A Sensitive Modification of the Quadrant Electrometer: Its Theory and Use

Abstract

New Quadrant electrometer of high sensitivity, with tilted needle and movable quadrants. Principle. If the needle is given a slight tilt about its long axis and one pair of quadrants is raised or lowered a small distance with respect to the other pair, electrical control forces are set up which add to or subtract from the restoring torque from the suspension. The theory of these forces is discussed, the equation giving the sensitiveness in terms of the potential of the needle and the geometrical arrangement of the quadrants is derived and the method of utilizing electrostatic control to the best advantage is described. Advantages: High sensitivity, nearly independent of the deflection, and quick adjustment of sensitivity through a great range. Results. By using a small needle, 4.5 mm. in radius, with a slight tilt, sensitivities as high as 60,000 mm. per volt have been obtained. Adjustments are difficult at these extreme sensitivities, but the electrometer can usually be used up to 15,000 mm. per volt without undue trouble. Practical suggestions are given regarding needles, suspensions and mirrors.