Experimental tests of the gravitational inverse-square law for mass separations from 2 to 105 cm

We report two experiments which test the inverse-square distance dependence of the Newtonian gravitational force law. One experiment uses a torsion balance consisting of a 60-cm-long copper bar suspended at its midpoint by a tungsten wire, to compare the torque produced by copper masses 105 cm from the balance axis with the torque produced by a copper mass 5 cm from the side of the balance bar, near its end. Defining Rₑₓₚₜ to be the measured ratio of the torques due to the masses at 105 cm and 5 cm, and RNewton to be the corresponding ratio computed assuming an inverse-square force law, we find {δ}{≡}(Rₑₓₚₜ/RNewton-1)=(1.2 ±{}7)×{}10^-4. Assuming a force deviating from an inverse-square distance dependence by a factor [1+{ε} lnr(cm)], this result implies {ε}=(0.5 ±{}2.7)×{}10^-4. An earlier experiment, which has been reported previously, is described here in detail. This experiment tested the inverse-square law over a distance range of approximately 2 to 5 cm, by probing the gravitational field inside a steel mass tube using a copper test mass suspended from the end of a torsion balance bar. This experiment yielded a value for the parameter {ε} defined above: {ε}=(1±{}7)×{}10^-5. The results of both of these experiments are in good agreement with the Newton- ian prediction. Limits on the strength and range of a Yukawa potential term superimposed on the Newtonian gravitational potential are discussed.