MA GNETO-ELECTRIC ILL UMINA TION. 



589 



the place where the equal and opposed currents generated in the 

 upper and lower halves of the ring neutralize each other. Conse- 

 quently the currents are collected and flow together along conducting 

 wires, which are fastened to the friction-pieces F. 



The perfect continuity of the current so obtained is secured by 

 causing the friction-pieces F to touch simultaneously several of the 

 radial conductors R ; consequently the metallic circuit is never broken. 



Fig. 5. 



Fig. 6. 



The effects produced by these machines vary with the rapidity of 

 rotation. Experience shows that the electro-motive force is sensibly 

 in proportion to the velocity ; but it is probable that this force tends 

 toward a limit, corresponding to a particular velocity, beyond which 

 the electromotive force would remain constant, or even diminish. 

 Moreover, the electromotive force is greater in proportion to the num- 



Fig. 7. 



ber of coils encircling the iron ring, but the relation between these 

 two quantities has not yet been determined. The theoretical resistance 

 of the machine should be one-fourth of the whole resistance of the 

 wire wound round the ring armature ; but the actual resistance is not 

 so great, since each friction-disk always touches several radii, R, and 



