=434. Directions.= (A) Fig. 139 shows simply the arrangement of coils. Coil H (No. 90) with core, is joined to the galvanoscope as in Fig. 136. Coil E, with short core, should be joined to key and cell as shown in Fig. 138. (B) Keeping in mind the polarity of the lower end of core E, quickly lower it to the core of H, the circuit being kept closed for a few seconds. Does the needle remain deflected after the motion ceases? (C) Quickly raise E, the circuit being still closed, then open the circuit. Compare the directions taken by the induced currents in (B) and (C). _=435. Discussion of Exps. 178, 179.=_ This motion in straight lines is not suitable for producing currents strong enough for commercial purposes. In order to produce currents of considerable strength, the coils of wire have to be pushed past magnets with great speed. Special machines (see Dynamos) are constructed in which the coils are wound so that they can be given a rapid _rotary motion_ as they fly past strong electromagnets. In this way the coil can keep on passing the same magnets, in the same direction, as long as force is applied to the shaft that carries them. [Illustration: Fig. 140.] =EXPERIMENT 180. To study the effect of starting or stopping a current near a coil of wire or other closed circuit.= =436. Directions.= (A) Arrange as in Fig. 140. Place the two coils, H and E, on the same core, L I C. Connect E with the key and cell as before (Fig. 138). Connect H with the astatic galvanoscope, A G, as in Fig. 136. Keep the coils 2 or 3 feet from A G, so that the needle will not be affected by them. (B) Close the circuit at the key, watching the needle, then as soon as the needle regains its former position, open the circuit again. Compare the direction of the induced current in H with that of the current in E, (1) when the main circuit is closed, and (2) when it is opened. Is any current induced in H by a steady current in E? (See Transformers.) [Illustration: Fig. 141.] =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.)