Brush Holders.
The adjustable (generally) clutch or clamps for holding the commutator
brushes of a dynamo, which keep them in contact with the commutator, and
admit of adjustment by shifting backward and forward of the brushes to
compensate for wear. They are connected to and form part of the rocker,
q. v. By rotating the latter the brush-holders and brushes are carried
in one direction or other around the commutator, so as to vary the lead
as required.
Brush, Pilot.
A third brush, used for application to different parts of a revolving
armature commutator to determine the distribution of potential
difference between its different members. (See Curve of Distribution of
Potential in Armature.) One terminal of a volt-meter is connected to one
of the regular brushes, A, of a dynamo; the other to a third brush, p,
which is pressed against different portions of the commutator of the
dynamo. The readings of the volt-meter are plotted in a curve of
distribution of potential.
Fig. 69. PILOT BRUSH.
Brush, Rotating.
Brushes for taking off the current from dynamo commutators, or giving
current connection to motors, whose ends are in the form of rollers
which rotate like little wheels, and press against the commutator
surface.
Brush, Third.
A third brush is sometimes provided in a dynamo for regulating purposes.
Applied to a series machine it adjoins one of the regular brushes and
delivers its current to a resistance, to whose further end the regular
circuit is connected. By a sliding connection the resistance is divided
between the third brush circuit and the regular circuit, and by varying
the position of this contact regulation is obtained.
It is to be distinguished from the pilot brush used for determining the
characteristic of the commutator, although based on the same general
principles.
Fig. 70. THIRD BRUSH REGULATION.