Susceptibility Servo-Balance for Measurements on Metals of Extremely Low Resistivities

Abstract

Although a great variety of methods have already been described for measuring magnetic susceptibility, the present instrument has the advantage that it makes possible measurements on weakly magnetic high‐purity metals having extremely small resistivities. Most servo‐balances for measuring susceptibility that have been described in the literature have been for strong paramagnetics or ferromagnetics and in none of the balances has the effect due to strong induced eddy currents been allowed for. The present instrument consists of a sensitive electrodynamic balance which is operated as a null instrument by allowing it to form part of a servomechanical network. The feedback system provides a stiffness of balance movement of 8×10⁴ d/deg deflection. The sensitivity of this instrument is such that changes of 10⁻⁹ emu/g may be detected in the susceptibility of large metallic samples having electrical resistances less than 10⁻⁹ ohm cm.