A Formula for the Specific Heat of Ferromagnetic Substances and its Discontinuity at the Critical Temperature

Abstract

THE loss of magnetism by ferromagnetic substances at their critical temperatures is accompanied by a striking change in the specific heat. Not only do ferromagnetic substances exhibit an abnormal rise in the specific heat up to the critical temperature, but also at this point there is a sudden diminution amounting to about one-half the increase. For example, the specific heat of iron is 0.119 at ordinary temperatures; it rises to 0.309 at the critical temperature, and at this point suddenly falls to about 0.189, so that there is an abrupt change of 0.120 in the true specific heat—with nickel the discontinuity amounts to 0.0285, and with magnetite to about 0.079. Attempts have been made to obtain a quantitative relation between the loss of magnetic energy and the energy of this thermal discontinuity, and a formula has been given by H. A. Lorentz, based on the theories of Langevin and Weiss, which when reduced to its simplest form is, where “C is the discontinuity in the specific heat and m is the molecular mass of the ferromagnetic body. With this formula the calculated values of “C are 0.089 for iron, 0.0282 for nickel, and 0.0644 for magnetite, and the disagreement with observed values, except in the case of nickel, is very marked.