Patent ID: 7451645

Claim:
In an apparatus which measures one of absolute gravity or a gravity field gradient, comprising a chamber, a test mass which freefalls within the chamber, a retro reflector connected to the test mass, and an interferometer which includes a laser beam which is projected into the chamber and reflected from the retro reflector during freefall of the test mass to obtain information indicative of the absolute gravity or the gravity field gradient, an improvement to the test mass which avoids inaccuracies arising from attractive and repulsive forces from nearby electromagnetic fields on the freefalling test mass, comprising in combination: a non-conductive material substantially forming the test mass, the nonconductive material inhibiting eddy current conduction resulting from movement of the test mass through a magnetic field to avoid any induced eddy current-induced magnetic field which attracts or repels the test mass during freefall; and a conductive layer on the nonconductive material which permits conduction of only substantially negligible eddy currents and which discharges any static electrical charge to avoid any induced electrostatic attraction or repulsion of the test mass during freefall.