An aluminum disc about 10 cm in diameter is hung from the pan holder of a Welch triple beam balance that is sensitive to about a hundredth of a gram. A rectangular plate provides a guard ring for this circular disc. The disc and the guard ring are coplanar when the beam is balanced. A second, insulated, rectangular plate is positioned from 3 to 6 millimeters below the disc. Potential differences up to 4 kilovolts are applied between the grounded disc and guard ring and this insulated plate. A stop is placed so that the pointer of the beam cannot rise above the equilibrium position. To measure the change in force when the potential is applied, the weights are adjusted till the beam just falls away from the stop. The stop is necessary since the force between the plates varies with the square of the separation and therefore the beam would be unstable when the force was greater than the equilibrium value. In addition to measuring the force between parallel charge plates as a function of potential difference and separation, a dielectric can be placed between the two plates.