Contact Electricity

Abstract

IF a Volta's condenser be formed of an iron and a copper plate having their surfaces of contact well ground together, it is found that, on placing them together and then separating them, the iron acquires a positive charge and the copper a negative. This occurs so long as the atmosphere surrounding the plates is the ordinary one containing watery vapour and other oxygen compounds. But if the atmosphere contain sufficient hydrogen sulphide, the iron will be found negatively and the copper positively electrified. Sir Wm. Thomson has shown that “a metal bar insulated so as to be movable about an axis perpendicular to the plane of a metal ring made up half of copper and half of zinc, the two halves being soldered together, turns from the zinc towards the copper when vitreously electrified, and from the copper towards the zinc when resinously electrified”.