Electrification following the contact of solids

The charge found on solids after they have been rubbed together can only be a direct consequence of friction when the rubbing is vigorous; more usually, the charging is the result of contact, rubbing complicating the phenomenon. Opinions differ regarding the origin of contact charging, but certain experiments suggest a working hypothesis, according to which the charging is due to electron transfer when both materials are either semiconductors or metals, but to ions when insulators take part. Electron transfer is associated with the setting up of a contact potential. Ion transfer can be caused either by a potential difference, in which case the current flows with the e.m.f., or it can be caused by forces that are not electrical in nature, in which case the current flows against the potential difference generated by the ‘mechanical transfer’ of charge.