8o ELECTRIC LIGHTING. 



by their similar poles. There results in the middle of the 

 system two consequent poles of great power, and it is 

 between these poles, prolonged and curved, that the ring is 

 enclosed, with its axle carrying the current-collectors as seen 

 in Fig. 19. 



In the first Gramme machines two rings were used, one 

 being reserved, as in Ladd's system, to excite the electro- 

 magnet. But, as we have already said, this ring had soon 

 to be suppressed, and the induced current in its entirety 



FIG. 21. 



was made to pass through the electro-magnetic system, by 

 which the effects were considerably increased. Henceforth 

 the machines had but a single ring, but it was made of a 

 large size, and, as shown in Fig. 21, the ring itself, instead of 

 being made of a solid piece of iron, was formed by a bundle 

 of many iron wires, as in the voltaic induction machines. 



Fig. 21 represents a section of this ring, and shows the 

 way in which the different sections of the induced coil are 

 wound upon it. The connecting wires a a a of the various 

 sections are, as shown in the upper part of the figure, sol- 

 dered to the plates R R, which are placed side by side round 

 the axle of rotation, and constitute a kind of drum on which 



