Capacitor : Electric Field :: Leyden Jar : ?

A capacitor stores energy in the electric field that builds up between its two plates; the field is what keeps the charge separated. A Leyden jar, which is an early form of capacitor, stores charge on the inner and outer surfaces of a glass tube coated with metal. The key idea is that the jar’s glass acts as a dielectric that allows charge to be held on the metal layers while keeping the charges physically separated. Thus the analogue of “electric field” for a Leyden jar is the “dielectric charge flow,” the movement of charge onto the glass’s inner and outer surfaces. For example, when a Leyden jar is charged, electrons accumulate on the inner metal coating, leaving a positive charge on the outer coating, just as a capacitor’s plates hold opposite charges.