Modulated-Source Eötvös Experiment at Little Goose Lock

A Cu-Pb torsion balance placed next to a large lock on the Snake River permitted setting limits on the coupling constant for a possible intermediate-range material-dependent gravitational force. A 1.7 × ${10}^8$-kg perturbing mass of water could be turned on or off within 12 min and had a well-known distribution. No evidence for a fifth force was observed. The experiment is primarily sensitive to isospin coupling, for which it yields limits on the intrinsic coupling coefficient ranging from ${\alpha }_0=(-2.2\mathrm{}\pm 4.6)\times {10}^{-3\mathrm{}}$ at $\lambda =10\mathrm{}$ m to ${\alpha }_0=(-0.5\mathrm{}\pm 1.0)\times {10}^{-3\mathrm{}}$ for $\lambda \ge 200$ m.