CHAPTER XXIII.

ELECTROMAGNETS.

_=396. Electromagnets=_ are important to the student of electricity. They form the principal part of nearly every electrical instrument. You have seen that a wire has a magnetic field about it the instant a current passes through it. A coil, or helix of wire, has a stronger field than a straight wire carrying the same current, because each turn, or convolution, adds its field to the fields of the other turns. By having a _core_ of soft iron instead of air, wood, or other non-magnetic material, the strength of the magnet is greatly increased. The central core may be permanently fixed in the coil, or it may be removable. (See Apparatus Book, Chapter IX, for Home-made Electromagnets.)

_=397. Cores of Electromagnets.=_ A strong magnet has more lines of force passing from its N pole through the air to its S pole than a weak magnet. By increasing the number of lines of force we increase the strength of a magnet. It has been seen, in experiments with permanent magnets, that lines of force do not pass as readily through air as through soft iron, and that lines of force will go out of their way to pass through iron. It was learned in Exp. 154 that inside of a helix (Fig. 123) the lines of force pass from the S to the N pole; they then spread out through the air and pass back on all sides of the coil to its S pole, as in the case of permanent magnets. The air around and inside of a helix offers a great resistance to the lines of force, and tends to weaken the magnetic field. When part of the circuit consists of an iron core, which is a splendid conductor of lines of force, the magnetic field is greatly increased in strength.

=EXPERIMENTS 158-163. To study straight electromagnets.=

_Apparatus._ A good dry cell or other source of a fairly strong current; coil with soft iron core; key; wires with connectors, etc.; small nails; iron-filings; compass; large wire nail; tin box (No. 94) to act as a base for the electromagnets.

=EXPERIMENT 158. Lifting power.=

=398. Directions.= (A) Join the cell, key, and coil, as explained in Exp. 154, so that the current will pass only when the key is pressed. Place the core inside of the coil (Fig. 125). Two good cells in series can be used to advantage.

(B) Hold the coil in a vertical position near small nails, iron filings, tin boxes, etc.; then press the key and raise coil; carry the clinging iron to another place, break the circuit at the key, and explain the result. Why do nails cling more strongly to the core than filings after the circuit is broken?

=EXPERIMENT 159. Residual magnetism of core.=

=399. Directions.= (A) After the current has passed through the coil with the core in place, remove the core and test it for magnetism with the compass. Will the small end of the core attract both poles of the compass-needle, or is it slightly magnetized?

(B) If there is any residual magnetism, strike the core with a hammer and test again.

(C) Use a soft steel wire nail for the core, and repeat (A) and (B). Why does soft iron make a better core than steel for electromagnets? Which should be the more easily magnetized?

=EXPERIMENT 160. Magnetic tick.=

=400. Directions.= (A) Join the electromagnet with the cell and key as before (Exp. 154). Hold one end of the core firmly against the top of a tin box which should stand upon the table and which should act as a sounding-board. The flat boxes used in the experiments on static electricity are good for this, or use the tin box, No. 94, for a base. Rapidly open and close the circuit by means of the key and listen for any clicks made by the core.

(B) Listen for this sound in telegraph sounders, electric bells, etc., if you have them. The armature should be held, of course, so that slight sounds can be heard.

_=401. Discussion.=_ A bar of iron becomes slightly longer when it is magnetized, the particles of iron being made to point in the same direction. As soon as the current ceases to flow through the coil the particles of the soft core nearly all resume their mixed positions. The click heard is supposed to be due to the changes in the molecules of iron. The core becomes gradually warmer when it is rapidly magnetized and demagnetized by a strong current.

=EXPERIMENT 161. Magnetic figures.=

=402. Directions.= (A) Arrange as in Fig. 126. The key should be used in case a dry cell acts as the source of the current. Two good cells joined in series can be used to advantage. Lay the coil flat upon the table and place on it a piece of stiff, smooth paper, or a sheet of glass.

(B) Sprinkle a few iron filings upon the glass, which may be held in place by books. Gently tap the glass with a pencil while you close the circuit at the key. Do the filings arrange themselves as in the case of permanent magnets? Make a sketch of the field, remembering that you have both N and S poles, and compare it with previous results.

=EXPERIMENT 162. Magnetic figures.=

=403. Directions.= (A) Arrange as in Fig. 126, but stand the coil on end, using the base as directed in § 407, to hold it firmly in position. Join the ends, O E and I E, to the key as before. Fig. 127 shows a top view of the coil and base.

(B) With books, etc., fix a piece of stiff, smooth paper, or glass just over the top of the core, and proceed as in Exp. 161 to study the field. See § 417 for making permanent pictures of magnetic fields.

=EXPERIMENT 163. Magnetic field.=

=404. Directions.= (A) Use same arrangement as for Exp. 162, except filings and glass, which are replaced by the compass.

(B) Hold the compass about 2 in. from the top pole of the electromagnet, close the circuit for a second or two and note action of needle. Is the top N or S, when the current enters the coil at O E? Compare result with § 392.

(C) Move the compass quickly about the pole, the circuit being closed, and note action of needle. Compare result with directions taken by particles of iron filings in Exp. 163.

(D) Reverse the direction of the current through the coil and test the nature of the pole at the top.

_=405. Horseshoe Electromagnets.=_ Fig. 128 shows a simple form of electromagnet with two coils which have a bent piece of iron as a core for both. The coils have to be wound on by hand in this form. As this is troublesome, the coils are generally wound on two separate cores which are joined by a _yoke_ (§ 406), which takes the place of the curved part in Fig. 128. The separate coils can be quickly made with a "winder" and joined to suit. (See Apparatus Book, Chapter IX, for Home-made Electromagnets.) Fig. 129 shows a top view of a home-made experimental horseshoe electromagnet. The coils are joined by an iron strap, called the _yoke_, which is screwed to a wooden base. A strip of iron placed above the magnets to be attracted by them, when the current passes, is called the _armature_. (See Telegraph Sounders.)

_=406. Use of Yoke.=_ It has been explained (§ 82) why horseshoe magnets are, in general, better than straight ones. The same is true of electromagnets; there are two poles to attract, and two to induce. The lines of force pass through the yoke on their way from one core to the other, and this reduces the resistance to them. The strength of the horseshoe magnet would be greatly reduced if the lines of force were obliged to pass through two air spaces instead of one; in fact, if there were no yoke we should have simply two straight magnets. The yoke should be made of soft iron.

=407. Experimental Magnets= are quickly joined to a tin base (No. 94), which has 3 holes punched in, through which screws can be put to hold the cores in place. Fig. 127 shows plan of tin. Fig. 130 shows how removable cores are fastened to the base, the coils being on the spools, and Fig. 131 shows how home-made coils on bolts can be used. The coils on bolts should be wound as directed in Apparatus Book, Chapter X. The tin base also serves as the yoke.

_Removable Cores._ Fig. 130. These are of soft iron (No. 92, 93). In one end of each is a hole for the screws, S. Part of the tin has been cut away in the Fig. The copper washer, C W, should be used. (See § 408.) Connectors are fastened to the ends of the coils (§ 226-230).

_Bolt Cores._ Fig. 131. After winding on the coils, as directed in Apparatus Book, remove the nut and put on an extra washer, E W, so that the ends of the coils will not be pressed against the tin, but come out between the two washers. Push the screw-end of the bolt through holes (about 2 in. apart) punched in the tin, then put on the nut, as shown. Do not force the nut on too far,--just far enough to hold the cores in place. The ends of the wires are not shown in Figs. 130, 131. Connectors are fastened to them (§ 408).

=408. Method of Joining Coils.= To produce the best results the poles of the horseshoe electromagnet should be unlike. As the coils are wound alike, their ends must be joined in such a manner that the current will pass around them in opposite directions; that is, if the current enters one coil at the outside end, O E, it must enter the other coil at the inside end, I E. Fig. 132 shows a plan of the connections, spring connectors being fastened to the coil-ends, to allow rapid and easy changes in the arrangement. L, M, and R are pieces of metal fastened to a strip of wood (No. 95), used to make connections from cells or other apparatus. They are turned up at each end as in Fig. 104, 3. Care should be taken not to get short circuits by allowing two wires to touch the tin base.

By changing the ends of the coils upon L, M, and R (left, middle, and right), and by changing the direction in which the current enters the "combination connecting plates" (No. 95), it is evident that the nature of the poles can be regulated to suit.

=EXPERIMENTS 164-173. To study horseshoe electromagnets.=

_Apparatus._ Coils of wire with cores and yoke like those explained in this chapter. Coils fastened to tin base or yoke with wires leading from them to the combination connecting plates (No. 95, Fig. 132), are very handy. Cells; iron filings; compass; iron strip (No. 76).

=EXPERIMENT 164. To test the poles.=

=409. Directions.= (A) Arrange as in Fig. 126, but use the experimental magnets and combination connections (Fig. 132) in place of the single coil shown in Fig. 126. Join O of the key with L, and Zn of the cell with R of Fig. 132. When the key is pressed the current will enter the magnets from L and leave at R.

(B) With the compass test the polarity of the cores as in Exp. 163, B, C. Make a sketch of the arrangement, and note which pole is N and which S.

(C) See which way the current must pass around each coil, by the way it is wound, and compare the results of (B) with Exp. 154, Fig. 123.

=EXPERIMENT 165. To test the poles.=

=410. Directions.= (A) Arrange as in Exp. 164, but reverse the direction of the current through the coils. Do this by joining O of the key (Fig. 126) with R of Fig. 132, and Zn of the cell with L.

(B) Repeat (B) and (C) of Exp. 164 and study results.

=EXPERIMENT 166. To test the poles.=

=411. Directions.= (A) Arrange all connections as in Exp. 164, then reverse the positions of O E and I E of coil A; that is, join O E to M, and I E to L, Fig. 132. This will make unlike ends come together at M; in other words, when the current enters at L and leaves at R it will pass around both coils in the same direction.

(B) Study the nature of the poles, as in Exps. 164, 165, and note results.

_Note._--Fig. 133 shows simply the two cores of a horseshoe electromagnet with arrows to indicate in which direction the current is passing in each coil to produce N and S poles.

=EXPERIMENT 167. To study the inductive action of one core upon the other.=

=412. Directions.= (A) Arrange as for Exp. 164, but join the wire from Zn of the cell to M (Fig. 132). In this way coil B will be cut out of the circuit. Place the coils in the E and W line.

(B) Find about how far the residual magnetism of the core of B can act upon the compass-needle, holding the compass on the side away from coil A, no current passing.

(C) Press the key for an instant, and note whether the magnetism of coil B has been made stronger or weaker. Explain the action of core A on core B.

=EXPERIMENT 168. Magnetic figures.=

=413. Directions.= (A) Arrange as in Exp. 164. With books, etc., fix a piece of smooth, stiff paper, or a sheet of glass, just above the poles of the electromagnets.

(B) Sprinkle iron filings upon the glass, and gently tap it while the circuit is closed at the key for a few seconds. Make a sketch of the magnetic figure produced. Do the lines of force from the opposite poles attract or repel each other? See § 417 for making permanent figures. (See "Things a Boy Should Know About Electricity" for drawings of magnetic figures.)

_Note._--If possible, use two or three good cells in series for making magnetic figures, as a fairly strong field is best.

=EXPERIMENT 169. Magnetic figures.=

=414. Directions.= (A) Arrange apparatus as for Exp. 165, and make the magnetic figure for this combination, as directed in Exp. 168. Sketch and study the results.

=EXPERIMENT 170. Magnetic figures.=

=415. Directions.= (A) Arrange the apparatus and connections as in Exp. 166, and make the magnetic figure of this combination as directed in Exp. 168. In this case the poles are alike. Sketch and study the results.

=EXPERIMENT 171. Magnetic figures.=

=416. Directions.= (A) Arrange apparatus and connections as in Exp. 167, and make the magnetic figure of the combination as directed in Exp. 168. Compare the figure produced with that of Exp. 168. In this case the current passes through but one coil.

=417. Permanent Magnetic Figures= can be made in several ways for future study and comparison.

(A) _Paraffine paper figures._ Make paraffine paper as directed in Apparatus Book, page 135. For this purpose smooth, stiff, _white_ paper is best, so that the filings will show plainly, and but a thin coating of paraffine should be given. Place the magnets upon the table, lay over them a piece of unparaffined paper, and fix the paraffine paper directly over this. This is necessary, as the coated paper sticks when heated. For electromagnets it will be necessary to support the edges of the paper with books, etc. Sprinkle on the filings and tap the paper to make them arrange themselves while the circuit is closed. After the lines of force show plainly, the current need not be used again, provided the paper be kept perfectly still. Pass the flame of a Bunsen burner over the paper to melt the coating. This will, no doubt, make the two pieces of paper stick together, and permanently fix the particles of filings in place. Do not heat the paper too much--just enough to melt the paraffine. If you have no gas, hold a fire-shovel, containing hot coals, over the paper. As soon as the paraffine cools, the figures will stand considerable handling.

_Blue print figures_ are very pretty, and last indefinitely. Get some blue-print paper at a photographer's, who will give you directions about "developing" it with water. Keep this in the dark, and take out but one sheet at a time for experiments. To make the figures, take your apparatus near a window where bright sunlight comes in. Pull down the curtain so that you have but a dim light when you make the magnetic figure, as directed before. After the lines of force show plainly, raise the curtain, and let the bright sunlight shine on it for 5 or 6 minutes, or until the surface of the paper has a rich, bronze color. The paper cannot be acted upon by the light under the particles of filings. Quickly shake the filings from the paper, and wash it in 3 changes of water to "develop" it, then pin the paper up to dry.

=EXPERIMENT 172. Lifting power.=

=418. Directions.= (A) Arrange the apparatus as in Exp. 164. Hold an iron strip (No. 76), a screw-driver, or other iron bar directly over and near the poles of the experimental electromagnet. Close the circuit at the key, then lift the magnets by the "armature," as the iron strip may be called, the circuit being kept closed for a few seconds. If your cell is good there should be no trouble in lifting the magnets by the armature. Open the circuit, and see whether the magnets drop.

(B) Hold the magnets upside down directly over nails, tin boxes, iron filings, or other pieces of iron. Close the circuit, move the attracted iron to another place on the table, and open the circuit. Can this principle be used for practical purposes?

_Note._--Some experiments illustrating practical uses of electromagnets will be given in a future chapter.

=EXPERIMENT 173. Residual magnetism when magnetic circuit is closed.=

=419. Directions.= (A) Arrange as in Exp. 164. You have already seen that each core retains some magnetism after the circuit is closed. Place the iron strip firmly across the poles, close the circuit for an instant, open the circuit, then see whether the armature still clings to the cores with some strength. The armature should fit well upon the cores for this experiment.

(B) Again press the armature upon the cores, no current being used; then lift it as in (A). Compare the attraction with that found in (A).

_=420. Closed Magnetic Circuits.=_ It was seen in the study of the permanent horseshoe magnet, that the armature clung strongly to the magnet. The armature closed the magnetic circuit, the lines of force having almost no resistance. In the case of electromagnets the magnetic circuit becomes closed when the armature touches both poles at the same time. The armature clings strongly to the poles even after the current ceases to flow. As soon as the magnetic circuit is broken, however, but little residual magnetism remains. The armatures of electromagnets are usually arranged so that they can not quite touch the cores, to avoid this sticking.