Читать книгу The Study of Elementary Electricity and Magnetism by Experiment - Thomas M. St. John - Страница 5

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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 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 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.