A vacuum balance that compares the weights of 10-kg stainless-steel masses suspended in evacuated tubes at different levels in a hydroelectric reservoir is being used to measure the gravitational attractions of layers of lake water up to 10 m in depth. The mean effective distance between interacting masses in this experiment is 22 m, making it the largest-scale measurement of G using precisely controlled moving masses. The experiment extends laboratory-type measurements into the range previously explored only by geophysical methods. Assuming purely Newtonian physics the value of the gravitational constant determined from data obtained so far is G=6.689(57)×${10}^{\mathrm{-}11}$ ${\mathrm{m}}^3$ ${\mathrm{kg}}^{\mathrm{-}1}$${\mathrm{s}}^{\mathrm{-}2}$, which agrees with laboratory estimates. The data admit at a 0.6 standard deviation level the parameters of non-Newtonian gravity inferred from geophysical measurements in mines and a tower. These measurements push the estimated ranges of non-Newtonian forces down to a scale accessible to our reservoir experiment, so that experimental improvements now at hand may provide a critical test of non-Newtonian effects.

• Received 4 April 1988

DOI:https://doi.org/10.1103/PhysRevD.38.1023