e base, where it is carried
up to a screw, 12, which holds it to U. Its bare end reaches out to
gently scrape against the commutator, 9, when it swings around. This
wire, 10, should not press against 9 during the entire revolution.

257. Adjustment. Suppose the current enters at X. When the "brush," 10,
presses against the commutator, 9, the current passes through X, 1, 2,
3, 4, 5, 6, 7, down 8 to 9, and out through 10 to Y. (The current, of
course, goes down into D and into the bolt-cores also; but it can go no
farther, if the coils are properly insulated, and A is not allowed to
touch the cores. It is better to have the end of the shaft rest upon a
piece of glass, having a slight depression made with a file, or in a
dent made in tin which rests upon wood, the tin having no connection
with D.) If 10 should continue to press against 9, the current would
continue to pass, and A would be held firmly in place, directly over 2
and 6, and, of course, the shaft could not revolve. If, however, the
brush leaves 9 (See plan of 9 at side of Fig. 117), just as A gets over
the coils, or an instant before it gets there, the weight of A will
carry it beyond the coils. No current should pass again, until A is at
least at right angles to a line drawn through the center of the coils.
If the current again passes, the ends of A will be attracted by the
bolt-cores.

In other words, the current should pass a little less than one-half of
the time, and this is divided into two parts. Suppose you start A with
your finger; the current should be shut off automatically just before
the center of A gets over the center of the bolt-cores. A makes 1/4 of a
revolution without current, and just after it gets beyond this, the
current passes for nearly 1/4 of a revolution, which brings the ends
over the poles again. The next 1/4 of a turn it has no current, because
the flat side of 9 is opposite the brush, 10, as during the first 1/4.
The last 1/4 the current passes again. The exact position of the
commutator will depend upon the way you arrange the brush. The positions
of 9 and 10 can be found by trial, so that the circuit will be promptly
opened and closed at the proper moment. Start the motor by turning the
armature.

258. Batteries. The amount of power needed will depend upon how well you
make the motor. One cell of App. 3 or 4 will run a well made one, but it
is better to use 2 cells. Join the wires to X and Y.

[Illustration: Fig. 118.]

AP