beaker into which fits a glass beaker and into that a second metal beaker. The latter may have a stem and discharge knob much in the same way as an ordinary Leyden jar, the only difference being that the inner and outer coatings are not pasted on to the glass, but are removable. If, after charging this jar, we take out the inner metal beaker by insulated tongs, and also remove the glass beaker from its outer envelope, we have completely dissected the jar. The two metal coatings may be handled, the glass may be picked up by the hand touching the outside, and yet when we put the Leyden jar together again, we find that it still contains the original charge, less a certain unavoidable leakage, since no insulator is perfect. We thus see that any body, whether insulator or conductor, may hold a static charge on its surface.
Why then do we make the electrodes of an electric machine, such as that diagrammatically represented by Fig. 5, of brass and not of glass? For the simple reason that a glass electrode, although quite capable of holding a charge, is very ill-adapted for receiving it. The charge must be conveyed to it by a wire, and from the point where the wire joins it the charge must be able to flow to all points of the surface.
