A gravity gradiometer is disclosed which has a sensor in the form of bars (41 and 42) which are supported on a mounting (5) which has a first mount section (10) and a second mount section (20). A first flexure web (33) pivotally couples the first and second mount sections about a first axis. The second mount has a first part (25), a second part (26) and a third part (27). The parts (25 and 26) are connected by a second flexure web (37) and the parts (26 and 27) are connected by a third flexure web (35). The bars (41 and 42) are located in housings (45 and 47) and form a monolithic structure with the housings (45 and 47) respectively. The housings (45 and 47) are connected to opposite sides of the second mount section 20. The bars (41 and 42) are connected to their respective housings by flexure webs (59). Transducers (71) are located in proximity to the bars for detecting movement of the bars to in turn enable the gravitational gradient tensor to be measured. A calibration sensor is provided for sensing whether the masses are balanced at room temperature so the balance of the masses can be adjusted by adjustable screws to balance the masses for cryogenic operation of the gradiometer. The calibration sensor comprises a resonant circuit (400, 410) and an oscillator (414). The resonant circuit includes a capacitor (400) which is formed by part of the sensor mass and a space plate (405).