Test mass for gravimeters and gradiometers

In an apparatus which measures one of absolute gravity or a gravity field gradient, comprising a chamber, an electrically conductive 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, wherein:

• the conductive test mass has a noncontiguous surface configuration which disrupts the natural path of electrons in the test mass caused by eddy currents resulting from movement of the test mass through a magnetic field and which causes the induced eddy currents to produce a magnetic field in a direction other than in the freefall direction of the test mass to thereby avoid attracting or repelling the test mass during freefall.

A method of measuring one of absolute gravity or a gravity field gradient while avoiding inaccuracies arising from attractive and repulsive forces from nearby electromagnetic fields on a freefalling test mass of an apparatus which measures one of absolute gravity or a gravity field gradient by freefalling the test mass in a chamber while a retro reflector connected to the test mass reflects a laser beam projected into the chamber from an interferometer to obtain information indicative of the absolute gravity or the gravity field gradient, wherein the method further comprises:

• using a test mass formed substantially of non-conductive material that substantially inhibits eddy current conduction resulting from movement of the test mass through a magnetic field to avoid any induced eddy current-induced magnetic field on the test mass during freefall;
• and simultaneously
• using the same test mass that also has a conductive layer 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 while in freefall.

A method of measuring one of absolute gravity or a gravity field gradient while avoiding inaccuracies arising from attractive and repulsive forces from nearby electromagnetic fields on a freefalling electrically conductive test mass of an apparatus which measures one of absolute gravity or a gravity field gradient by freefalling the test mass in a chamber while a retro reflector connected to the test mass reflects a laser beam projected into the chamber from an interferometer to obtain information indicative of the absolute gravity or the gravity field gradient, wherein the method further comprises:

• using an electrically conductive test mass with a noncontiguous surface configuration which disrupts the natural path of electrons in the test mass caused by eddy currents resulting from movement of the test mass through a magnetic field and which causes the induced eddy currents to produce a magnetic field in a direction other than in the freefall direction of the test mass to avoid attracting or repelling the test mass during freefall.