Page:Elementary Text-book of Physics (Anthony, 1897).djvu/294

280 at which it was originally magnetized, its magnetization also temporarily diminishes.

Any mechanical disturbance, such as jarring or friction, which increases the freedom of motion among the molecules of a magnet, in general brings about a diminution of its magnetization. On the other hand, similar mechanical disturbances facilitate the acquisition of magnetism by any magnetizable body placed in a magnetic field.

251. Theories of Magnetism.—It has been shown by mathematical analysis that the facts of magnetic interactions and distribution are consistent with the hypothesis, which we have already made, that the ultimate molecules of iron are themselves magnets, having north and south poles which attract and repel similar poles in accordance with the law of magnetic force. Poisson's theory, upon which most of the earlier mathematical work is based, is that there exist in each molecule indefinite quantities of north and south magnetic fluids, which are separated and moved to opposite ends of the molecule by the action of an external magnetizing force. Weber's view, which is consistent with other facts that Poisson's theory fails to explain, is that each molecule is a magnet, with permanent poles of constant strength; that the molecules of an iron bar are, in general, arranged so as to neutralize one another's magnetic action, but that, under the influence of an external magnetizing force, they are arranged so that their magnetic axes lie more or less in some one direction. The bar is then magnetized. On this hypothesis there should be a limit to the possible intensity of magnetization, which would be reached when the axes of all the molecules have the same direction. Direct experiments by Joule, J. Müller, and Ewing indicate the existence of such a limit. An experiment of Beetz, in which a thin filament of iron deposited electrolytically in a strong magnetic field becomes a magnet of very great intensity, points in the same direction. The coercive force is, on this hypothesis, the resistance to motion experienced by the molecules. The facts that magnetization is facilitated by a jarring of the steel brought into