The precision measurement of capacitance
N F Astbury, L.H. Ford · Proceedings of the Physical Society · 1939
The paper describes a method for the measurement of capacitance in terms of mutual inductance and resistance. The circuit is developed from the Carey-Foster bridge, and a combination of two bridge circuits used successively leads to the elimination of small correcting terms. The power factor also of the condenser under test is determined. The values of capacitance and power factor are deduced entirely in terms of two readings of a mutual inductance and two readings of a resistance box. The screening of the bridge, including the mutual inductance, has been carried out in great detail, and the whole circuit is enclosed in an earthed screen within which certain of the components are again screened independently. The effects of stray capacitances within the network and between the network and earth are thus reduced to negligible proportions. The equipment is designed for measurements on capacitances ranging from 1000 μμF. to 2 μF. at frequencies ranging from 50 to 3000 c./sec., and the probable errors are estimated as 5 parts in 10 5 on capacitance and 0.00005 on power factor at 1000 c./sec. At higher frequencies and for capacitances outside the stated range, the accuracy may be halved. The circuits are very adaptable and the bridges, as finally constructed, can be used also for the measurement of self-inductance and other quantities.