A Change of Capacitance Method for the Measurement of Mechanical Displacements
Eric Bradshaw · Journal of Scientific Instruments · 1945
The paper describes a method for the measurement of changes of capacitance which was evolved for the determination of mechanical displacements. It is suitable for the measurement of static or alternating displacements and may be adapted for the investigation of any phenomenon with which a changing capacitance is, or may be, associated. The circuit includes two radio-frequency oscillators feeding a mixer valve. Variation in the frequency of one of the oscillators, and therefore in the frequency of the mixer output, is effected by the changing capacitance. A frequency-amplitude converter circuit of the phase-discriminator type is employed to produce from the mixer output a direct voltage which is proportional to the change of capacitance. The sensitivity of the design described is in excess of 1 mm. per 12 × 10 -16 F, using a cathode-ray oscillograph as an indicator, and 1 μ A per 8 × 10 -16 F, using a moving-coil meter as an indicator.