Abstract: A method and apparatus for detecting magnetic fields in which a string (1) carries an alternating current. The displacement of the string from its unperturbated position due to a magnetic gradient across the string is detected by pick-up coils (L1, L2). The signals from the pick-up coils are demodulated using a signal derived from the alternating current to produce an output.

Abstract: Apparatus for measuring gravitational fields comprising a superconducting string (1) fixed at both ends and forming part of a closed superconducting loop inductively coupled to two driving solenoids (L.sub.d1, L.sub.d2). Displacement of the string in response to a gravitational field is sensed by two magnetic flux transformers each comprising a signal coil and two pick-up coils ((L.sub.p1, L.sub.p2). Pairs of pick-up coils lie in two perpendicular planes providing two independent channels of measurements. The two arms of each flux transformer are balanced to convert only the amplitudes of the string's antisymmetric natural modes into an output voltage. The output voltage of each channel is used to produce a feed-back current distribution (L.sub.y1, L.sub.y2) proximate and parallel to the string.