POLYPHASE CURRENTS.

35

thereon, take off the alternating currents that operate the motor.It is preferable in this case to employ a motor or transformerwith three energizing coils, a" b" c", placed symmetrically withthose of the generator, and the circuits from the latter are con-nected to the terminals of such coils either directly—as whenthey are stationary—or by means of brushes e and contact ringse. In this, as in the other cases, the ordinary commutator maybe used on the generator, and the current taken from it utilizedfor exciting the generator field-magnets or for other purposes.