| A liquid-supported torsion balance (LSTB) has been developed which is designed to measure variations in the curvature of the gravity level-surface (i.e., in the "horizontal gravity gradients"). The historical background of LSTB's and gradiometers is reviewed. The theoretical background for an understanding of the essential features of LSTB's is laid. The LSTB is contrasted with the classical fiber-supported torsion balance. The substitution of an electrode array for electrostatic centering and torquing in place of the traditional fiber eliminates many problems associated with fibers and provides an easily-adjustable torsion constant. The apparatus developed is described in detail, with specific reference to temperature-control and liquid-purity issues. We currently obtain temperature stability of (+OR-) 1/2 (mu)(DEGREES)C over a week, with drifts (in the horizontal temperature gradient) as low as 3(mu)(DEGREES)C/hour. The optimal liquid to use in a LSTB is one with a maximum density point, which will minimize the thermal convection currents. In our case the liquid chosen is water. The purification of the water, especially the maintenance of the purity of the free surface, is discussed in detail. The previously-inevitable corrosion of aluminum (an otherwise optimal float material) in water, which seriously compromised the purity of the water, has been studied and its rate reduced by over two orders of magnitude. This allows continuous operation now for at least eight months. Detailed noise analyses are presented and all noise sources are shown to be tractable or small. The noise due to thermal convection currents is presently believed to be the limiting noise source. Progress has been made toward its minimization. Preliminary data are presented and discussed. Plans for future improvements are given. |
