CHAPTER XXVI.

THE PRODUCTION OF MOTION BY CURRENTS.

_=445. Currents and Motion.=_ We have seen, in the experiments on induced currents, that a current of electricity can be generated by properly moving magnets near coils of wire. (See Dynamo-electric Machines.) Can we reverse this process? Can motion be produced by the electric current?

=EXPERIMENTS 183-190. To study the production of motion by means of the electric current.=

_Apparatus._ The support, including base, rod, and support wire, S W (Fig. 144.) Coils of wire (No. 89, 90); iron cores for coils; cell; key; connecting wires; compass; current reverser; bar magnet; horseshoe magnet.

=EXPERIMENT 183. Motion produced with a hollow coil and a piece of iron.=

=446. Directions.= (A) Arrange as in Fig. 144. Coil H (No. 90) is to be used as a pendulum, and can be supported by fastening a string to it, the upper end of which should be tied to S W.

Connect the ends of H with K and D C. There will be a slight magnetic field about H as soon as the circuit is closed.

(B) Hold I C near the end of the coil. Close the circuit for an instant. Is there any motion produced in H? While the motion will be slight, there should be enough to be noticed if the cell is strong.

(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?

=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.

=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.

=EXPERIMENT 188. Motion with two electromagnets.=

=451. Directions.= (A) Arrange as in Fig. 147. Join the two coils, H and E, in parallel. Connect their two outside ends O E to a metal plate A, and their inside ends I E to B. Join wires 1 and 6 to K, D C, A and B, as shown. When the circuit is closed at K, the current will pass along wire 1 and divide at A, entering E and H at the same time by wires 2 and 4 and returning through 3 and 5 to B, and thence to D C.

(B) Close the circuit for an instant with wires arranged as in Fig. 147. Do the electromagnets attract or repel each other? Study out the direction in which the current passes around the coils, and see whether they _should_ attract or repel.

(C) Change wire 4 to B, and wire 5 to A. The polarity of H, only, will be changed when this circuit is closed. Press the key for an instant and study the results.

_=452. Discussion of Exps. 183-188.=_ From the results it is evident that motion can be produced with the aid of the electric current in many different ways. It can be produced at the ends of wires which simply reach across the room, or which reach miles from the source of the current. To get practical results for commercial purposes we require a proper source of current, proper conductors, and proper apparatus to convert the motions into useful work. The motions given to the parts of the apparatus in the previous experiments are not suitable for commercial purposes, as they are in straight lines. A rotary motion is needed to do good work; and when this is applied to a shaft, belts can be used to run all sorts of machinery. (See Electric Motors.)

=EXPERIMENT 189. Rotary motion with a hollow coil of wire and a permanent magnet.=

=453. Directions.= (A) Arrange as in Fig. 148. A key can be used instead of the reverser. The coil of the galvanoscope, G V, has a magnetic field about it when the circuit is closed. The needle has a permanent field.

(B) Close the circuit for an instant, let the needle swing back past the zero mark, close the circuit again, etc., until the added impulses give the needle a complete turn.

(C) Keep the needle turning on its axis by opening and closing the circuit at the proper time. With a little practice you can make it turn rapidly.

(D) Reverse the motion of the needle. (See § 455.)

=EXPERIMENT 190. Rotary motion with an electromagnet and a permanent magnet.=

=454. Directions.= (A) Arrange as in Fig. 149. Place the compass a short distance from the end of the core of the coil H (No. 89). Close the circuit, and as soon as the needle gets part way around open it again, closing it at the proper time to give the needle a new impulse. The speed can be regulated, somewhat, by changing its distance from the core. A key may be used in place of a reverser.

(B) Reverse the direction of rotation.

_=455. Discussion of Exps. 189-190.=_ We have, in these experiments, the key to the action of electric motors. By properly opening and closing the circuit, the rotary motion can be kept up as long as current is supplied. If a small pulley were attached to the top of the compass-needle in Exp. 190, a tiny belt could be attached, and we should have a machine that could do, perhaps, a fly-power of work. (See Electric Motors.)