An unusually sensitive modulated-source Eotvos experiment will be conducted next to a large lock on the Snake River in eastern Washington to put limits on the existence of an intermediate-range material-dependent "gravitational" force. A prototype experiment conducted by the author during the summer of 1988 placed limits on a positive "Fifth Force" effect. It is important to settle this possibility with a definitive measurement. A portable laboratory already assembled at Yale University and already used in the prototype experiment will be moved to the Little Goose Lock during the summer of 1990 for further measurements. Improvements in the pendulum design will make the experiment at least one order of magnitude more sensitive than the prototype experiment and it is expected that the new measurements will set definitive limits on the existence of this effect.
