(C) Swing the suspended coil back and forth like a pendulum for a minute, until you get in mind the rapidity of its vibrations. Stop it, then repeat (B), closing and opening the circuit at regular intervals, so that the little impulses given by the attraction for I C will gradually cause H to vibrate. The wires leading from H should not drag upon the table. =EXPERIMENT 184. Motion with hollow coil and bar magnet.= =447. Directions.= (A) Substitute the bar magnet M (No. 97) for the iron of Exp. 183 (Fig. 144). Get clearly in mind the polarity of the coil from the way the current flows through it, then test it with the compass to find whether you are right. (B) Hold the N pole of M near the left-hand end of the coil, close the circuit for an instant and study results. (C) Reverse the magnet and repeat (B). Compare the results with those of Exp. 183. Try to make the coil vibrate. =EXPERIMENT 185. Motion with electromagnet and piece of iron.= =448. Directions.= (A) Arrange as described in Exp. 183, Fig. 144. Place a short core inside of the coil and repeat. (See § 446 for directions.) Why is the motion produced much larger than that given by a hollow coil? (B) The coil can gradually be made to swing through quite a little space by closing and opening the circuit regularly (§ 446, C). Could any use be made of such a motion, if it were on a large scale? Could it be made to run a machine? [Illustration: Fig. 145.] =EXPERIMENT 186. Motion with electromagnet and bar magnet.= =449. Directions.= (A) Arrange as in Fig. 145, the coil being suspended and connected as in Exp. 183 (Fig. 144). (B) Study the effect of closing the circuit when the N pole of M is held near the core of H. Reverse M, and repeat. [Illustration: Fig. 146.] =EXPERIMENT 187. Motion with electromagnet and horseshoe magnet.= =450. Directions.= (A) Arrange as in Fig. 146. The ends of H (No. 89) are joined to X and Y of the current reverser C R (No. 57). It is evident, then, that the direction of the current through H can be easily and rapidly reversed by C R. (See Exp. 103.) Either pole of the horseshoe magnet H M will attract I C when it is not magnetized. (B) Place the end of I C near the N pole of H M so that it will be attracted to it. You have learned that like poles repel each other, so press the lever of C R that will produce a N pole at the left-hand end of I C. The core I C should be repelled by the N pole of H M and be instantly attracted by its S pole. (C) Rapidly reverse the current and make I C jump back and forth from one pole to the other. The results of this experiment should be remembered, as they will aid in understanding motors. A core 1/4 in. in diameter can be placed in between the poles and be made to vibrate rapidly as the current is reversed.