e needles, as shown. Be sure
to place the poles of the little magnets as in the Fig. This combination
may be supported by a fine thread. It is used for Astatic Detectors.
(See Index.)

CHAPTER VIII.

YOKES AND ARMATURES.

_94. Yokes_ are used to fasten two straight electro-magnets together to
form a horseshoe electro-magnet. The reasons for using them should be
understood. Soft iron should be used for yokes and armatures, as this is
the best conductor of lines of magnetic force. Sheet-tin is made of thin
iron, which is coated with tin. (Try a magnet upon a tin can.) This soft
iron is very easily handled, bent, and punched, and is very useful for
many purposes. The tin from old tomato cans, cracker boxes, etc., is
just as good as any. The method of making your yokes will depend
entirely upon the tools at your command. Several ways are given. Y, Fig.
47, shows the position of the yoke.

APPARATUS 70.

_95. Yoke._ For the experimental magnets (App. 89) a fairly large yoke
is required in order to have the magnets far enough apart. If you have
only a nail punch (App. 26) with which to make holes in tin, you will be
obliged to punch but one thickness at a time. (See method of punching
sheet-metal, App. 26.) Cut 5 or 6 pieces of the tin, 3-1/4 x 1 in. With
a center punch (tools) or sharp-pointed nail make small dents (2 in.
apart) in each piece to mark the places where the holes are to be
punched. Punch 5/16 in. holes in each piece. If you do this carefully,
the holes in the different pieces will match, and the bolts can be
pushed or screwed into these. When screwing in the bolt magnets turn
them by their heads; do not pinch the coils, as this loosens the wire.

If you have a good punch, it is better to make the yoke as in App. 27,
instead of using separate pieces of tin.

APPARATUS 71.

_96. Yoke._ Fig. 45 and 46. Cut a strip of tin 6 in. long by 3-1/4 in.
wide. Bend one end of it so that it will lap over 3/4 in. (Fig. 46);
hammer it down gently, then bend this over and over until the whole tin
is used. The final result will be a flat roll, 3-1/4 by about 1 in. This
should be hammered flat.

[Illustration: Fig. 45.]

[Illustration: Fig. 46.]

If you have the tools it is easy to drill two 5/16 holes in this strip.
They should be 2 in. apart; that is, 2 in. from the center of one to
that of the other. Start the holes with a center punch.

_97._ If you have no way of drilling the holes, they must be p