ut stout thread will do. Wind 3 or 5
layers of No. 24 insulated copper wire upon the soft iron core. This is
useful for simple experiments, and this idea may be applied to magnets
to be used in pieces of apparatus. Hold the bundle of wires in a vise,
and file the ends smooth, before winding on the wire. Paraffine should
be used to hold the turns of insulated wire together.

[Illustration: Fig. 55.]

[Illustration: Fig. 56.]

[Illustration: Fig. 57.]

APPARATUS 83.

_112. Electro-Magnet._ Fig. 57. An electro-magnet with a removable core
may be made by winding the wire on a spool. The core is made, as in
App. 82, of soft iron wires, bound together with stout thread. A bolt
may be used instead of the wire, but the wire loses its magnetism much
quicker than a soft steel bolt would. (Study residual magnetism.) This
magnet is strong enough for many purposes, but the wire is too far from
the core, on account of the thickness of the wood, to make it efficient.
The wire may be wound on by hand, but a winder (App. 93) will do much
better and quicker work.

APPARATUS 84.

[Illustration: Fig. 58.]

[Illustration: Fig. 59.]

_113. Horseshoe Electro-Magnet._ Fig. 58. Bend soft iron wires, and make
a bundle of them. If you wish to wind the wire around spools, the bundle
cannot be very large. It will be found best to make the bundle about 3/8
in. in diameter, and not to use the spools. Strong paper should be
wrapped once or twice around the legs of the horseshoe, and the
insulated wire, say 4 layers, can then be wound directly upon this. (See
Sec. 115 for method of making connection between the coils.) It is a
little troublesome to wind wire upon a horseshoe like this, and for
App. 85. Spools are handier, because each can be wound separately, and
then be slipped in place. The ends of the horseshoe should be filed
smooth.

APPARATUS 85.

_114. Electro-Magnet._ Fig. 59. An ordinary iron staple is useful as the
core of a small magnet. One like this is shown also in Fig. 94, used as
a telegraph sounder. It takes some time to wind 4 layers of wire on to
each leg of the staple, so be sure to see Sec. 115 about the method of
winding. In Fig. 59 the half-hitches (Sec. 110) are not shown. Coat the
finished coils with paraffine.

115. Method of Joining Coils. Fig. 60. If A and B represent the two
cores of a horseshoe electro-magnet, the coils must be joined in such a
manner that the current will pass around them in opposit