The influence of hydrostatic pressure on the critical temperature of magnetization for iron and other materials

Abstract

This cooperative investigation by L. H. Adams and J. W. Green, of the Geophysical Laboratory and Department of Terrestrial Magnetism, respectively, both of the Carnegie Institution of Washington, was undertaken principally because of its bearing on the Earth's magnetic field and its relation to the problem of the central iron‐core. The object was to determine whether or not the temperature at which iron and other ferromagnetic substances pass from the magnetic to the non‐magnetic state is affected by an increase of pressure.The specimens of the materials investigated were made up as the core of a miniature transformer or induction‐unit, and placed in an electrically heated pressure‐bomb. A six‐volt alternating‐current was supplied to the primary of the transformer and the output from the secondary was amplified, rectified, and carried to a direct‐reading galvanometer. The temperature of the inversion‐point, indicated by the galvanometer‐reading dropping to zero, was measured by means of platinum‐platinrhodium thermocouples.Five ferromagnetic metals were used. Determinations were made at pressures up to 2,000 atmospheres for iron and magnetite, 2,200 atmospheres for nickel, 2,600 atmospheres for nickel‐steel, and, in the case of meteoric iron, 3,600 atmospheres. The results indicate that pressure has practically no effect on the inversion‐point, although the possibility of a slight decrease is not excluded, as there was a slight tendency toward depression, especially in the case of nickel‐steel and meteoric iron.On the whole, it seems a fair inference that the pressure‐coefficient of the magnetic inversion‐point remains zero or negative even at the high pressures in the Earth's interior, and that consequently the permeability of the nickel‐iron core of the Earth is not significantly higher than that of ordinary rocks.