The measurement of true contact charge density using soft rubber

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Abstract
When a metal and an insulator touch and become charged, the total charge on one surface divided by the apparent area of touching is not generally equal to the true charge density, but is less because the area of true contact is a fraction of the apparent area. The paper reports an experiment to determine the true contact charge density by contact measurements between a metal and an insulator in the form of a very soft rubber. The rubber is able to deform elastically around the asperities on the surface of a metal, so there is intimate contact between the two. For single contacts of smooth spheres on smooth flats of metal and rubber the true charge density on the rubber is independent of the metal work-function and is -(15+or-5) nC cm-2, i.e. equivalent to about 1 electron per 104 atoms of surface.

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