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.
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 × 4 × ⅞ in.; the magnets are made of 5⁄16 bolts, 2½ 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 3¼ in. long, and place the magnets 2 in. apart center to center.
