_=44. Retentivity or Coercive Force; Residual Magnetism.=_ Soft iron loses _most_ of its magnetism when simply removed beyond the action of a magnet. We say that it does not retain magnetism; that is, it has very little _retentivity or coercive force_. This is an important fact, the action of many electric machines and instruments depending upon it. A slight amount of magnetism remains, however, in the softest iron, after removing it from a magnet. This is called _residual magnetism_. A piece of iron may show poles, when tested with the compass, although it may have almost no pointing power. =EXPERIMENT 20. To test the retentivity of soft steel.= _Apparatus._ A wire nail, W N (No. 19); horseshoe magnet, H M; iron filings; flat cork, F C; the dish of water (Exp. 10, Fig. 4). =45. Directions.= (A) With H M magnetize the nail; this is made of soft steel. (B) Test the lifting and pointing powers of W N (Exp. 19). (C) Strike W N several times with a hammer to jar it. (D) Again test its lifting and pointing powers. _=46. Discussion.=_ Soft steel has a greater retentivity than soft iron. It contains less carbon than cast or tool steel, and is called mild steel or machinery steel. You do not want soft steel for permanent magnets. =EXPERIMENT 21. To test the retentivity of hard steel.= _Apparatus._ A hard steel sewing-needle (No. 1); other articles used in Exp. 20. =47. Directions.= (A) Magnetize the needle with H M. (B) Test its lifting and pointing powers (Exp. 19). (C) Hammer the needle and test again as in (B). =EXPERIMENT 22. To test the effect of heat upon a magnet.= _Apparatus._ A magnetized sewing-needle; the candle, cork, etc., of Exp. 2. (See Fig. 1.) =48. Directions.= (A) Test the needle for magnetism. (B) Stick the needle into the cork (Fig. 1), and heat it until it is red-hot. (C) Test the needle again for magnetism. (D) See if you can again magnetize the needle. _=49. Discussion.=_ Heating a body is supposed to thoroughly stir up its molecules. Jarring or twisting a magnet tends to weaken it. (See Exp. 19.) The molecules of steel do not move about or change their relative positions as readily as those of soft iron. When the molecules of hard steel are once arranged, by magnetizing them, for example, they strongly resist any outside influences which tend to mix them up again. A magnet does not attract a piece of red-hot iron. The particles of the hot iron are supposed to vibrate too rapidly to be brought into line; that is, the iron cannot become polarized by induction. (See Exp. 24.) =EXPERIMENT 23. To test the effect of breaking a magnet.= _Apparatus._ A magnetized sewing-needle; iron filings; compass, O C (No. 18). [Illustration: Fig. 10.] =50. Directions.= (A) Break the little bar magnet (needle), and test the two new ends produced for magnetism, with the iron filings. (Fig. 10).