Measurement of small electrical forces using a torsion balance and capacitive position transducer

A method for measuring the force arising from the attraction between a charged electrode and another electrode on which charges have been induced has been developed using a torsion balance operated in the ac mode far above the resonance frequency of the balance. The movement of the balance is sensed by a capacitive position transducer which uses two lock‐in amplifiers to demodulate the position signal. The balance itself and the position transducer are described. The sensitivity of the position transducer is given, and results from three electrode geometries are discussed.