Sensitive detection of superconductivity by changes in inductance and losses of a resonant bridge

Abstract

THE method of Schawlow and Devlin I is at present the most commonly used for the detection of superconductivity. This method employs the change in selfinductance of a solenoid when a sample located inside the coil becomes superconducting. In order to increase the sensitivity, the solenoid is made a part of a parallel resonant circuit that controls the frequency of an oscillator, and changes in frequency corresponding to small changes in inductance are measured on a frequency counter. The method requires no physical connections to the sample, and irregularly shaped samples may be used. Frequency stability of the oscillator is a problem, however, and, when the solenoid is in a cryogenic bath being cooled by pumping, the oscillator frequency changes--probably because bubbling of the liquid affects the dielectric material surrounding the solenoid. In the method to be described, pumping effects are reduced considerably by a balanced-bridge technique. Also, sensitivity is increased by utilizing both the inductance change and the change in coil loss 2 making it possible to detect superconductivity in less than 1 mg quantities of material. While both of these effects have been noted in the literature, 1-4 they have not previously been used simultaneously for the detection of superconductivity.