(grād’ ē om’ ∂t ∂r) A device for measuring a component of the gradient of a potential or electromagnetic field. 1. An arrangement of two magnetometers (or gravimeters), one above the other, so that the difference in their readings is proportional to the vertical gradient of the magnetic (gravity) field, or of magnetometers on opposite wingtips and in a tail stinger of an aircraft, so that the difference in their readings is proportional to the horizontal gradients of the magnetic field, or some similar arrangement. The direction with respect to the Earth’s magnetic field is a factor in interpretation. 2. A three-arm torsion balance that is sensitive to gravity gradients but not to curvature. 3. Borehole gravimeter (q.v.).