(D) Remagnetize the needle, and reverse its poles so that it will again balance.
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.