A new method for recording minute changes of capacitance
W D Oliphant · Journal of Scientific Instruments · 1937
The paper describes a method for highly magnifying and for recording oscillographically high-frequency minute transient changes of capacitance, and consists in using the capacitance change to tune a simple oscillatory circuit through its resonant point. This circuit is loosely coupled to another oscillatory circuit tuned advantageously to some point on the side of its resonance curve and the effect of the former circuit is to alter the effective impedance of the latter according to a definite law, and this is indicated by means of a valve voltmeter, which indication may, for example, be recorded on an electromagnetic or cathode-ray oscillograph, a direct current valve amplifier being incorporated to obtain the necessary current. The circuits are excited by a valve oscillator which is loosely coupled to the valve voltmeter oscillatory circuit. While the method only has been used for recording transient fluid pressures, it has nevertheless a wide field of useful applications where measurement or observation of small changes of capacitance is important. The method facilitates the recording of any physical or mechanical phenomenon which can conveniently be adapted to produce a small change of capacitance of the order of a few micro-microfarads.