Measuring the torque of a sealed electrostatic voltmeter
E H W Banner · Journal of Scientific Instruments · 1936
A method is described of measuring the torque of an electrostatic voltmeter by measuring the capacitance C at different voltages at supply frequency. The capacitance can be measured for any voltage except zero, and from these figures are found values of dC/d θ, where θ is the deflexion, and so the torque is determined. When two air condensers are in series the voltage ratio is inversely proportional to the capacitances, neglecting leakage; if one of the condensers is an electrostatic voltmeter and the other a calibrated air condenser the capacitances are determined for given voltages across the voltmeter. Supply frequency is used, with special precautions to eliminate large possible errors due to stray capacitances and leakage. If the capacitance is measured at practically zero voltage by some bridge method the electrostatic efficiency may be determined in addition. An example is given.