(D) Without moving or jarring A, take H M away from it and note result upon the filings.
53. Temporary Magnetism; Induced Magnetism. The armature, A, was induced to become a magnet without even touching H M. Its magnetism was temporary, however, as the filings dropped as soon as the inductive action of H M was removed. A small amount of residual magnetism (44) remained in A. Soft iron is exceedingly valuable, because it has very little retentivity (44), and because it can be easily magnetized by induction. The armature was made of soft iron. It had induced magnetism. It was a temporary magnet.
EXPERIMENT 25. To find whether a piece of steel can be permanently magnetized by induction.
Apparatus. An unmagnetized sewing-needle; horseshoe magnet, H M; iron filings; sheet of stiff paper.
54. Directions. (A) Test the needle for magnetism.
(B) Place the unmagnetized needle upon the paper, then move H M about immediately under it, so that the needle will be attracted.
(C) Test the needle again for permanent magnetism.
