the place where the equal and opposed currents generated in the upper and lower halves of the ring neutralize each other. Consequently 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.
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 number
Fig. 7.
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
