H represents the commutator, with the plates of ordinary construction. When the auxiliary brush c occupies such a position upon the commutator that the electro-motive force between the brushes a and c is to the electro-motive force between the brushes c and b as the resistance of the circuit a M c' c A is to the resistance of the circuit b M' c' 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 and 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', and partly neutralizing the current in 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', partly neutralized.
By combining with the brushes a, b, and c any usual automatic regulating mechanism, the current developed can be regulated in proportion to the demands in the working circuit. The parts M and M' of the field wire may be wound in the same direction. In this case they are arranged as shown in Fig. 253; or the part M may be wound in the opposite direction, as shown in Figs. 254 and 255.
Fig. 254.
It will be apparent that the respective cores of the field-magnets are subjected to neutralizing or intensifying effects of the current in the shunt through c', and the magnetism of the cores will be partially neutralized, or the points of greatest magnetism shifted, so that it will be more or less remote from or approaching to the armature, and hence the aggregate energizing actions of the field magnets on the armature will be correspondingly varied.
In the form indicated in Fig. 253 the regulation is effected by shifting the point of greatest magnetism, and in Figs. 254 and 255 the same effect is produced by the action of the current in the shunt passing through the neutralizing helix.
The relative positions of the respective brushes may be varied by moving the auxiliary brush, or the brush c may remain stationary and the core P be connected to the main-brush holder A, so as to adjust the brushes a b in their relation to the brush c. If, however, an adjustment is applied to all the brushes, as seen in Fig. 257, the solenoid should be connected to both a and c, so as to move them toward or away from each other.
There are several known devices for giving motion in proportion to an electric current. In Figs. 256 and 257 the moving cores are shown as convenient devices for obtaining the required extent of motion with very slight changes in the current passing through the helices. It is understood that the adjustment of the main brushes causes variations in the strength of the current independently of the relative position of those brushes to the auxiliary brush. In all cases the adjustment should be such that no current flows over the auxiliary brush when the dynamo is running with its normal load.
In Figs. 256 and 257 A A indicate the main-brush holder, carrying the main brushes, and C the auxiliary-brush holder, carrying the auxiliary brush. These brush-holders are movable in arcs concentric with the centre of the commutator-shaft. An iron piston, P, of the solenoid S, Fig. 256, is attached to the auxiliary-brush holder C. The adjustment is effected by means of a spring and screw or tightener.
In Fig. 257 instead of a solenoid, an iron tube inclosing a coil is shown. The piston of the coil is attached to both brush-holders A A and C. When the brushes are moved directly by electrical devices, as shown in Figs. 256 and 257, these are so constructed that the force exerted for adjusting is practically uniform through the whole length of motion.