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The low temperatures produced by the adiabatic demagnetization of a paramagnetic salt have been used to cool specimens of aluminium, cadmium, gallium and zinc, and their critical fields have been measured down to 0·1° K. The magnetic measurements on the metal and the salt were made independently and the experimental arrangement had the further advantage that the quality of the thermal contact between the metal and the salt could be studied. The results could be accurately expressed by relations of the form H c =H0(1-(T/T c )2); values of H0 and T c for each metal are tabulated.
Goodman et al. (1951) studied this question.