nches from the bolt. As soon as you reach the left side or head end of
the bolt, feed the wire towards the right. If at any time the layers
become rough on account of one turn slipping down between turns of the
previous layer, fasten a piece of paraffine paper around the coil as
soon as the imperfect layer is completed. Wind on 8 layers, and count
the number of turns in one or two of them, so that you can tell about
how many turns in all you have around the core. Make a "half-hitch" (see
Sec. 110) with the wire when the last layer is finished, to keep it from
unwinding, and leave a 6 in. end.

The coil should be protected by fastening around it a piece of
dark-colored stiff paper. Paraffine paper is good for this purpose. With
a little practice you will be able to rapidly and neatly wind on the
wire. The winder-nut, W N, must hold the bolt solidly to keep it from
wobbling.

123. We shall call the starting end of the wire which passes through P
H, the inside end, and the end of the last layer the outside end. This
can pass out between the washer and the paper covering.

APPARATUS 89.

_124. Experimental Horseshoe Electro-Magnet._ Fig. 66. Among the most
useful pieces of apparatus for home use, is a good horseshoe
electro-magnet. Fig. 66 shows a very convenient and practical form. With
this, alone, can be shown all the principles of telegraph sounders,
electric bells, etc. They are excellent for making magnetic figures (See
text-book). You are supposed to be looking down on the App. in Fig. 66.
The bolts are 2 in. apart center to center.

[Illustration: Fig. 66.]

The bolt magnets are fully described in App. 88; the binding-posts, as
App. 46; the yoke, as App. 71; the method of fastening to the base, as
App. 90; the base is 5 x 4 x 7/8 in.; the magnets are made of 5/16
bolts, 2-1/2 in. long.

125. To Join the Coils, fasten the two inside ends (Sec.123) of the wire
to a middle binding-post, and carry the outside ends to the two outside
binding-posts. In this way you can use either magnet alone, if desired
(See experiments in text-book), or change the polarity at will by
changing the connections. (See Sec. 115 and 123.)

APPARATUS 90.

_126. Fastenings for Electro-Magnet._ Fig. 67. When both electro-magnets
are to be permanently fastened to a base, especially if tin yokes are to
be used, as in App. 89, it is best to use a nut on each side of the
yoke. It is important to have a perfectly tight connection betwe