Page:Electricity (1912) Kapp.djvu/119

Rh surface, as is the case with a charged conductor; hence, if we touch a particular point of the electrified part, we take off a little of the charge at that point, but the rest of the rod remains charged. We thus see that a charge can reside on the surface of an insulator, and it can be proved experimentally that the charge even penetrates a little way into the body of the insulator.

If a Leyden jar or a paper condenser be discharged and left standing a little while, a second though very much weaker discharge may be taken from it. It is as though the electricity had soaked into the body of the dielectric and some of it was thus prevented from getting out at the first discharge. When the jar has apparently been completely discharged, there is still left a small residual charge, which slowly leaks out on to the surface of the glass and is then ready to produce a second discharge spark. If the dielectric is mica, this phenomenon of soaking is much less pronounced, and if air is used as a dielectric, it is absent.

A very striking experiment may be made to show that the charge in a Leyden jar does not reside on the surface of the metal coatings, but on the surface of the glass. Imagine a metal