It is now to be understood that when the auxiliary brush c occupies such a position upon the commutator that the electro-motive force between the brushes c and b as the resistance of the circuit a, M, c-1, c, A, to the resistance of the circuit b, M-1, c-1, c, B, the potentials of the points X and Y will be equal and no current will flow over the auxiliary brush, but when the brush c occupies a different position, the potentials of the points X and Y will be different and a current will flow over the auxiliary brush to or from the commutator, according to the relative position of the brushes. If for instance the commutator space between the brushes a and c, when the latter is at the neutral point, is diminished, a current will flow from the point Y over the shunt c. to the brush b thus strengthening the current in the part M-1, and partly neutralising the current in the part M; but if the space between the brushes a and c is increased, the current will flow over the auxiliary brush in an opposite direction and the current in M will be strengthened and in M-1 partly neutralised. By combining with the brushes a, b, and c any known automatic regulating mechanism, the current developed can be regulated in proportion to the demands in the working circuit.
The parts M and M-1 of the field wire may be wound in the same direction, (in this case they are arranged as shown in Fig. 1 or the part M may be wound in the opposite direction as shown in Figs. 2 and 3.)
