observation originally made and published by Volta, namely, that the electric force between any two metals in the series is equal to the sum of the electric forces between all intermediate pairs. Thus, if in any arbitrary scale the electric force between zinc and lead is , and that between lead and copper , then the electric force between zinc and copper is .
The series given above ends with the most negative metal—gold; but Volta found that another substance, not a metal, but graphite, which is a special form of carbon, is still more negative than gold, and since Volta's time the series has been enlarged and extended by the addition of other metals and also sulphates and oxides, so that we must consider the phenomenon of electrification by contact to extend over a great variety of substances, and not to be restricted to a combination of metals.
Whether electrification is produced by friction or by contact, the process is in either case the separation of charges of electricity of opposite sign. We know that such charges attract each other, and that if accumulated on conductors sufficiently near, the conductors themselves will experience an attracting force.
