6. Are all magnets produced by induction? Explain.
7. In what magnetic devices is a high retentivity desirable?
(2) The Theory of Magnetism and Magnetic Fields
205. The Theory of Magnetism.—If a magnetized watch spring is broken in two, each part is found to be a magnet. If one of these parts be broken and this process of breaking be continued as far as possible, the smallest part obtained has two poles and is in fact a complete magnet. (See Fig. 176.) It is supposed that if the division could be continued far enough that each of the molecules of the steel spring would be found to have two poles and to be a magnet. In other words, magnetism is believed to be molecular. Other evidence supporting this idea is found in the fact that when a magnet is heated red hot, to a temperature of violent molecular motion, its magnetism disappears. Also if a long, fine soft iron wire be strongly magnetized, a light jar causes its magnetism to disappear. This would lead us to believe that magnetism is not a property of the surface of the body, but that it depends upon molecular structure or the arrangement of the molecules.
Fig. 176.—Effect of breaking a magnet.
Fig. 177.—Possible arrangement of molecules in an unmagnetized iron bar.
It is believed also that the molecules of a magnetic substance are magnets at all times; that before the body is magnetized the molecules are arranged haphazard (see Fig. 177) but that when a magnet is brought near, the molecules tend to arrange themselves in line, with their north-seeking poles pointing in the same direction. (See Fig. 178.) If the magnet is jarred some of the molecules tend to get out of line, perhaps to form little closed chains of molecules. (See Fig. 177.)
