are formed
through this bar, to receive round-headed wood screws (16), these screws
being 3 inches long, so they will pass through the wooden pieces (I)
and enter the base (J). Midway between the ends, each bar (15) has an
iron bearing block (17), 3/4" x 1/2" and 1-1/2 inches high, the 1/4-inch
hole for the journal (7) being midway between its ends.

COMMUTATOR BRUSHES.--Fig. 28 shows the base, armature and commutator
assembled in position, and to these parts have been added the commutator
brushes. The brush holder (18) is a horizontal bar made of hard rubber
loosely mounted upon the journal pin (7), which is 2-1/2 inches long. At
each end is a right-angled metal arm (19) secured to the bar (18) by
screws (20). To these arms the brushes (21) are attached, so that their
spring ends engage with the commutator (12). An adjusting screw (22) in
the bearing post (17), with the head thereof bearing against the
brush-holder (18), serves as a means for revolubly adjusting the brushes
with relation to the commutator.

DYNAMO WINDINGS.--There are several ways to wind the dynamos. These
can be shown better by the following diagrams (Figs. 30, 31, 32, 33):

THE FIELD.--If the field (A, Fig. 30) is not a permanent magnet, it must
be excited by a cell or battery, and the wires (B, B') are connected up
with a battery, while the wires (C, C') may be connected up to run a
motor. This would, therefore, be what is called a "separately excited"
dynamo. In this case the battery excites the field and the armature
(D), cutting the lines of force at the pole pieces (E), so that the
armature gathers the current for the wires (C, C').

[Illustration: _Fig. 30._ FIELD WINDING]

[Illustration: _Fig. 31._ SERIES-WOUND]

SERIES-WOUND FIELD.--Fig. 31 shows a "series-wound" dynamo. The wires of
the fields (A) are connected up in series with the brushes of the
armature (D), and the wires (G, G') are led out and connected up with a
lamp, motor or other mechanism. In this case, as well as in Figs. 32 and
33, both the field and the armature are made of soft gray iron. With
this winding and means of connecting the wires, the field is constantly
excited by the current passing through the wires.

SHUNT-WOUND FIELD.--Fig. 32 represents what is known as a "shunt-wound"
dynamo. Here the field wires (H, H) connect with the opposite brushes
of the armature, and the wires (I, I') are also connected with the
brushes, these two wires being provided to perform