Improvements in the precision measurement of capacitance

All measurements of capacitance and resistance at the N.P.L. depend ultimately upon the Campbell standard of mutual inductance and upon frequency. Hitherto capacitance has been obtained from mutual inductance and resistance using a Carey-Foster type of network. The precision obtained with this in the Astbury bridge has been considerably improved since it was constructed 20 years ago, and an accuracy of repetition of about five parts in 106 is now obtained. Capacitance can also be derived from resistance and frequency; a Wien bridge has recently been constructed for this purpose. It has been designed to make a single precise measurement of capacitance (the repetition accuracy is two parts in 106) and thereby to complete a system of three measurements relating capacitance, resistance and mutual inductance in conjunction with frequency.