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? [Illustration: Fig. 137.] =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.) [Illustration: Fig. 138.] =EXPERIMENT 178. To find whether a current can be generated with an electromagnet and a hollow coil of wire.= =433. Directions.= (A) The hollow coil (No. 90) should be joined to the astatic galvanoscope, as shown in Fig. 136. Instead of the bar magnet in Fig. 136, an electromagnet is to be used, and this should be joined in series with a cell and key, as shown in Fig. 138. The current from the cell will pass only when K is pressed. (B) Note from the winding which way the current must pass around the coil when the circuit is closed at K, and determine whether the lower end of the long iron core, L I C (No. 96) should be N or S. With the compass test the poles of the core to be sure you are right. (C) Quickly lower the end of L I C into the hollow coil (H, Fig. 136), the circuit being kept closed long enough to allow the needle to partially come to rest again. Withdraw L I C before you open the circuit. Explain action of needle. (D) Reverse the direction of the current through the electromagnet, by changing the connections, and repeat (C). Does any induced current pass through A G when the core is held still in the coil H, even though a current passes through coil E? [Illustration: Fig. 139.] =EXPERIMENT 179. To find whether a current can be generated with an electromagnet and a coil of wire having an iron core.=