The coil of wire temporarily magnetizes the iron core; it can permanently magnetize a piece of steel used as a core. (See "Study," Chapter XXII., for experiments.)
Fig. 95.
97. Forms of Electromagnets. Fig. 95 shows a straight, or bar electromagnet. Fig. 96 shows a simple form of horseshoe electromagnet. As this form is not easily wound, the coils are generally wound on two separate cores which are then joined by a yoke. The yoke merely takes the place of the curved part shown in Fig. 96. In Fig. 97 is shown the ordinary form of horseshoe electromagnet used for all sorts of electrical instruments. (See "Apparatus Book," Chapter IX., for home-made electromagnets.)
98. Yokes and Armatures. In the horseshoe magnet 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, instead of going through the air. This reduces the resistance to them. If we had no yoke we should simply have two straight electromagnets, and the resistance to the lines of force would be so great that the total strength would be much reduced. Yokes are made of soft iron, as well as the cores and armature. The armature, as with permanent horseshoe magnets, is strongly drawn toward the poles. As soon as the current ceases to flow, the attraction also ceases.
| Fig. 96 | Fig. 97. |
Fig. 98.
Beautiful magnetic figures can be made with horseshoe magnets. Fig. 98 shows that the coils must be joined so that the current can pass around the cores in opposite directions to make unlike poles. (See "Study," Exp. 164 to 173.)