Читать книгу The Study of Elementary Electricity and Magnetism by Experiment - Thomas M. St. John - Страница 8
ОглавлениеCHAPTER V.
TERRESTRIAL MAGNETISM.
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.
Fig. 23.
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.
EXPERIMENT 45. To study the lines of force above and below a bar magnet placed horizontally.
Apparatus. A bar magnet, B M (No. 21); compass, O C (No. 18).
85. Directions. (A) Lay B M upon the table and place O C upon its center. Note the position of the compass-needle.
(B) Slide O C along from one end of B M to the other, and study the effect upon its needle. Do lines of force curve over B M as well as around its sides, as shown in Exp. 31?
(C) Place O C upon the table. Hold B M horizontally above O C, and move O C back and forth under B M. Does the needle remain horizontal, or does it show that lines of force pass under B M on their way from its N to its S pole?
Fig. 24.
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.
EXPERIMENT 46. To study the dip or inclination of the magnetic needle, due to the action of the earth.
Apparatus. Fig. 25. Our compass, O C (No. 18); horseshoe magnet, H M (No. 16); piece of paper.
87. Directions. (A) Place O C upon the table, and mark upon a piece of paper the height of the N pole of its needle above the table. (Fig. 25.) The paper should be held in a vertical position, and near the pole.
Fig. 25.
(B) With H M reverse the poles of the compass-needle (Exp. 13), so that its former N pole shall become a S pole.
(C) Place the needle upon its pivot again, and mark upon the paper, as before, the height of its new N pole above the table. Does the needle remain horizontal?
(D) Remagnetize the needle, and reverse its poles so that it will again balance.
Fig. 26.
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.
EXPERIMENTS 47–48. To study the inductive influence of the earth.
Apparatus for Exps. 47–48. Compass, O C, (No. 18); an iron stove poker, or other rod of iron; a hammer. (The iron and hammer are not furnished.)
89. Note. You have seen (Exp. 24), that iron becomes magnetized by induction when placed near a magnet. As the earth acts like a huge magnet, having poles, lines of force, etc., will it magnetize pieces of iron which are in the air or upon its surface?
90. Directions. (A) Test the poker for poles with O C, remembering that repulsion is necessary to prove that it is polarized. If the poker has very weak poles, proceed; but if it shows some strength, hold it in an east and west direction, and hit it several sharp blows on the end with the hammer. Test for polarity again.
(B) With one hand hold the poker in the N and S line, give it a dip toward the north, and strike it several times with the hammer to thoroughly stir up its molecules.
(C) Test again for poles with O C, and note especially whether the lower end (of the poker) became a N or a S pole.
EXPERIMENT 48.
91. Directions. (A) Turn the poker end for end (See Exp. 47); repeat the striking, and test again the pole produced at the lower and north end of it.
(B) Now hold the poker horizontally in the east and west line, and pound it.
(C) Test for poles. Has this strengthened or weakened the poker 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. 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.
EXPERIMENT 49. To test the effect of twisting a wire held north and south in the earth's magnetic field.
Apparatus. Compass, O C (No. 18); a piece of soft iron wire, 6 in. (15 cm.) long (No. 15). Bend up about an inch of the wire at each end so that it may be firmly held when twisting it.
Note. You have seen that we can pound magnetism into or out of a piece of iron at will. Can we twist it into a wire and out again without the use of magnets?
94. Directions. (A) Test the wire for poles with O C.
(B) Hold the wire in a N and S direction, dipping it at the same time, as directed in Exp. 47 for the poker, and twist it back and forth.
(C) Test again for poles with O C. As the poles of the wire may be very weak, bring them slowly toward the compass-needle (see Exp. 14), and note the first motions produced upon the needle.
(D) Hold the wire horizontally east and west, twist and test again. Has its magnetism become weaker or stronger than before?
EXPERIMENT 50. To test for magnetism in bars of iron, tools, etc.
Apparatus. Steel drills; files; chisels; bars or rods of iron that have been standing in an upright position; stove-lid lifters; stove pokers, etc., etc.; a compass.
95. Directions. (A) With the compass test the ends of the above for magnetism, and note which ends are S.
Notes.
