=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 1/4 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. [Illustration: 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. 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. [Illustration: 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).