A modified form of the magnetic balance previously designed by one of us, is described. In this apparatus the gradient is made independent of the magnetizing field. With such an apparatus measurements have been made on the variation of the intensity of magnetization with magnetic field for single-crystal specimens cut along appropriate crystal axes, both for the hexagonal close-packed and face-centred cubic cobalt. Such measurements enable the magnetic anisotropy constants to be determined. The nature of the phase change from close-packed hexagonal to face-centred cubic occurring at about 400° C is such that single crystals can be cut at room temperature, which will transform to give single crystals of known orientation in the face-centred cubic phase. The values of the magnetic anistropy constants for the hexagonal phase for various temperatures are compared with those of earlier determinations and fair agreement is found. Measurements on the face-centred cubic phase were made every 50° C in the temperature range 500 to 1000° C; at the latter temperature the material became magnetically isotropic. For face-centred cubic cobalt it was found that the crystal directions in order of increasing difficulty of magnetization are [111], [110] and [100], as is the case for nickel. Moreover, the value of the anisotropy constant k 1 obeys the same empirical law, giving the variation with temperature, as nickel. The similarity in the magnetic behaviour of these two ferromagnetics having the same crystal structure is thus evident.

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The paper describes how measurements of the spontaneous magnetization of pure specimens of single-crystal close-packed hexagonal and polycrystalline face-centred cubic cobalt have been made over the temperature range -183 to 1121° C. A single crystal of cobalt possessing a close-packed hexagonal structure was prepared by the process of slow cooling of the melt. Owing to the presence of a phase change in cobalt occurring c . 400° C when the metal changes from a close-packed hexagonal structure below 400° C to a face-centred cubic structure above 400° C, it was necessary to determine two critical rates of cooling to prepare a crystal. Magnetic test specimens of single-crystal cobalt, cut so that they were capable of being magnetized along the easy axis of magnetization (the hexagonal axis), and of polycrystalline cobalt, of approximate dimensions 4 x 2 x 2 mm., were studied using the magnetic balance devised by Sucksmith (1939). The spontaneous magnetization of the hexagonal form was accurately measured from — 183 to 431° C, this latter temperature being determined as the upper transformation temperature of the cobalt. Complete tables of these measurements are given. By extrapolation the value 162.5 5 was determined for the spontaneous magnetization of hexagonal cobalt at absolute zero of temperature. Measurements on the polycrystalline face-centred cubic material were made from 394° C (this temperature being determined as the lower transformation temperature) to the Curie temperature, determined by extrapolation to be 1121° C. The face-centred cubic material possessed greater magnetization than the hexagonal, and in the temperature range 431 to 950° C it was found to be dependent upon the heat treatment the specimen had received; the significant factor in this heat treatment being the speed with which the specimen was allowed to cool through the lower transformation temperature. The Curie temperature was not affected by this heat treatment. To determine the spontaneous magnetization at very high temperatures where no saturation was observed the extrapolation technique devised by Weiss & Forrer (1926) was used. This procedure is described. When reduced values of magnetization and temperature were calculated it was found that the hexagonal and face-centred cubic materials obeyed different reduced equations of state although excellent agreement was obtained when the reduced values for face-centred cubic cobalt were compared with similar values for nickel. These results given in detailed tabulated form show that the reduced magnetic equation of state is structure dependent.

Measurements on the temperature variation of spontaneous magnetization by both the magnetothermal and magnetic methods have hitherto been con6ned to nickel; the extension of these measurements, calhng for greater sensitivity, is discussed.To the direct method devised by Weiss and I'orrer has been added a direct current ampli6er, which allows a small magnetocaloric rise of temperature to be measured accurately. The original ferromagnetic balance devised by Sucksmith has been improved and now. is capable of wider application to measurements of magnetic intensity in 6elds which are due to either air core coils or electromagnets. The method is not complicated by the presence of any image effect so that measurements of do/dP in high 6elds can be made with considerable precision.Measurements on a copper-nickel alloy (27.5 percent Cu) have been made, and it is shown that the two methods give identical results except in the immediate neighborhood of the Curie temperature, where the thermal measurements give a steeper descent to the temperature axis with little "tail, " while the magnetic data indicate a higher Curie temperature with a signi6cant tail. Below the Curie temperature the linear relationship between the magnetocaloric rise of temperature and the square of the intensity magnetization is found as in nickel and iron; above this temperature there is pronounced curvature.Measurements have also been carried out on a mixed ferrite (MgO ZnO 2Fe203). Satisfactory agreement between the results by the two methods are obtained, both curves diGering markedly from the law of corresponding states for the ferromagnetic elements.

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