(D) After the magnet, M, has been placed in the coil, as in (C), and the needle has come to rest, quickly pull M from the coil, watching the needle. If a current is produced, does it pass from the coil in the same direction as before, in (C)?
(E) Turn M end for end, repeat (C) and (D), and study the results. Are lines of force made to cut the turns of the coil?
(F) Repeat (C) and (D), moving M slowly.
428. Discussion. An induced current, produced as in the above experiment, is a momentary one. No current passes when the magnet and coil are still; at least one of them has to be in motion. When the magnet is inserted, the induced current is said to be an inverse one, as it passes in a direction opposite to that which would be necessary to give the magnet its poles, it being considered[180] a core magnetized by the current. A direct current is produced when the magnet is withdrawn from the coil. Rapid movements produce stronger currents than slow ones. (See [§ 439].)
429. Induced Currents and Work. It takes force to move a magnet through the center of a coil, and it is this work that is the source of the induced current. When the coil is pushed on to the magnet, or when it is moved through a magnetic field, force is also required. We have, in this simple experiment, the key to the action of the dynamo and other important electrical machines. These will be discussed later.
EXPERIMENT 176. To find whether a current can be generated with a bar magnet and a coil of wire having an iron core.
430. Directions. (A) Arrange as in [Exp. 175], [Fig. 136], and, in addition, place an iron core (No. 92) inside of the coil (No. 90).
(B) Hold the bar magnet (No. 97) as in [Fig. 136], and quickly lower it until it touches the core, at the same time watching the needle. Study results, direction of current, etc., as before.
(C) Suddenly withdraw M from the core. Is the current produced in the same direction as that from (B)?
(D) Turn M end for end and repeat (B) and (C).