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, bv 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 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. 334. 
