On the significance of charge decay measurements in insulators

The significance of measurements of surface voltage decay on insulators is discussed. In the configuration discussed, the sample rests on a conducting, grounded plate, and its floating interface is charged by rubbing it against a different material or by exposure to a DC corona discharge, which proves to be the most reproducible charging technique. The charging source is removed to avoid back-discharge, and the potential of the free, charged surface is measured by a noncontacting probe, preferably of the vibrating type. The potential of the probe is feedback-monitored in such a way that the potential adjusts itself to that of the sample surface, so that the field between sample and probe is kept negligibly small. The case of homogeneous samples is considered first. So-called local models applicable to electrically inhomogeneous samples, i.e., samples in which the injected charge density exceeds the intrinsic charge density, are then presented