A UXILIAR Y BR USII REG ULA TION.

441

tion to an electric current. In Figs. 256 and 257 tlie movingcores are sliown as convenient devices for obtaining the requiredextent of motion with very slight changes in the current passingthrough the helices. It is understood that the adjustment ofthe main brushes causes variations in the strength of the currentindependently of the relative position of those brushes to theauxiliary brush. In all cases the adjustment should be such thatno current flows over the auxiliary brush when the dynamo isrunning 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 movablein arcs concentric with the centre of the commutator-shaft. Aniron piston, p, of the solenoid s, Fig. 256, is attached to the aux-iliary-brush holder c. The adjustment is effected by means of aspring and screw or tightener.

In Fig. 257 instead of a solenoid, an iron tube inclosing a coilis shown. The piston of the coil is attached to both brush-holders a a and c. When the brushes are moved directly byelectrical devices, as shown in Figs. 256 and 257, these are soconstructed that the force exerted for adjusting is practicallyuniform through the whole length of motion.

It is true that auxiliary brushes have been used in connectionwith the helices of the field-wire; but in these instances the

Fig. 255.

helices receive the entire current through the auxiliary brush orbrushes, and these brushes could not be taken off without break-ing the circuit through the field. These brushes cause, move-over, heavy sparking at the commutator. In the presentcase the auxiliary brush causes very little or no sparking, andcan be taken off without breaking the circuit through the field-