o E. S is a small screw that holds E in place, if the
paper does not hold it tight enough.

[Illustration: Fig. 121.]

[Illustration: Fig. 122.]

Fig. 122 shows a section of the spool and tin sections with the brushes
pressing against them. The sections do not touch each other, and the
brushes touch opposite sections. It is evident, then, that the current
must pass through the coils 12 and 16 in order to get from one section
of the commutator to the other, provided you have no short circuits
through the shaft or elsewhere. The slots in the commutator must be
directly under the center line of the yoke, 14, as seen in Fig. 121.

267. The brushes, 9 and 19, Fig. 120, are made of very thin tin or
copper. They are cut to the shape shown, the narrow part being about 1/8
in. wide, and long enough to reach at least to the center-line of the
apparatus. The foot, or bottom part of the brushes, should be about
1-1/4 x 3/4 in. These are used to fasten them to the base and to make
connections. If you have no thin metal for brushes, use copper wires,
and arrange them so that they will press gently against the commutator.

268. Connections. The inside ends (Sec. 123) of the field-magnets are
held at 4. The outside end of coil 2 is joined to X, and that of coil
6 to 8, the foot of the brush which presses against 10. The section,
10, of the commutator is joined to 11, the outside end of coil 12, its
inside end being fastened to the inside end of coil, 16, either by
twisting them together, or by fastening them under the top nuts of the
armature yoke, 14. The outside end of coil 16 is joined to the other
commutator section, 18. The brush, 19, completes the circuit. In the
foot of 19 is the binding-post, Y.

If the current enters at X, it will pass through 1, 2, 3, 4, 5, 6, 7, 8,
9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, and out at Y, provided 10 and
18 are in contact with 9 and 19. Be careful not to have any short
circuits. If, for example, the wire 7 touches 4, or if 3 touches 8, or
if the wires 11 and 17 touch the shaft, your current will not pass where
you expect, and you will have trouble.

269. Adjustment. The armature cores should just clear the poles of the
field-magnets as they turn. This must be regulated by the thickness of A
and the position of the nuts on the shaft, S B. The slots in the
commutator must be under the center of the yoke, 14. The brushes, 9 and
19, must touch 10 and 18, but not so hard that they will stop