Читать книгу The Study of Elementary Electricity and Magnetism by Experiment - Thomas M. St. John - Страница 7

Table of Contents

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

Apparatus. A bar magnet, B M (No. 21); a compass (No. 18); a sheet of stiff paper about 1 ft. (30 cm.) square, with a center line, C L, drawn parallel to one of its sides (Fig. 16½), and with another line, E W, drawn perpendicular to C L. (See Apparatus Book, Chap. VI., for various ways of making home-made permanent magnets.)

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.

Fig. 16½.

(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.

Fig. 17.

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.

EXPERIMENT 32. To study the magnetic field of a bar magnet.

Apparatus. A sheet of stiff paper; iron filings, I F; bar magnet, B M (No. 21); a sifter for the filings (No. 24); (See Apparatus Book, §48, 49, 50, for home-made sifters.)

63. Directions. (A) Place B M upon the table, and lay the paper over it.

(B) With the sifter sprinkle some filings upon the paper directly over B M, then tap the paper gently, to assist the particles to take final positions. Study the results.

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.

EXPERIMENTS 33–37. To study the magnetic fields of various combinations of bar magnets.

Apparatus for Exps. 33–37. Two bar magnets, B M (Nos. 21, 22); an iron ring, I R (No. 23); iron filings, I F; a sheet of stiff paper; the sifter (No. 24).

65. Note. The student will find it very helpful to make the magnetic figures of the combinations given. Thoroughly magnetize the bar magnets upon an electro-magnet, or upon a strong horseshoe magnet, and mark their N poles in some way. The N poles may be marked by sticking a small piece of paper to them.

66. Directions. (A) Arrange the two magnets, B M, as in Fig. 18, with their unlike poles about an inch apart. (The dotted circle indicates the iron ring to be used in the next experiment. About a quarter, only, of the magnets are shown.)

(B) Place the paper over the magnets, and sift filings upon it immediately over the unlike poles. Note particularly the lines of filings between N and S.

(C) Make a sketch of the result. (See experiments with electromagnets, and the illustrations of magnetic figures with them.)

EXPERIMENT 34.

67. Directions. (A) Leaving the opposite poles an inch apart, as in Exp. 33, place the iron ring, I R (No. 23), between them (Fig. 18, dotted circles).

(B) Place the paper over it all, and sprinkle filings upon it to get the magnetic figure.

(C) Make a sketch of the resulting figure, and compare it with the figure made in Exp. 33. Why do the lines of force appear indistinct in the center of the ring and around it? (See §74.)

Fig. 18.

Fig. 19.

EXPERIMENT 35.

68. Directions. (A) Arrange the two bar magnets, as in Exp. 33, but with their two N poles an inch apart.

(B) Make the magnetic figure of the combination. Do the lines of force flow from one N pole directly to the N pole of the other? Do the particles of filings reaching out from one B M attract or repel those from the other B M?

EXPERIMENT 36.

69. Directions. (A) Place the two bar magnets side by side, so that their unlike poles shall be arranged as in Fig. 19.

(B) Make the magnetic figure.

EXPERIMENT 37.

70. Directions. (A) Turn one B M end for end, so that their like poles shall be near each other, but otherwise arranged as in Fig. 19.

(B) Make and study the magnetic figure.

EXPERIMENTS 38–39. To study the lifting power of combinations of bar magnets.

Apparatus for Exps. 38–39. Two bar magnets, B M (No. 21, 22), of about equal strength; iron filings, I F.

71. Directions. (A) Find out about how many filings you can lift with the N pole of one magnet.

(B) Place the two magnets together (Fig. 20), their like poles being in contact; then see whether the two N poles will lift more or less filings than one pole.

Fig. 20.

EXPERIMENT 39.

72. Directions. (A) Remove all filings from the two magnets just used, and hold them tightly together (Fig. 20), with their unlike poles in contact.

(B) Compare the amount of filings you can lift at one end of this combination with that lifted in Exp. 38 (A) and (B).

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. 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.)

EXPERIMENTS 40–42. To study the magnetic field of the horseshoe magnet.

Apparatus for Exps. 40–42. Horseshoe magnet, H M; iron filings, I F; sheet of stiff paper.

75. Directions. (A) Place H M, with its armature removed, flat upon the table, and cover it with the paper; then make the magnetic figure. (Exp. 32.)

(B) Compare the number of well-defined curves at the poles with the number at the equator.

EXPERIMENT 41.

76. Directions. (A) Make the magnetic figure of H M with its armature in place.

(B) Is the attraction for outside bodies increased or decreased by placing the armature on H M?

EXPERIMENT 42.

77. Directions. (A) Lay H M flat upon the table, and place one or two matches between its poles and the armature; cover with paper as before, and make the magnetic figure. Do lines of force still pass through the armature?

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.

EXPERIMENT 43. To show that lines of force are on all sides of a magnet.

Apparatus. Our compass, O C (No. 18); horseshoe magnet, H M; glass tumbler, G T; sheet of stiff paper; iron filings, I F. Arrange as in Fig. 21. H M may be supported in a vertical position by placing paper, or a handkerchief, under it. The poles should just touch the stiff paper placed over the tumbler.

Fig. 21.

79. Directions. (A) Sprinkle iron filings upon the paper, and study the resulting magnetic figure.

(B) Place O C upon the paper in different positions. Does the magnetic needle always come to rest about parallel to the lines of filings?

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 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.

EXPERIMENT 44. To study a horseshoe magnet with movable poles.

Apparatus. A narrow strip of spring steel, S S (No. 25); iron filings, I F.

81. Directions. (A) Magnetize the spring steel, S S.

(B) Bend S S until its poles are about ¼ in. apart, then using it as a horseshoe magnet, and keeping its poles the same distance apart, see about how many filings you can lift.

(C) Clean the poles of S S, press them tightly together, then again test its lifting power with filings.

Fig. 22.

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.