Sir Ambrose Fleming's method of electrification and Alfred Coehn's electrostatic experiments

Silica flour poured from a glass beaker or a copper trough carries negative charges away. The silica particles acquire these charges while in contact with glass or copper walls at room temperature. With the sole exception of what is known as "waterfall electrification", cases of electrification through friction or through impact are in fact cases of contact electrification. The sign of the charges separated depends on the work function and the solution pressure of the metal with respect to the dielectric. The action of rubbing an insulator against a metal, for instance, silica in bulk against copper in bulk, can lead to an inversion of the sign of the static charges acquired; the silica can assume a positive charge. This is ascribed to the dependence upon temperature of the migration of positive ions from the metal to the insulator. The action of rubbing thus has two functions: firstly it helps to establish intimate contact between two solids, and secondly, in the atmosphere it abrades particles from noble metals. In addition, in the case of less noble metals the rise of temperature of the surface irregularities increases the ability of the surface of the dielectric to accept positive ions.