Eddy current measurements on ultrapure molybdenum and rhenium

An extension of the well-known eddy current method for measuring the resistivity ρ of low-resistivity specimens is described. The induced voltage in a pickup coil around a cylindrical sample is given, for a sudden field change, as V(t)∝∑n=1∞exp[−λn2α(ρ)t]. The usual procedure requires the measurement of V(t) for values of the time t such that only the first term in the sum need be considered. For low-resistivity metals, this method requires a long observation time (several seconds) and a low-noise high-gain amplifier. By including the first M terms in this expansion, the amplifier requirements and the observation time may be reduced. Resistivity data with an estimated error of less than 5% have been obtained for several ultrapure samples of Mo and Re at 4.2 K.