Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/256

 diamagnetism," wrote Weber in 1852, "the hypothesis of electric molecular currents in the interior of bodies is corroborated, and the hypothesis of magnetic fluids in the interior of bodies is refuted." The latter hypothesis is, moreover, unable to account for the phenomena shown by bodies which are strongly magnetic, like iron: for it is found that when the magnetizing force is gradually increased to a very large value, the magnetization induced in such bodies does not increase in proportion, but tends to a saturation value This effect cannot be explained on the assumptions of Poisson, but is easily deducible from those of Weber; for, according to Weber's theory, the magnetizing force merely orients existing magnets, and when it has attained such a value that all of them are oriented in the same direction, there is nothing further to be done.

Weber's theory in its original form is, however, open to some objection. If the elementary magnets are supposed to be free to orient themselves without encountering any resistance, it is evident that a very small magnetizing force would suffice to turn them all parallel to each other, and thus would produce immediately the greatest possible intensity of induced magnetism. To overcome this difficulty, Weber assumed that every displacement of a molecular circuit is resisted by a couple, which tends to restore the circuit to its original orientation. This assumption fails, however, to account for the fact that iron which has been placed in a strong magnetic field does not return to its original condition when it is removed from the field, but retains a certain amount of residual magnetization.

Another alternative was to assume a frictional resistance to the rotation of the magnetic molecules; but if such a resistance existed, it could be overcome only by a finite magnetizing force; and this inference is inconsistent with the observation that some degree of magnetization is induced by every force, however feeble.

The hypothesis which has ultimately gained acceptance is that the orientation is resisted by couples which arise from the