The precise measurement of capacitance

The method described is designed for industrial application and its purpose is to enable capacitance to be conveniently measured with a precision approaching that achieved by the National Physical Laboratory¿on whom reliance is placed as the standardizing authority. Advantage is taken of the fact that resistance and frequency standards of great stability can now be produced, and all measurements are referred to such standards. The apparatus essentially comprises a bridge network with inductively coupled ratio arms which may be switched into the form of either a ¿comparison¿ or a Wien bridge. A novel feature is the combined use of the comparison bridge with a 2 : 1 ratio and the Wien bridge with a 1 : 1 ratio, whereby the phase-angle of the frequency-dependent arms is made 45°. This enables the unknown capacitance in one arm to be determined in terms of the resistance in the other arm at a given frequency, the values of the other capacitance and resistance not being required. In this way capacitor standards having values ranging from 0.01 to 1.0 ¿F may be precisely calibrated at 1 kc/s and 2 kc/s. The comparison bridge then enables any capacitor within the range 100 ¿¿F¿1 ¿F to be measured in terms of one or more of these capacitor standards with an accuracy of about ±100 parts in 106, or ±0.1 ¿¿F. A simplified version of the Wien bridge is proposed which is specifically designed to calibrate a standard capacitor of value 0.1 ¿F at 1 kc/s in terms of a standard resistor having a nominal value of 3 180 ohms.