMENTS OF SCIENCE. 



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 alter- 

 ation 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.* 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 charzicter of the influencing pole. The result was that 

 the currents in the two semicircles of the coil surrounding 

 the ring flowed in opposite directions. 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 alter- 

 nating machines are employed to produce the lights now 

 exhibited on the Holborn Viaduct and Thames Embank- 

 ment. 



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 

 instantly evokes in its surrounding coil an induced current 

 of corresponding strength. The currents excited in ap- 

 proaching to and retreating from a pole, 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 



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

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



