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.
_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.
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 × 4 × 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 (§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 § 115 and 123.)
_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 between bolt and yoke. Several ways of fastening the bolts and yokes are shown; but it will be found best to cut holes in the base for the lower nuts, and to screw the yoke directly to the base. This makes a solid and pleasing arrangement. For the experimental magnets (App. 89) make the yoke 3-1/4 in. long, and place the magnets 2 in. apart center to center.
_127. Winder._ Fig. 68. In case you do not have any means of making a smooth hole for the "bearings" of the winders of App. 93 and 94, you can use a spool for the purpose. B is the end of a piece of board about 1 in. thick, 3 in. wide, and 6 in. long. The spool, A, is laid upon this, a band of tin, T, being used to hold it down firmly upon the end of B. Screws, S, hold T down. A stove-bolt axle (See App. 93) is shown, and by using a nut, as explained, bolt magnets may be wound. By using the handle of App. 92, this arrangement can be used to wind almost anything, when used together with the attachment of App. 95.
