T VIE apparatus described in this paper was designed for the measurement with high precision of the static magnetic susceptibility of weakly dia-and paramagnetic metals and alloys in the temperature range of approximately 4.2-300 ° K. It was designed primarily for a study of effective Bohr magneton values of solutes in the Group IB metal solvents. In its present form the apparatus is capable of the determination of the susceptibility of solids, including high conductivity metals, with mass susceptibilities of approximately 10 -7 c.g.s.e.m.u, per gram with a precision of better than + 0.5 per cent in the range 6-300 ° K. However, many of its features could be applied to the determination of much larger susceptibilities. Measurements have been made in the range 6-300 ° K of the mass susceptibility of polycrystalline samples of copper with purities ranging to 99-9999 per cent. The purest samples do not show the tendency towards paramagnetism at low temperatures which all previous measurements have shown, and this tendency is attributed to ferromagnetic impurities, the concentration of which has been determined to +0.05 p.p.m. (wt) by carrier-distillation emission spectroscopy.
The apparatus is a modified low temperature version of the Faraday device described by Garber et al. 1 (hereafter referred to as I). In principle it is not fundamentally different from others described in the literature, 2-4 but it is rather more sensitive: the minimum detectable change in force on the sample is approximately 1.5 x 10 -a dyn and this corresponds to a minimum detectable change in mass susceptibility of approximately 4 x 10 -12 c.g.s.e.m.u, per gram.