same potential hold exactly the same quantity of electricity as if it were made of solid brass or lead or any other metal. Electricity at rest resides entirely on the outside of a metal conductor. We might reduce the thickness of a shell to any degree, and still the charge is not altered.
Now let us follow this fact to its logical conclusion. What will happen if we reduce the thickness of the shell to zero? If thickness has nothing to do with the capacity, then by reducing it to nothing at all we should not alter the capacity. In other words, the body which holds a charge need not be a conductor at all. Its conducting property is only necessary to let the charge distribute itself over the whole of its surface, in fact to get a charge on to it at all. If, however, the charge is not transferred to the surface from outside, but actually produced on the surface, then there is no need that the surface should be the surface of a conductor. This will be clearly seen if we reflect that a glass rod, although a very good insulator, may be electrified by rubbing. Its insulating property is a positive advantage, since we may hold one end in our hand and yet electrify the surface at the other end. The charge is not able to slip about freely over the whole
