the plates and the distance between them vary inversely. When the faces of the two plates are in contact, that is, are separated by molecular distances, these charges become very great. Such an arrangement of equal and opposite charges, distributed over the surfaces of two bodies in contact and separated by a distance comparable with the distance between the molecules, was called by Helmholtz an electrical double-sheet. It evidently presents some analogies to the magnetic shell.
The charges making up the double-sheet cannot be detected by separating a plate of zinc from a plate of copper with which it has been in contact and examining the separate plates, because the separation cannot be effected so uniformly that no discharge takes place between the two bodies. If, however, those faces of the zinc and copper plates which are contiguous be insulated from each other by a thin layer of shellac and contact made between the plates by means of a metallic wire, so that a difference of potential is set up between them, on removal of the wire and separation of the plates they are found to possess charges of considerable magnitude.
We may explain in this way electrification by friction. We may assume that the two bodies rubbed together acquire different potentials by contact; the friction forces large areas of their surfaces into close proximity, and the charges upon those surfaces become very great; because the bodies ordinarily used for producing electrification by friction are nonconductors, the charges on their surfaces are not recombined as the bodies are separated, so that each body retains a large free charge.
A similar electrical double-sheet will exist on the surfaces of contact between a liquid and a metal. An arrangement by which the effects due to this double-sheet may be observed was invented by Lippmann. It consists of a vertical glass tube drawn out at its lower end in a capillary tube. The capillary tube dips into dilute sulphuric acid, which rests on mercury in the bottom of the vessel containing it. Mercury is poured into the vertical tube until its pressure is such that the capillary portion of the tube is nearly filled with it. When the mercury in the vessel is joined with the
