Measurement of Volume and Resistivity Changes in High-Pressure Experiments by an A-C Inductive Technique

A coil wound on a sample core subjected to high pressures may be used to measure compressibility and electrical resistivity of the core. In a first approximation, the coil reactance reflects core volume, and the resistance reflects core resistivity. The exact impedance of the coil in the general case depends on the ratio of skin depth to sample size. With large skin depths, the sensitivity to volume changes is high, and with small skin depths, sensitivity to resistivity is high. Operation in these two cases was demonstrated for four materials of widely different hardness: barium, bismuth, titanium, and tungsten. In addition, operation was demonstrated at high temperatures by detection of the alteration of pyrophyllite to coesite plus kyanite. Permeability as a function of pressure was shown for an iron-nickel alloy. The chief advantage of the coil method is the remote, nonmechanical measurement of volume changes. In addition, it offers greater versatility to favor many different types of sample material.