Instrumentation and measurements

The calibration of capacitors at the National Physical Laboratory in Britain is discussed in a paper by Rayner and Ford.¹ Their primary standard of reactance is a standard inductor which is used to calibrate capacitors with a Carey-Foster Bridge. Then a precision Schering Bridge is used for comparing capacitors with an error of ± 1 in 10,000 and ± .00002 in power factor. L. H. ford² has measured the effect of humidity on the capacitance end power factor of precision air capacitors and notes that changes in capacitance of 3 in 10,000 are noted at relative humidity of 65%. This is more than the amount predicted by the change in the dielectric constant of air by the water vapor. The power factor also increases at relative humidity above 40% reaching 0.00008 at 65% R.H. The measurements were made so as to eliminate the effect of insulators. New standards of very small capacitance from 0.001 to 100 F were developed by Moon and Sparks³ in which they placed the measuring electrode at the bottom of a well with the guard electrode surrounding it and much closer to the opposite electrode.