Applications of a zero-frequency superconducting inductance bridge

An inductance bridge has been constructed which operates at zero frequency to compare superconducting inductors. The bridge uses a point contact SQUID as a current null detector. The unknown and standard inductors are at liquid helium temperature, but the bridge is balanced by resistance adjustments at room temperature. The present bridge can detect a change of 1 part in 10⁴in a μH inductance and has various advantages over ac bridges. The bridge can be applied in a number of ways. Currently we are using it to determine the kinetic inductance, penetration depth and critical current of thin film circuits. The bridge is particularly adapted to measuring the quantum phase difference vs current of current-dependent inductances. Susceptibility and precision measurements of inductance are other possible applications. Although the bridge was developed to measure small inductances, the principle can be easily applied to larger inductances and may be useful in large scale applications.