(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.
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.
CHAPTER 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.