EXPERIMENT 181. To study the effect of starting or stopping a current in a coil placed inside of another coil.
437. Directions. (A) Arrange as in [Fig. 141]. Join coil H with the astatic galvanoscope, A G. Place the small coil P (No. 98) with core, inside of H, and connect the ends of P with the key and cell, as shown.
(B) Close the circuit at K; watch the needle, and as soon as it regains its position, open the circuit again.
Compare the direction of the induced current in H with that of the inducing current in P, (1) when the inducing circuit is closed, and (2) when it is broken. (See Induction Coils.)
438. Discussion of Exps. 180, 181. When a current suddenly begins to flow through a coil, the effect upon a neighboring coil is the same as that produced by suddenly bringing a magnet near it; and when the current stops, the opposite effect is produced.
We may consider that when the inducing circuit is[184] closed, the lines of force shoot out through the turns of the outside coil. Upon opening the circuit the lines of force cease to exist; that is, we may imagine them drawn in again.
439. Direction of Induced Current. [Fig. 142] shows the magnet on its way into the coil; the number of lines of force is increasing in the coil, and the induced current passes in an anti-clockwise direction when looking down into the coil along the lines of force. This produces an indirect current. If a current from a cell were passed through the coil in the direction of this indirect current, the lower end of a bar of iron would become a S pole. (See [§ 428].)