14 Professor Tyndall [Jan. 17, 



resistance and large quantity must be employed. If 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 magnetism of the 

 bar may be, as long as its condition remains permanent no current is 

 developed. Conceive the pole of a magnet placed near one end of the 

 bar to be moved along it to 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 fundamental conception of M. Gramme which 

 led 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. By a suitable 

 mechanism he caused the various parts of the ring to pass in succes- 

 sion close to the poles of a horse-shoe magnet. The direction of the 

 current varies with the motion, and with the character of the influ- 

 encing pole, the result being that the currents in the two semicircles 

 of the coil surrounding the ring flow in opposite directions. But it 

 is easy by a suitable mechanical arrangement to conduct them away 

 from the places where they meet, and to 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 alter- 

 nating currents. Such alternating 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 coj)per wire. The wheel is 

 caused to rotate between the poles of powerful electro-magnets. On 

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

 instantly evokes in the surrounding coil an induced current of 

 corresponding strength. The currents excited in approaching and 

 retreating, and in passing different poles flow in opposite direc- 

 tions, but by means of a commutator these conflicting electric 



* ' Comptcs Eeiidus,' 1871, p. 176. See also Gaugain on the Gramme 

 machine, 'Ann. de Chem. ot do Phys.,' vol, xxviii. p, 324. 



