Thomas M. St. John

1. Introduction. We should know something about iron and steel at the start, because we are to use them in nearly every experiment. The success with some of the experiments will depend largely upon the quality of the iron and steel used.

When we buy a piece of iron from the blacksmith, we get more than iron for our money. Hidden in this iron are other substances (carbon, phosphorus, silicon, etc.), which are called "impurities" by the chemist. If all the impurities were taken out of the iron, however, we should have nothing but a powder left; this the chemist would call "chemically pure iron," but it would be of no value whatever to the blacksmith or mechanic. The impurities in iron and steel are just what are needed to hold the particles of iron together, and to make them valuable. By regulating the amount of carbon, phosphorus, etc., manufacturers can make different grades and qualities of iron or steel.

When carbon is united with the pure iron, we get what is commonly called iron.

2. Kinds of Iron and Steel. Cast iron is the most impure form of iron. Stoves, large kettles, flatirons, etc., are made of cast iron. Wrought iron is the[4] purest form of commercial iron. It usually comes in bars or rods. Blacksmiths hammer these into shapes to use on wagons, machinery, etc. Steel contains more carbon than wrought iron, and less than cast iron.

Soft steel is very much like wrought iron in appearance, and it is used like wrought iron.

Hard steel has more carbon in it than soft steel. Tools, needles, etc., are made of this.

EXPERIMENT 1. To study steel.

Apparatus. A steel sewing-needle (No. 1). [A]

[A] NOTE. Each piece of apparatus used in the following experiments has a number. See "Apparatus list" at the back of this book for details. The numbers given under "Apparatus," in each experiment, refer to this list.3. Directions. (A) Bend a sewing-needle until it breaks. Is the steel brittle?

(B) If you have a file, test the hardness of the needle.

4. Discussion. "Needle steel" is usually of good quality. It will be very useful in many experiments. Do you know how to make the needle softer?

EXPERIMENT 2. To find whether a piece of hard steel can be made softer.

Fig. 1.

Apparatus. Fig. 1. A needle; a cork, Ck (No. 2); lighted candle (No. 3). The bottom of the candle should be warmed and stuck to a pasteboard base.

5. Directions. (A) Stick the point of the needle into Ck, Fig. 1, then hold the needle in the flame until it is red-hot. (The upper part of the flame is the hottest.)

(B) Allow the needle to cool in the air.

(C) Test the brittleness of the steel by bending it. Test its hardness with a file (Exp. 1).

6. Annealing. This process of softening steel by first heating it and then allowing it to cool slowly, is called annealing. All pieces of iron and steel are, of course, hard; but you have learned that some pieces are much harder than others.

EXPERIMENT 3. To find whether a piece of annealed steel can be hardened.

Apparatus. The needle just annealed and bent; cork, etc., of Exp. 2; a glass of cold water.

7. Directions. (A) Heat the bent portion of the needle in the candle flame (Exp. 2) until it is red-hot, then immediately plunge the needle into the water.

(B) Test its brittleness and hardness, as in Exp. 2.

8. Hardening; Tempering. Good steel is a very valuable material; the same piece may be made hard or soft at will. By sudden cooling, the steel becomes very hard. This process is called hardening, but it makes the steel too brittle for many purposes. By tempering is meant the "letting down" of the steel from the very hard state to any desired degree of hardness. This may be done by suddenly cooling the steel when at the right temperature, it not being hot enough to produce extreme hardness. (The approximate temperature of hot steel can be told by the colors which form on a clean surface. These are due to oxides which form as the steel gradually rises in temperature.)

EXPERIMENT 4. To test the hardening properties of soft iron.

Apparatus. A piece of soft iron wire about 3 in. (7.5 cm.) long (No. 4); the candle, water, etc., of Exp. 3.

9. Directions. (A) Test the wire by bending and filing.

(B) Heat the wire in the candle flame as you did the needle (Fig. 1), then cool it suddenly with the water. Study the results.

10. Discussion. Soft iron contains much less carbon than steel. The hardening quality which steel has is due to the proper amount of carbon in it. If you have performed the experiments so far, you will be much more able to understand later ones, and you will see why we are obliged to use soft iron for some parts of electrical apparatus, and hard steel for other parts.

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11. Kinds of Magnets. Among the varieties of magnets which we shall discuss, are the natural, artificial, temporary, permanent, bar, horseshoe, compound, and electro-magnet.

Fig. 2.

The Horseshoe Magnet, H M (Fig. 2), is the most popular form of small magnets. The red paint has nothing to do with the magnetism. The piece, A, is called its armature, and is made of soft iron, while the magnet itself should be made of the best steel, properly hardened. The armature should always be in place when the magnet is not in use, and care should be taken to thoroughly clean the ends of the magnet before replacing the armature. The horseshoe magnet is artificial, and it is called a permanent magnet, because it retains its strength for a long time, if properly cared for.

EXPERIMENT 5. To study the horseshoe magnet.

Apparatus. Fig. 2. The horseshoe magnet, H M (No. 16).

12. Directions. (A) Remove the armature, A, from the magnet, then move A about upon H M to see (1) if the curved part of H M has any attraction for A, and (2) to see if there is any attraction for A at points between the curve and the extreme ends of H M.

13. Poles; Equator. The ends of a magnet are called its poles. The end marked with a line, or an N, should be the north pole. The unmarked end is the south pole. N and S are abbreviations for north and south. The central part, at which there seems to be no magnetism, is called the neutral point or equator.

EXPERIMENT 6. To ascertain the nature of substances attracted by a magnet.

Apparatus. The horseshoe magnet, H M (Fig. 2); silver, copper, and nickel coins; iron filings (No. 17), nails, tacks, pins, needles; pieces of brass, lead, copper, tin, etc. (Ordinary tin is really sheet iron covered with tin.) Use the various battery plates for the different metals.

14. Directions. (A) Try the effect of H M upon the above substances, and upon any other substances thought of.

15. Magnetic Bodies; Diamagnetic Bodies. Substances which are attracted by a magnet are said to be magnetic. A piece of soft iron wire is magnetic, although not a magnet. Very strong magnets show that nickel, oxygen, and a few other substances not containing iron, are also magnetic. Some elements are actually repelled by a powerful magnet; these are called diamagnetic bodies. It is thought that all bodies are more or less affected by a magnet.

16. Practical Uses of Magnets. Many practical uses are made of magnets, such as the automatic picking out of small pieces of iron from grain before it is ground into flour, and the separation of iron from other metals, etc. The most important uses of magnets are in the compass and in connection with the electric current, as in machines like dynamos and motors. (See experiments with electro-magnets.)

EXPERIMENT 7. To study the action of magnetism through various substances.

Apparatus. Horseshoe magnet, H M; a sheet of stiff paper; pieces of sheet glass, iron, zinc, copper, lead, thin wood, etc.; sewing-needle. (A tin box may be used for the iron, and battery plates for the other metals.)

17. Directions. (A) Place the needle upon the paper and move H M about immediately under it.

(B) In place of the paper, try wood, glass, etc.

(C) Invent an experiment to show that magnetism will act through your hand.

(D) Invent an experiment to show that magnetism will act through water.

18. Magnetic Transparency; Magnetic Screens. Substances, like paper, are said to be transparent to magnetism. Iron does not allow magnetism to pass through it as readily as paper and glass; in fact, thick iron may act as a magnetic screen.

EXPERIMENT 8. To find whether a magnet can give magnetism to a piece of steel.19. Note. You have seen that the horseshoe magnet can lift nails, iron filings, etc.; you have used this lifting power to show that the magnet was really a magnet, and not merely an ordinary piece of iron painted red. Can we give some of its magnetism to another piece of steel? Can we pass the magnetism along from one piece of steel to another?

Apparatus. The horseshoe magnet, H M; two sewing-needles that have never been near a magnet; iron filings.

20. Directions. (A) Test the needles for magnetism with the iron filings, and be sure that they are not magnetized.

(B) Remove the armature, A, from H M, then touch the point of one of the needles to one pole of H M.

(C) Lay H M aside, and test the point of the needle for magnetism.

(D) If you find that the needle is magnetized, rub its point upon the point of the other needle; then test the point of the second needle for magnetism.

21. Discussion; Bar Magnets. A piece of good steel will attract iron after merely touching a magnet. To thoroughly magnetize it, however, a mere touch is not sufficient. There are several ways of making magnets, depending upon the size, shape, and strength desired. For these experiments, the student needs only a good horseshoe magnet, or, better still, the electro-magnets described later; with these any number of small[10] magnets may be made. Straight magnets are called bar magnets.

EXPERIMENT 9. To make small magnets.

Apparatus. Fig. 3. The horseshoe magnet, H M; sewing-needles; iron filings. (See Apparatus Book, Pg. 140, for various kinds of steel suitable for small magnets.)

22. Directions. (A) Hold H M (Fig. 3) in the left hand, its poles being uppermost. Grasp the point of the needle with the right hand, and place its point upon the N or marked pole of H M.

(B) Pull the needle along in the direction of its length (see the arrow), continuing the motion until its head is at least an inch from the pole.

(C) Raise the needle at least an inch above H M, lower it to its former position (Fig. 3), and repeat the operation 3 or 4 times. Do not slide the needle back and forth upon the pole, and be careful not to let it accidentally touch the S pole of H M.

(D) Test the needle for magnetism with iron filings, and save it for the next experiment.

Fig. 3.

Fig. 4.

EXPERIMENT 10. To find whether a freely-swinging bar magnet tends to point in any particular direction.

Apparatus. Fig. 4. A magnetized sewing-needle (Exp. 9); the flat cork, Ck (No. 2); a dish of water. (You can use a tumbler, but a larger dish is better.)

23. Note. An oily sewing-needle may be floated without the cork by carefully lowering it to the surface of the water. All magnets, pieces of iron and steel, knives, etc., should be removed from the table when trying such experiments. Why?24. Directions. (A) Place the little bar magnet (the needle) upon the floating cork, turn it in various positions, and note the result.

25. North-seeking Poles; South-seeking Poles; Pointing Power. It should be noted that the point swings to the north, provided the needle is magnetized as directed in Exp. 9. This is called the north, or north-seeking pole. The N-seeking pole is sometimes called the marked pole. For convenience, we shall hereafter speak of the N-seeking pole as the N pole, and of the S-seeking pole as the S pole. We shall hereafter speak of the tendency which a bar magnet has to point N and S, as its pointing power. An unmagnetized needle has no pointing power.

26. The Magnetic Needle; The Compass. A small bar magnet, supported upon a pivot, or suspended so that it may freely turn, is called a magnetic needle. When balanced upon a pivot having under it a graduated circle marked N, E, S, W, etc., it is called a compass. These have been used for centuries. (See Apparatus Book for Home-made Magnetic Needles.)

EXPERIMENT 11. To study the action of magnets upon each other.

Apparatus. Two magnetized sewing-needles (magnetized as in Exp. 9); the cork, etc., of Exp. 10.

27. Directions. (A) Float each little bar magnet (needles) separately to locate the N poles.

(B) Leave one magnet upon the cork, and with the hand bring the N pole of the other magnet immediately over the N pole of the floating one. Note the result.

(C) Try the effect of two S poles upon each other.

(D) What is the result when a N pole of one is brought near a S pole of the other?

EXPERIMENT 12. To study the action of a magnet upon soft iron.

Apparatus. A magnetized sewing-needle; cork, etc., of Exp. 10; a piece of soft iron wire, 3 in. long; iron filings.

28. Directions. (A) Test the wire for magnetism with filings. Be sure that it is not magnetized. If it shows any magnetism, twist it thoroughly before using. (Exp. 19.)

(B) Float the magnetized needle (Exp. 10), then bring the end of the wire near one pole of the needle and then near the other pole.

(C) Place the wire upon the cork, hold the needle in the hand and experiment.

29. Laws of Attraction and Repulsion. From experiments 11 and 12 are derived these laws:

(1) Like poles repel each other; (2) Unlike poles attract each other; (3) Either pole attracts and is attracted by unmagnetized iron or steel.

The attraction between a magnet and a piece of iron or steel is mutual. Attraction, alone, simply indicates that at least one of the bodies is magnetized; repulsion proves that both are magnetized.

EXPERIMENT 13. To learn how to produce a desired pole at a given end of a piece of steel.

Apparatus. Same as in Exp. 9.

30. Directions. (A) Magnetize a sewing-needle (Exp. 9) by rubbing it upon the N pole of H M from point to head. Float it and locate its N pole.

(B) Take another needle that has not been magnetized, and rub it on the same pole (N) from head to point. Locate its N pole.

(C) Magnetize another needle by rubbing it from point to head upon the S pole of H M; locate its N pole. Can you now determine, beforehand, how the poles of the needle magnet will be arranged?

31. Rule for Poles. The end of a piece of steel which last touches a N pole of a magnet, for example, becomes a S pole.

32. Our Compass (No. 18). While the floating magnetic needle described in Exp. 10, and shown in Fig. 4, does very well, it will be found more convenient to[13] use a compass whenever poles of pieces of steel are to be tested. Fig. 5 shows merely the cover of the box which serves as a base for the magnetic needle furnished. We shall hereafter speak of this apparatus as our compass, O C. (See Apparatus Book, Chap. VII, for various forms of home-made magnetic needles and compasses.)

33. Review; Magnetic Problems. To be sure that you understand and remember what was learned in Exp. 11, do these problems:

1. Using the S pole of the horseshoe magnet, magnetize a needle so that its head will become a N pole. Test with floating cork, as in Exp. 11.

2. Using the N pole of the horseshoe magnet, magnetize a needle so that its head shall be a S pole. Test.

3. Magnetize two needles, one on the N and one on the S pole of the horseshoe magnet, in such a way that the two points will repel each other. Test.

If the student cannot do these little problems at once, and test the results satisfactorily to himself, he should study the previous experiments again before proceeding.

Fig. 5.

Fig. 6.

EXPERIMENT 14. To find whether the poles of a magnet can be reversed.

Apparatus. Fig. 6. The horseshoe magnet, H M; a thin wire nail, W N, 2 in. (5 cm.) long; a piece of stiff paper, cut as shown, to hold W N; thread with which to suspend the paper; compass, O C (No. 18).

34. Directions. (A) Magnetize W N so that its point shall be a S pole. Test with O C to make sure that you are right.

(B) Swing W N in the paper (Fig. 6), then slowly bring the S pole of H M near its point. Note result.

(C) Quickly bring the S pole of H M near the point. Is W N still repelled? Has its S pole been reversed?

35. Discussion; Reversal of Poles. The poles of weak magnets may be easily reversed. This often occurs when the apparatus is mixed together. It is always best, before beginning an experiment, to remagnetize the pieces of steel which have already served as magnets. The same may be shown by magnetizing a needle, rubbing it first in one direction, and then in another upon the magnet, testing, in each case, the poles produced.

EXPERIMENT 15. To find whether we can make a magnet with two N poles.

Apparatus. The horseshoe magnet, H M; an unmagnetized sewing-needle; compass, O C (No. 18).

36. Note. You have already learned that the polarity of a weak magnet can be changed (Exp. 14). Can you think of any method by which two N poles can be made in one piece of steel?37. Directions. (A) Place the needle upon H M, as in Fig. 7.

(B) Keeping the part, C, in contact with the N pole of H M, and using the N pole of H M as a pivot, turn the needle end for end so that its head will be in contact with the S pole of H M.

(C) Pull the needle straight from H M, being careful not to slide it in either direction.

(D) Test the polarity of the ends with O C (Fig. 5), and save it for the next experiment.

Fig. 7.

Fig. 8.

EXPERIMENT 16. To study the bar magnet with two N poles.

Apparatus. The strange magnet just made (Exp. 15); iron filings; compass, O C (No. 18).

38. Directions. (A) Sprinkle filings over the whole length of the needle and then raise it (Fig. 8).

(B) Break the needle at its center, and test, with O C, the two new ends produced at that point. Remember that repulsion is the test for polarity.

39. Discussion; Consequent Poles. Iron filings cling to a magnet where poles are located. In this case, two small magnets were made in one piece of steel; they had a common S pole at the center. The pointing power (§ 25) of such a magnet is very slight; would it have any pointing power if we could make the end poles of equal strength? Intermediate poles, like those in the needle just discussed, are called consequent poles. Practical uses are made of consequent poles in the construction of motors and dynamos.

EXPERIMENT 17. To study consequent poles.

Apparatus. An unmagnetized sewing-needle; horseshoe magnet, H M (No. 16); iron filings (No. 17); compass (No. 18).

40. Directions. (A) Let w, x, y, and z stand for four places along the body of the needle, w being at its point and z at its head.

(B) Touch w with the N pole of H M, x with the S pole, y with the N pole, and z with the S pole. Do not slide H M along on the needle, just touch the needle as directed.

(C) Cover the needle with filings, then lift it.

EXPERIMENT 18. To study the theory of magnetism.

Apparatus. A thin bar magnet, B M (No. 21); iron filings; a sheet of paper. Fig. 9 shows simply the edge of B M and the paper. B M should be magnetized as directed in Exp. 9.

Fig. 9.

41. Directions. (A) Sprinkle some iron filings upon a sheet of paper.

(B) Bring one pole of B M in contact with the filings (Fig. 9), and lightly sweep it through them several times, always in the same direction. Are the filings simply pushed about?

(C) Do the same with a stick, and compare the result with that produced with B M.

42. Theory of Magnetism; Magnetic Saturation. This bringing into line the particles of iron indicates that each particle became a magnet. This experiment should aid in understanding what is thought to take place when steel is magnetized. The pile of filings represents the body to be magnetized, and each little filing stands for a particle of that body. A bar of steel is composed of extremely small particles, called molecules. They are very close together and do not move from place to place as easily as the pieces of filings. A magnet, however, when properly rubbed upon the steel, seems to have power to make the molecules point in the same direction. This produces an effect upon the whole bar.

Each molecule of the steel is supposed to be a magnet. When these little magnets pull together, the whole bar becomes a strong magnet. When a magnet is jarred, and the little magnetized molecules are mixed again, they pull in all sorts of directions upon each other. This lessens the attraction for outside bodies.

Steel is said to be saturated, when it contains as much magnetism as possible. A piece of steel becomes slightly longer when magnetized.

It is thought, by many, that there is a current of electricity around each molecule, making a little magnet of it. (See electro-magnets.)

EXPERIMENT 19. To find whether soft iron will permanently retain magnetism.

Apparatus. A piece of soft iron wire, 3 or 4 in. (7.5 to 10 cm.) long (No. 4); the horseshoe magnet, H M; iron filings; flat cork, F C (No. 2), and the dish of water used in Exp. 10 (Fig. 4).

43. Directions. (A) Magnetize the wire (Exp. 9). Notice that the wire clings strongly to H M.

(B) Test the lifting power of the little wire magnet by seeing about how many iron filings its poles will raise.

(C) Test the pointing power (§ 25) of the wire by floating it on F C (Fig. 4).

(D) Holding one end of the wire in the hand, thoroughly jar it by striking the other end several times against a hard surface.

(E) Test the lifting and pointing powers, as in B and C.

44. Retentivity or Coercive Force; Residual Magnetism. Soft iron loses most of its magnetism when simply removed beyond the action of a magnet. We say that it does not retain magnetism; that is, it has very little retentivity or coercive force. This is an important fact, the action of many electric machines and instruments depending upon it. A slight amount of magnetism remains, however, in the softest iron, after removing it from a magnet. This is called residual magnetism. A piece of iron may show poles, when tested with the compass, although it may have almost no pointing power.

EXPERIMENT 20. To test the retentivity of soft steel.

Apparatus. A wire nail, W N (No. 19); horseshoe magnet, H M; iron filings; flat cork, F C; the dish of water (Exp. 10, Fig. 4).

45. Directions. (A) With H M magnetize the nail; this is made of soft steel.

(B) Test the lifting and pointing powers of W N (Exp. 19).

(C) Strike W N several times with a hammer to jar it.

(D) Again test its lifting and pointing powers.

46. Discussion. Soft steel has a greater retentivity than soft iron. It contains less carbon than cast or tool steel, and is called mild steel or machinery steel. You do not want soft steel for permanent magnets.

EXPERIMENT 21. To test the retentivity of hard steel.

Apparatus. A hard steel sewing-needle (No. 1); other articles used in Exp. 20.

47. Directions. (A) Magnetize the needle with H M.

(B) Test its lifting and pointing powers (Exp. 19).

(C) Hammer the needle and test again as in (B).

EXPERIMENT 22. To test the effect of heat upon a magnet.

Apparatus. A magnetized sewing-needle; the candle, cork, etc., of Exp. 2. (See Fig. 1.)

48. Directions. (A) Test the needle for magnetism.

(B) Stick the needle into the cork (Fig. 1), and heat it until it is red-hot.

(C) Test the needle again for magnetism.

(D) See if you can again magnetize the needle.

49. Discussion. Heating a body is supposed to thoroughly stir up its molecules. Jarring or twisting a magnet tends to weaken it. (See Exp. 19.)

The molecules of steel do not move about or change their relative positions as readily as those of soft iron. When the molecules of hard steel are once arranged, by magnetizing them, for example, they strongly resist any outside influences which tend to mix them up again.

A magnet does not attract a piece of red-hot iron. The particles of the hot iron are supposed to vibrate too rapidly to be brought into line; that is, the iron cannot become polarized by induction. (See Exp. 24.)

EXPERIMENT 23. To test the effect of breaking a magnet.

Apparatus. A magnetized sewing-needle; iron filings; compass, O C (No. 18).

Fig. 10.

50. Directions. (A) Break the little bar magnet (needle), and test the two new ends produced for magnetism, with the iron filings. (Fig. 10).

(B) Touch the two new poles together to see whether they are like or unlike.

(C) Test the nature of the poles with O C (Fig. 5)

(D) Break one of the halves and test its parts.

51. Discussion. The above results agree with the theory that each molecule is a magnet (Exp. 18). No matter into how many pieces a magnet is broken, each part becomes a magnet. (Fig. 10). This shows that those molecules near the equator of the magnet really have magnetism. Their energy, however, is all used upon the adjoining molecules; hence no external bodies are attracted at that point.

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EXPERIMENT 24. To find whether we can magnetize a piece of iron without touching it with a magnet.

Apparatus. Horseshoe magnet, H M; iron filings, I F (Fig. 11).

52. Directions. (A) Hold the armature of the magnet in a vertical position (Fig. 11), its lower end being directly in a little pile of iron filings.

(B) Bring the N pole of H M near the upper end of A, but do not let them touch each other.

(C) Keeping A and the pole of H M the same distance apart, lift them. Do any filings cling to A?

(D) Without moving or jarring A, take H M away from it and note result upon the filings.

53. Temporary Magnetism; Induced Magnetism. The armature, A, was induced to become a magnet without even touching H M. Its magnetism was temporary, however, as the filings dropped as soon as the inductive action of H M was removed. A small amount of residual magnetism (44) remained in A. Soft iron is exceedingly valuable, because it has very little retentivity (44), and because it can be easily magnetized by induction. The armature was made of soft iron. It had induced magnetism. It was a temporary magnet.

56. Polarization; Pole Pieces. The wire, I W (Fig. 12), was acted upon by induction (Exp. 24) and behaved like a magnet. Poles were produced in it, so we say that the wire was polarized. Pieces of iron, placed upon the poles of a magnet, are called pole pieces. It should be noted that the lower end of the wire has a pole like the pole of H M, to which it is attached.

EXPERIMENT 31. To study the space around a magnet, in which pieces of iron become temporary magnets by induction.

61. Directions. (A) Lay the paper upon the table, and place the compass over the center of the line, C L, previously drawn.

(B) Place the eye directly over the compass-needle, then turn the paper until the line is N and S; that is, until the line is parallel to the length of the needle. Pin the paper to the table to hold its center line N and S.

(C) Place B M upon the paper, as shown (Fig. 16½), its N pole to the north, and its center at the cross line, E W.

(D) Slowly move the compass entirely around and near B M, and note the various positions taken by the needle. Note especially the way in which its N pole points. This is to get a general idea of the action of the needle.

(E) Place the compass in the position marked 1, which is on E W, about 1 in. from the line, C L. Press the wooden support down firmly upon the paper to show, by the dent made in the paper by the pin-head, the exact place on the paper that is under the center of the compass-needle. Before removing the compass from this position, look down upon it again, and make a dot on the paper with a pencil directly under each end of the needle. Remove the compass, and draw a line through the dent and the two dots just made. This will show a plan of the exact position of the needle.

(F) Repeat this for the various points marked 2, 4, 6 in. from C L, always marking on the plan the position of the N pole of the needle. Do the same with the other points marked on Fig. 16½ by dots, and study the resulting diagram.

62. Discussion; The Magnetic Field. The compass-needle was decidedly affected all around B M (Fig. 17), showing that induction can take place in a considerable space around a magnet; this space is called the magnetic field of the magnet. Let us consider one position taken by the compass-needle in the field of B M (Fig. 17), as, for example, the one in which the needle has been made black. The S pole of the black needle is attracted by the N pole of B M, and is repelled by the S pole of B M. The N pole of the compass-needle is attracted by the S pole of B M, and is repelled by B M's N pole. The position which it takes, therefore, is due to the action of these 4 forces, together with its tendency to point N and S.

Every magnet has a certain magnetic field, with its lines of force passing through the surrounding air in certain definite positions. As soon, however, as a piece of iron or another magnet is brought within the field, the original position of the lines of force is changed. This has to be considered in the construction of electrical machinery.

64. Magnetic Figures; Lines of Magnetic Force. The filings clearly indicated the extent and nature of the magnetic field of B M. You should notice how the filings radiate from the poles, and how they form curves from one pole to the other. They make upon the paper a magnetic figure. Each particle of the filings becomes a little magnet, by induction (Exp. 24), and takes a position which depends upon attractions and repulsions, as discussed in Exp. 31.

Magnetism seems to reach out in lines from the poles of a magnet. The position and direction of some of the lines are shown by the lines of filings. They are very distinct near the poles, and are considered, for convenience, to start from the N pole of a magnet, where they separate. They then pass through the air on all sides of the magnet, and finally enter it again at the S pole. These lines are called lines of force or lines of magnetic induction.

The poles must not be considered mere points at the ends of a magnet. As shown by magnetic figures, the lines of magnetic induction flow from a considerable portion of the magnet's ends.

73. Discussion; Compound Magnets. Many lines of force pass into the air from two like poles. Such a combination is called a compound magnet. A piece of thin steel can be magnetized more strongly in proportion to its weight than a thick piece, because the magnetism does not seem to penetrate beyond a certain distance into the steel. Thin steel may be magnetized practically through and through. A thick magnet has but a crust of magnetized molecules; in fact, a thick magnet may be greatly weakened by eating the outside crust away with acid. By riveting several thin bar or horseshoe magnets together, thick permanent magnets of considerable strength are made.

74. Lines of force, in passing from the N to the S pole of a magnet, meet a resistance in the air, which does not carry or conduct them as easily as iron or steel. In the arrangement of Exp. 39 the lines of force are not obliged to push their way through the air, as each magnet serves as a return conductor for the lines of force of the other.[28] Either magnet may be considered an armature for the other.

To show in another way that few lines of force pass into the air, the student may lay the above combination upon the table and make a magnetic figure. (See Apparatus Book, p. 38, for method of making home-made compound magnets.)

In the case where a ring was placed between the poles of two bar magnets (Exp. 34), the lines of force from the N pole jumped across the first air-space. They then disappeared in the body of the ring, until they were obliged to jump across the second air-space, to get to the S pole. The weakness of the field in the central space was clearly shown by the filings. There were no stray lines of force passing through the air, because it was easier for them to go through the iron ring. This will be discussed again under "Dynamos and Motors." (See also § 78.)

78. Discussion; Resistance to lines of Force. It is evident, from the last 3 experiments, that lines of force will pass through iron whenever possible, on their way from the N to the S pole of a magnet. When the armature of a horseshoe magnet is in place, most of the lines of magnetic induction crowd together and pass through it rather than push their way through the air. Air is not a good conductor of lines of force; and the magnet has to do work to overcome the resistance of the air, when the armature is removed, in order to complete the magnetic circuit. This work causes a magnet to become gradually weaker. The soft iron armature is an excellent conductor of lines of force; it completes the magnetic circuit so perfectly that very little work is left for the magnet to do.

80. Discussion. The student should keep in mind the fact that the filings in the magnetic figure show the approximate extent and form of the magnetic field simply[30] in one plane. If the paper were held in some other position near the magnet (in a tilted position, for example,) the lines of filings would not be the same as those produced in Exp. 40–42. The lines of force come out of every side of the N pole. When a magnetic needle is placed in any magnetic field, its N pole points in the direction in which the lines of force are passing; that is, it points towards the S pole of the magnet producing the field.

82. Discussion; Advantages of Horseshoe Magnets. When the opposite poles of the flexible magnet are pressed together, the lines of force do not have to pass through the air; there is very little attraction for outside bodies. The same effect is produced with the armature (Exp. 41). A horseshoe magnet has a strong attraction for its armature, because it has a double power to induce and to attract. Suppose the N pole of a bar magnet, B M (Fig. 22), be placed near one end of a piece of iron, as, for example, the armature, A. A will become a temporary magnet by induction (Exp. 24). The S pole of A, polarized by induction, will be attracted by B M, while its N pole will be repelled by B M; so, you see, that a bar magnet does not pull to advantage.

83. The Magnetism of the Earth. The student must have guessed, before this, that the earth acts like a magnet. It causes the magnetic needle to take a certain position at every place upon its surface, and this position depends upon the earth's attractions and repulsions for it. The earth has lines of force which flow from its N magnetic pole, and these lines, before they can get to the earth's S magnetic pole, must spread out through the air on all sides of the earth.

As the magnetic needle points to the earth's N magnetic pole (which is more than 1,000 miles from its real N pole), it is evident that the compass-needle does not show the true north for all places upon the earth's surface. In fact, the N pole of the needle may point E, W, or even S. This effect would be seen by carrying a compass around the earth's N magnetic pole.

84. Declination. For convenience, we shall represent the true N and S, at the place where you are experimenting, by the full line, N S, in Fig. 23. The dotted line shows the direction taken by the compass-needle. The angle, A, between them, is called the angle of variation or the declination. This angle is not the same for all places; and, in fact, it changes slowly at any given place; so it becomes necessary to construct magnetic maps for the use of mariners and others.

86. The Dip or Inclination of the Magnetic Needle. The needle is said to dip when it takes positions like those in Fig. 24. Compass-needles should be horizontal, when properly balanced, and entirely free from all effects other than those of the earth. The excessive dip shown (Fig. 24) is due, of course, to the efforts of the magnetic needle to place itself in the direction in which the lines of force of B M pass.

88. Discussion; Balancing Magnetic Needles. If a piece of unmagnetized steel be balanced and then magnetized, it will no longer remain horizontal; it will dip. Try this. Compass-needles are balanced after they are magnetized. Can you now see why the needle did not remain horizontal after its poles were changed? A piece of steel first balanced and then magnetized, has to have its S pole slightly weighted, as suggested by the line at S (Fig. 26 x), to make it horizontal. The magnetic needle does not tend to dip at the earth's equator, because the lines of force of the earth are nearly horizontal at the equator. As we pass toward the north or south on the earth, the lines of force slant more and more as they come from or enter the earth's magnetic poles. What position would the needle take if we should hold it directly over the earth's N magnetic pole? Fig. 24 shows what the needle does when held near the poles of a bar magnet.

92. Discussion. Dipping the poker places it nearly in the same direction as that taken by the earth's lines of force. The magnetic influence of the earth acts to advantage upon the poker, by induction, only when the poker is properly held.

It no doubt occurs to the student that the end of a magnetic needle which points to the north is really opposite in nature to the north magnetic pole of the earth. The N pole of a needle, then, must be in reality a S pole to be attracted by the earth's N pole. It has been agreed, for convenience, to call the N-seeking pole of a magnet its N pole.

93. Natural Magnets. Nearly all pieces of iron become more or less magnetized by the inductive action of the earth's magnetism. Your poker was slightly magnetized at the start, perhaps, from standing in a dipping position.

Induction takes place along lines of force. In northern latitudes the earth's lines of force have a dip to the north.[35] You should now see why the greatest effect was produced upon the poker when it, also, was made to dip.

Parts of machinery, steel frames of bridges and buildings, tools in the shop, and even certain iron ores, become polarized by this inductive action. These might all be called natural magnets. Magnetic iron ore, called lodestone, is referred to, however, when speaking of natural magnets. Lodestone was used thousands of years ago to indicate N and S, and it was discovered, later, that it could impart its power to pieces of steel when the two were rubbed together.

Notes.