A gradiometer is disclosed which has a pair of sensor bars 41, 43 supported in housings 45, 47. Transducers 71 are located adjacent the bars 41, 43 to detect movement of the bars in response to the gravity gradient tensor. At least one of the transducers 71 comprises a first coil 510 and a second coil 516 arranged in parallel and a switch 362 for proportioning current between the coils 510 and 516 so as to create a virtual coil at a position D between the coils 510 and 516.