When one insulating solid is rubbed along and perpendicular to an identical one they acquire charges, -ve and +ve, respectively. Continued rubbing so affects the surfaces of rubber and rubbed that their properties change, with reversal of sign to +ve and -ve respectively. The effect 2 is permanent, unless the surfaces be restored by heat treatment. Oblique impact of two identical solids generates on them unequal charges. The net charge is, in general, -ve.
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Peter Shaw (1926) studied this question.
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