CHAPTER XXII.

ELECTROMAGNETISM.

_=383. Electromagnetism=_ is the name given to magnetism that is developed by electricity. You have already seen that if a magnetic needle be placed in the magnetic field of a _magnet_, its N pole will point in the direction in which the lines of force pass on their way from the N to the S pole of the magnet. You have also seen that in the galvanoscope, etc., a coil of wire acts like a magnet when a current passes through it. Can we not, then, use the needle to study the lines of force about wires and coils?

=EXPERIMENT 152. To study the lines of magnetic force about a straight wire carrying a current.=

_Apparatus._ The compass, O C; key, K; dry cell, D C. Arrange as in Fig. 116.

=384. Directions.= (A) Arrange the wire so that the current will flow through it from N to S over the compass-needle as soon as the circuit is closed (Fig. 117, A). Press K for an instant only, and note the direction in which the N pole is deflected. Repeat two or three times until you get clearly in mind the direction taken by the needle. Sketch the result in your note-book, and compare with Fig. 118, A. The arrow shows the direction of the current.

(B) Let the current pass for an instant from N to S and _under_ the needle, as shown in Fig. 117, B. Sketch result.

(C) Let the current pass for an instant from S to N _above_ the needle (Fig. 117, C). Sketch result.

(D) Let it pass from S to N _under_ the needle (Fig. 117, D). Sketch result.

(E) Let it pass through the wire from east to west (Fig. 117, F) above the needle, then under it, and note result. Compare the results with those indicated in Fig. 118.

_=385. Lines of Force About a Wire.=_ When a current passes through a wire, the needle, over or under it, tends to take a position at right angles to the wire. This shows that the lines of force pass _around_ the wire and not in the direction of its length. The needle does not swing entirely perpendicular to the wire; that is, to the E and W line, because the earth is at the same time pulling its N pole towards the N. If the needle had no pointing power, and at the same time retained its magnetic field, it would point exactly at right angles to the wire as soon as the current passed.

If you look along the wire, Fig. 119, from the point, C, towards the positions, A and B, you will see (A) that _under_ the wire the lines of force pass to the left, and that _above_ the wire (B) they pass towards the right. This is because the N pole points in the directions mentioned. (See Fig. 118.) Looking along the wire from Z towards position, D and C, you will see just the opposite to the above, as the current comes _towards_ you.

_Rule._--When the current goes from you, the lines of force pass around the wire in a clockwise direction, and when the current comes toward you they pass around it in an anti-clockwise direction.

_=386. Ampere's Rule=_ may be used to remember what has been learned in Exp. 152.

_If you imagine yourself swimming in the wire with the current, always facing the needle, the N-seeking pole of the needle will always be deflected towards your left hand._

When the needle is above the wire you must imagine that you swim upon your back, in order to _face_ the needle.

_Another Rule._--Hold the right hand with the thumb extended and with the fingers pointing in the direction of the current, the palm being towards the needle and on the opposite side of the wire from the needle. The N-seeking pole will then be deflected in the direction in which the thumb points.

_=387.=_ If a wire carrying a strong current be dipped in iron filings, the magnetic field about the wire acts by induction upon the particles of filings, making magnets of them. These cling to each other simply because they are little magnets.

_=388. Lines of Force about Parallel Wires.=_ When a current passes in the same direction in two parallel wires the lines of force pass around the wires in the same direction in both, and the magnetic fields attract each other. When the currents flow in opposite directions the magnetic fields repel each other.

=EXPERIMENT 153. To study the lines of force about a coil of wire like that upon the galvanoscope.=

_Apparatus._ Galvanoscope, G V; dry cell; key; compass. Arrange as in Fig. 116, using G V instead of the compass shown. The coil of G V should be placed in the E and W line. The current can pass only when the key is pressed. Connect the wires with G V, so that the current will pass through the 15-turn coil from W to E on top of the coil; that is, so that the current will have a "clockwise" motion. Fig. 120 represents a front view of the coil.

=389. Directions.= (A) Hold the compass in the various places marked with a dot (Fig. 120) and note the directions taken by its N pole. Make a circle similar to the one shown to represent the coil, and sketch upon it the way in which the lines of force pass around it according to your observations.

(B) Make a diagram like Fig. 121, which represents a cross-section of the coil through the center. Imagine that you have removed the top half of the coil and that you are looking down upon the ends of the wire of the lower half. Draw curved arrows about the coil at W and E to show which way the lines of force are passing. Compare your results with those in Fig. 119, remembering that at E, Fig. 121, the current is going away from you.

(C) Move O C back and forth on the center-line that runs N and S through the coil, and note the positions of the compass-needle. Does the coil seem to have poles?

(D) Reverse the current through the coil and repeat your observations.

=EXPERIMENT 154. To study the magnetic field about a small coil of wire.=

_Apparatus._ A coil of wire (No. 89), described in § 390; current reverser, C R (No. 57); dry cell; connecting wires, etc.

=390. Coils= of wire for some of the following experiments should be wound upon wooden spools that have been turned down thin, so that the wire will be as near the central hole as possible. They should be wound with a winder. (See Apparatus Book, Chapter X.)

For convenience we shall call the starting end of the coil, that is, the end that comes from the wire that is near the center, the _inside end_, I E. The end of the last layer of the coil we shall call the _outside end_, O E. These letters should be noted in the diagrams. See Apparatus List for details of the special coils used in these experiments.

=391. Directions.= (A) Arrange as in Fig. 122, so that the axis of the coil will lie in the E and W line. Place O C about 2 in. from the E end of the coil. Press one lever of C R so that the current will pass around the coil for an instant in a clockwise direction; that is, so that it will enter the coil at O E. Note the action of the needle. If the needle is not affected move it nearer the coil and press the lever again. Get clearly in mind the connections, the direction in which the N end of the needle is deflected, etc. Is the E end of the coil a N or a S pole?

(B) Reverse the current through the coil. What effect has it upon the polarity of the E end of the coil?

(C) Place O C at the west end of the coil and repeat (A) and (B).

(D) Place O C in various positions about the coil and note the action of the needle when the current passes. Does this coil act like a magnet, having poles, magnetic field, etc.?

_=392. Polarity of Coils.=_ It is evident from Exps. 153 and 154 that a coiled conductor has poles, magnetic field, etc., when a current passes, and that it strongly resembles a magnet, even though no iron enters into its construction. We may say that the coil becomes magnetized by the electric current. Fig. 123 shows a right handed coil or helix of wire, the current passing as shown by the small arrows. The left-hand end is a S pole because the current passes around it in a clockwise direction. When you face the right-hand end of the coil the current is seen to pass around it in an anti-clockwise direction; this produces a N pole. As the N pole of the magnetic needle is attracted toward the S pole of the coil, it is clear that the lines of force pass through the inside of the coil as shown by the large arrows. They then curve through the air and return to the S pole as with magnets.

=EXPERIMENT 155. To test the attracting and "sucking" power of a magnetized coil or helix.=

_Apparatus._ The coil, battery, etc., used in Exp. 154, Fig. 122; a sewing-needle.

=393. Directions.= (A) Arrange the coil, etc., as described in Exp. 154. The coil need not lie in the E and W line, however, and a key may be used instead of the current reverser.

(B) Magnetize the needle so that its point will be a N pole.

(C) Tie a thread about the center of the magnetized needle, hold the thread in the hand so that the S pole of the needle will swing freely at the hole at the right-hand end of the coil (Fig. 124). If the current passes as directed, the right-hand end of the coil will be a N pole. What happens to the needle when the key is pressed for an instant.

(D) Change the needle to the left end of the coil and repeat.

(E) Try a nail, pen, iron, etc., instead of the needle.

=EXPERIMENT 156. To find whether a piece of steel can be permanently magnetized by an electric current.=

_Apparatus._ Same as for last experiment; an unmagnetized sewing-needle; the compass.

=394. Directions.= (A) Be sure that the needle is not magnetized. It should attract both ends of the compass-needle. How can any magnetism in the needle be removed?

(B) Place the needle inside of the coil with its _point_ to the east; that is, with its point at the N pole of the coil, and its head at the S pole. Close the circuit for an instant. Test the needle again for poles. Is the point a N or a S pole?

(C) Turn the needle end for end in the coil, and see whether its polarity can be reversed.

(D) Experiment with iron wire, nails, steel pens, spring steel, etc.

=EXPERIMENT 157. To study the effect of a piece of iron placed inside of a magnetized coil of wire.=

_Apparatus._ Same as in Exp. 154; a short rod or iron _core_, I C, of soft iron (No. 92) that will fit inside of the coil. This combination is called an electromagnet.

=395. Directions.= (A) Arrange first as for Exp. 154, Fig. 122, with the coil in the E and W line, no core being used, and place O C about 6 in. from the right-hand end of the coil.

(B) Press the lever for an instant to see whether the field of the coil is strong enough to move the compass-needle at that distance. Move O C a little nearer or farther from the coil until the needle _just_ moves, when the circuit is closed.

(C) Place I C inside of the coil (Fig. 125), and repeat (B) to see whether the magnetic field of the coil is stronger or weaker than before.

(D) Study the location of the poles. Can they be reversed?