A XJXJLIA R Y BR USH BEG ULA TION.

439

the electric devices that may be employed for automaticallyadjusting the brushes, and Fig. 258 is a diagram illustrating thepositions of tlie brushes when the machine is being energized atthe start.

a and b are the positive and negative brushes of the main orworking-circuit, and c the auxiliary brush. The working-circuitn extends from the brushes a and b, as usual, and contains elec-tric lamps or other devices, r/, either in series or in multiplearc.

m m' represent the iield-helices, the ends of which are con-nected to the main brushes a and b. The branch or shunt wirec' extends from the auxiliary brush c to the circuit of the field-helices, and is connected to the same at an intermediate point, x.

h represents the commutator, with the plates of ordinary con-

Fig. 253.

struction. When the auxiliary brush <; occupies such a positionupon the commutator that the electro-motive force between thebrushes a and c is to the electro-motive force between the brushesc and b as the resistance of the circuit a m e' c a is to the resistanceof the circuit b W o' o b, the potentials of the points x and y willbe equal, and no current will flow over the auxiliary brush; butwhen the brush c occupies a different position the potentials ofthe points x and y will be different, and a current will flow overthe auxiliary brush to and from the commutator, according to therelative position of the brushes. If, for instance, the commu-tator-space between the brushes a and o, when the latter is at theneutral point, is diminished, a current will flow from the point yover the shunt c to the brush b, thus strengthening the currentin the part m', and partly neutralizing the current in part m ; butif the space between the brushes a and c is increased, the cur-