The use of eddy currents for measurements of bulk modulus: Application to potassium

Abstract

The eddy current technique for the study of the resistivity of cylindrical conductors has been adapted for high precision measurements at the level of a few ppm. By using a mutual inductance bridge at frequencies where the skin depth is less than about one fifth of the radius of the sample, the asymptotic expansions for the complex susceptibility as a function of frequency may be used to obtain both the resistivity and the area of cross-section for samples measured under contactless conditions, and more important here, also their variation with pressure.Several normal metals and superconductors have been measured at liquid helium temperatures and at pressures of up to 150 bar in order to assess the reliability of the technique. A detailed discussion of these tests is given, and they suggest that it should be practicable to measure the initial bulk modulus of even stiff metals to an accuracy of 0.1% over the temperature range 4-300 K.The method has been applied to potassium, and to dilute K Rb alloys. The mean value of the initial bulk modulus of potassium at 4.2 K is BT (4.2 K) = 37.83 ± 0.07 kbar, which is consistent with ultrasonic and bulk compression measurements, but with a ten-fold reduction in the uncertainty: however, this value is inconsistent with the value inferred from measurements of the de Haas-van Alphen effect, at a level of more than five times their combined standard error estimates.