A DECAMETRIC SCALE INVESTIGATION OF THE GRAVITATIONAL CONSTANT

Gravity measurements were made simultaneously at two locations at the top reservoir of the Ffestiniog pumped-storage power station in N. Wales, one above and one 'below' the water, as the water level changed. Measurements were made for eight weeks, using digital recording of LaCoste & Romberg earth-tide gravimeters fitted with electrostatic feedback. The reservoir geometry was well defined, the water level measured accurately, and the water density known, so we were observing the gravitational effect of accurately known slices of water, having three effective (weighted) distances from the gravimeters in the range 26-94 m. The gravity signal due to the Moon and Sun (the Earth tide) was removed by two methods: by subtracting a theoretical value, and by differencing the records of the two gravimeters. Assuming the Newtonian inverse-square law, the results give values for gravitational constant G differing from the laboratory value by only about (0.1 +/- 0.2) per cent, so there is no evidence for any 'fifth force' deviation from Newtonian gravity. The main contributions to the uncertainty come from the uncertainties in the gravimeter calibrations and positions, and in the reservoir geometry.