405. Horseshoe Electromagnets. [Fig. 128] shows a simple form of electromagnet with two coils which have a bent piece of iron as a core for both. The coils have to be wound on by hand in this form. As this is troublesome, the coils are generally wound on two separate cores which are joined by a yoke ([§ 406]), which takes the place of the curved part in [Fig. 128]. The separate coils can be quickly made with a "winder" and joined to suit. (See Apparatus Book, Chapter IX, for Home-made Electromagnets.) [Fig. 129] shows a top view[169] of a home-made experimental horseshoe electromagnet. The coils are joined by an iron strap, called the yoke, which is screwed to a wooden base. A strip of iron placed above the magnets to be attracted by them, when the current passes, is called the armature. (See Telegraph Sounders.)
406. Use of Yoke. It has been explained ([§ 82]) why horseshoe magnets are, in general, better than straight ones. The same is true of electromagnets; there are two poles to attract, and two to induce. The lines of force pass through the yoke on their way from one core to the other, and this reduces the resistance to them. The strength of the horseshoe magnet would be greatly reduced if the lines of force were obliged to pass through two air spaces instead of one; in fact, if there were no yoke we should have simply two straight magnets. The yoke should be made of soft iron.
407. Experimental Magnets are quickly joined to a tin base (No. 94), which has 3 holes punched in, through which screws can be put to hold the cores in place. [Fig. 127] shows plan of tin. [Fig. 130] shows how removable cores are fastened to the base, the coils being on the spools, and [Fig. 131] shows how home-made coils on bolts can be used. The coils on bolts should be wound as directed in Apparatus Book, Chapter X. The tin base also serves as the yoke.
Removable Cores. [Fig. 130]. These are of soft iron (No. 92, 93). In one end of each is a hole for the screws, S. Part of the tin has been cut away in the Fig. The copper washer, C W, should be used. (See [§ 408].) Connectors are fastened to the ends of the coils ([§ 226]–[230]).