It is shown that the statistical theory of Parts I and II can be extended so as to give formulas for the magnetization curve and other properties of an ideal reversible specimen, over the range of field values in which magnetization proceeds by a displacement of the boundaries between domains. The application to actual specimens is made by assuming that the reversible properties of an actual specimen are identical with the properties, at the same magnetization, of an ideal specimen having the same initial susceptibility. This leads to a formula for the reversible susceptibility of nickel and iron that was originally suggested by Gans and verified by him experimentally, but for which it is believed no satisfactory derivation has hitherto been offered. In the case of one nickel specimen and of cobalt, where the data do not follow the Gans curve, formulas agreeing better with experiment are obtained by taking account of the anisotropy of the domains. Reasons are given for believing that the replacement of variable by fixed domain volumes, an arbitrary step taken in order to simplify the problem, is probably not essential to the result.

• Received 20 June 1938

DOI:https://doi.org/10.1103/PhysRev.54.279