Electrometer arrangement with amplitude stabilized oscillator drive means for detector element

Claim 1

In an electrometer apparatus to measure unknowns such as the electrostatic potential of a surface in noncontacting manner or an electrostatic field, having a capacitive detector positionable in electrostatic coupling relationship with the surface or in the electrostatic field to produce a detector signal representative of the magnitude and polarity of the unknown being measured for long-term static measurement, the improvement comprising: an electromechanical resonant transducer having first and second terminals, operative to vibrate the capacitive detector at a predetermined frequency to vary the coupling relationship and produce modulated detector signals having a carrier frequency equal to the predetermined frequency, an amplitude stabilized reference oscillator circuit to produce reference signals at the predetermined frequency, a detector connected to receive the reference signals and modulated detector signals at a fixed phase relationship to demodulate the latter and produce an output signal indicative of the magnitude and polarity of the unknown being measured, said amplitude stabilized reference oscillator circuit including, a differentially connected operational amplifier having plus and minus inputs, the first terminal of the electromechanical resonant transducer being connected to the plus terminal of the operational amplifier, the second terminal being connected to ground, a light emitting source connected between the output of the operational amplifier and ground, and responsive to the output of the operational amplifier to supply a corresponding amount of energy to a light responsive element connected to the output of the operational amplifier, and a negative feedback connection connected between the light responsive element and the minus terminal of the operational amplifier.