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An accurate method for determining the surface charge density on foil electrets is described. The method is based on the measurement of an induction charge on a capacitive probe parallel to, and at a certain distance from, one of the charged surfaces of the electret. The method does not interfere electrically or mechanically with the electret charges. It is simple to implement and capable of measuring surface charge densities from 5×10−11 to some 10−6 C/cm2. For a surface charge density of 10−8 C/cm2, the reproducibility error is less than 1% and the systematic errors are estimated to be of the same order.
Sessler et al. (1971) studied this question.