Page:Elementary Text-book of Physics (Anthony, 1897).djvu/295

§ 251] the magnetic field, that a bar of iron or steel after being removed from the magnetic field retains some of its magnetic properties, that the dimensions of an iron bar are altered by magnetization, the bar becoming longer and diminishing in cross-section, and that a magnetized steel bar loses its magnetism if it be highly heated, are all best explained by Weber's hypothesis.

The phenomena of hysteresis led Ewing to form an extension of Weber's theory. Ewing called attention to the fact that the neutral state of a mass of iron or other magnetizable matter cannot be due to an indiscriminate or unordered arrangement of its constituent magnetic molecules, as was assumed in Weber's form of the theory, but that the interactions of the molecular magnets occasion the formation of groups of molecnles, definitely ordered, and such that they separately produce no external magnetic effect. Such groups may be roughly represented by an assemblage of small magnets floated on water or mounted on needle points, and left free to arrange themselves under the influence of their magnetic forces. If a mass of iron be placed in a field in which the magnetizing force continually increases, the first effect will be a slight development of the magnetism of the iron, due to slight changes in the directions of its molecules. When these changes have progressed so far that the original groups of molecules break down, a very rapid increase of magnetism results and new groups are formed, which are in equilibrium under the magnetic forces of the molecules and the forces of the field. As the force in the field still further increases, the increase in the magnetization of the iron still goes on, but more slowly, this increase being due to slight changes in the positions of the molecules in the new groups, which do not, however, destroy those groups. Saturation is reached, on this theory, when the molecules are arranged in parallel lines; no increase in the intensity of magnetization will then be produced by any further increase in the magnetizing force. This limit has been practically reached by Ewing in some of his experiments. If the magnetizing force be now diminished, the intensity of magnetization also diminishes, but at first only slightly. The magnetic