Simple method for checking the balance of gradiometers used with SQUIDs

A simple technique for determining the degree of balance of the external pair of oppositely wound sensing coils of a superconducting flux transformer before the connection of the flux transformer to a SQUID is presented. The technique involves the placing of the center of the gradiometer near the central position of a Helmholtz coil excited by a sinusoidal generator and determining the displacement from the Helmholtz coil center at which no voltage is induced in the gradiometer. If there is even a small imbalance in the gradiometer, a considerable displacement would be required to obtain a zero induced signal condition, with a displacement of 1 mm implying an imbalance of 25 ppm in a Helmholtz coil of 20 cm average radius. The technique has been used to obtain real-time measurements of the degradation in gradiometer balance caused by thermal cycling.