(E) Repeat (C) and (D), moving magnet slowly.
How does the strength of the current compare with that of [Exp. 175]? Are lines of force made to cut the turns of the coil?
EXPERIMENT 177. To find whether a current can be generated with a horseshoe magnet and a coil of wire having an iron core.
431. Directions. (A) Arrange the apparatus as in [Exp. 176], but use the horseshoe magnet, H M, instead of the bar magnet. [Fig. 137] shows the coil (No. 90) with one pole of H M held over the core.
(B) Study the effect of quickly lowering and raising first one pole and then the other over the core, as with the bar magnet. Get clearly in mind the direction in which the induced current flows in each case.
432. Induced Currents and Lines of Force. In the experiments just given, it should be remembered that the permanent magnets are sending out thousands of lines of force from their N poles, and receiving them again at their S poles. As the magnet is pushed into the coil ([Exp. 175]), the lines of force not only cut through the turns of the coil, but the number of lines of force that cut the coil at any instant varies rapidly as the magnet is moved.
Motion is necessary, with this arrangement, to make a change in the number of cutting lines of force. The current passes only while the magnet moves; and the direction of the current at any moment depends upon whether the number of lines of force is increasing or decreasing at that moment. (See [§ 438], 439.)