Apparatus. A piece of soft iron wire, 3 or 4 in. (7.5 to 10 cm.) long (No. 4); the horseshoe magnet, H M; iron filings; flat cork, F C (No. 2), and the dish of water used in Exp. 10 ([Fig. 4]).
43. Directions. (A) Magnetize the wire ([Exp. 9]). Notice that the wire clings strongly to H M.
(B) Test the lifting power of the little wire magnet by seeing about how many iron filings its poles will raise.
(C) Test the pointing power ([§ 25]) of the wire by floating it on F C ([Fig. 4]).
(D) Holding one end of the wire in the hand, thoroughly jar it by striking the other end several times against a hard surface.
(E) Test the lifting and pointing powers, as in B and C.
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]).