the motor.
Wire brushes are more easily adjusted than tin or sheet-copper ones. The
tin arm, T, must hold the shaft properly. The point of the shaft must
allow it to turn easily. The motor will turn clockwise if the
attachments are made as shown. Use 1 or 2 good bichromate cells, like
App. 3 or 4.

270. Operation. The current will pass through the field-coils in the
same direction, as long as the battery wires are not changed. The
current is reversed in the armature-coils every time the brushes change
from one section to the other of the commutator; that is, it flows in
one direction during one-half of a revolution, and in the opposite
direction during the other half. This reverses the poles of the
armature-magnets every 1/2 revolution. (See text-book for full
explanations and for simple experiments with electric motors.)

APPARATUS 147.

_271. Attachment for Winder._ In winding small electro-magnets for
armature, etc., in which cores are used that are not 5/16 in. in
diameter, your winder will have to be slightly changed. Its 5/16
stove-bolt will have to be removed, and a 1/4 in. one put in instead.
This may be done by making a handle for the 1/4 in. bolt. To keep this
from wobbling in the 5/16 hole, wind stiff paper around the bolt until
it fits quite tightly. The whole winder is explained as App. 93.

CHAPTER XX.

ODDS AND ENDS.

APPARATUS 148.

_272. Graduated Circles._ Fig. 123. For compasses (App. 67), and for use
in connection with tangent galvanometers (App. 116), a graduated circle
is necessary. Fig. 123 is a reduced drawing from an original that is 4
in. in diameter. The long lines are 10 degrees apart, the smallest
divisions shown being 5 degrees apart. Single degrees can be determined
with considerable accuracy with the eye.

[Illustration: Fig. 123.]

To divide the circle. Divide the circumference into 4 equal parts; these
will be 90 degrees from each other, there being 360 degrees in every
circle. Divide each quarter into nine equal parts with a pair of
dividers; these will be for the long lines, 10 degrees apart. Divide
each of these into two equal parts. If you are used to drawing, you can
divide the circle still more, but 5-degree divisions will do.

APPARATUS 149.

_273. Adjustable Table._ Fig. 124. A table that can be raised or lowered
is useful. The one shown at D, Fig. 124, is used for the galvanometer of
App. 117. The dimensions are given in the figure. The upright piece