=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. [Illustration: FIG. 176.--Effect of breaking a magnet.] [Illustration: 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.) [Illustration: FIG. 178.--Arrangement of molecules in a saturated magnet.] =206. Magnetic Fields and Lines of Force.=--The behavior of magnets is better understood after observing and studying the _lines of force_ of a magnet. The earliest descriptions of these are by William Gilbert, the first Englishman to appreciate fully the value of making experimental observations. He wrote a book in 1600 called _De Magnete_ in which he published his experiments and discoveries in magnetism. (See p. 217.) Magnetic lines of force may be observed by placing a magnet upon the table, then laying upon it a sheet of paper and sprinkling over the latter fine iron filings. On gently tapping the paper, the filings arrange themselves along curved lines extending from one end of the magnet to the other. These are called the _magnetic lines of force_. (See Fig. 179.) The space about a magnet in which the magnetic lines are found is called the _magnetic field_. (See Fig. 180.) [Illustration: FIG. 179.--Iron filings on paper over a bar magnet.] Many interesting things have been discovered concerning the lines of force. Some of the facts of magnetic action are given a simple explanation if we think of them as due to the magnetic lines of force. A summary of several discoveries concerning magnetic fields follows: