Mechanical measurements by capacitive means

An electrode placed above a surface can be used to measure the distance to the surface by the capacitance of the intervening air gap. The measurement frequency is selected to meet the required precision. The capacitance variations are converted into voltage fluctuations measured at the selected passband. The fluctuations are compared to a reference signal obtained by a method of zero or the signals of a calibrated electrode. Laboratory accuracies of one part in 100 million have been obtained, while field data has reached one part in 10,000. The capacitance measurements have been made at picofarad levels, with the voltage fluctuations being on the order of 10-10,000 V/pF. Sample data are provided for surface irregularities, the distance between interferometer facets and the displacements of a mechanical oscillator.