Abstract: A gravity meter comprises a casing, a vacuum tube mounted in the casing in a vibration-free manner, a sensor mechanism mounted within the vacuum tube, the sensor mechanism comprising two masses of different size acting on the respective arms of a beam balance comprising a material whose shape is arranged to change in response to changes in gravity and whose shape can be restored by the application of an electrical current thereto, and detector means arranged to provide from the restoring current an output representative of changes in gravity. The material is preferably a piezoelectric material.