The attraction between a magnet and a piece of iron or steel is mutual. Attraction, alone, simply indicates that at least one of the bodies is magnetized; repulsion proves that both are magnetized. =EXPERIMENT 13. To learn how to produce a desired pole at a given end of a piece of steel.= _Apparatus._ Same as in Exp. 9. =30. Directions.= (A) Magnetize a sewing-needle (Exp. 9) by rubbing it upon the N pole of H M from _point to head_. Float it and locate its N pole. (B) Take another needle that has not been magnetized, and rub it on the same pole (N) from _head to point_. Locate its N pole. (C) Magnetize another needle by rubbing it from _point to head_ upon the S pole of H M; locate its N pole. Can you now determine, beforehand, how the poles of the needle magnet will be arranged? _=31. Rule for Poles.=_ The end of a piece of steel which last touches a N pole of a magnet, for example, becomes a S pole. _=32. Our Compass=_ (No. 18). While the floating magnetic needle described in Exp. 10, and shown in Fig. 4, does very well, it will be found more convenient to use a compass whenever poles of pieces of steel are to be tested. Fig. 5 shows merely the cover of the box which serves as a base for the magnetic needle furnished. We shall hereafter speak of this apparatus as _our compass_, O C. (See Apparatus Book, Chap. VII, for various forms of home-made magnetic needles and compasses.) =33. Review; Magnetic Problems.= To be sure that you understand and remember what was learned in Exp. 11, do these problems: 1. Using the S pole of the horseshoe magnet, magnetize a needle so that its head will become a N pole. Test with floating cork, as in Exp. 11. 2. Using the N pole of the horseshoe magnet, magnetize a needle so that its head shall be a S pole. Test. 3. Magnetize two needles, one on the N and one on the S pole of the horseshoe magnet, in such a way that the two points will repel each other. Test. If the student cannot do these little problems at once, and test the results satisfactorily to himself, he should study the previous experiments again before proceeding. [Illustration: Fig. 5.] [Illustration: Fig. 6.] =EXPERIMENT 14. To find whether the poles of a magnet can be reversed.= _Apparatus._ Fig. 6. The horseshoe magnet, H M; a thin wire nail, W N, 2 in. (5 cm.) long; a piece of stiff paper, cut as shown, to hold W N; thread with which to suspend the paper; compass, O C (No. 18). =34. Directions.= (A) Magnetize W N so that its point shall be a S pole. Test with O C to make sure that you are right. (B) Swing W N in the paper (Fig. 6), then _slowly_ bring the S pole of H M near its point. Note result. (C) _Quickly_ bring the S pole of H M near the point. Is W N still repelled? Has its S pole been reversed?