English: Drawing of a Leyden jar from a 1913 physics book. A Leyden jar is an antique piece of physics apparatus that can store electric charge; it was the first form of capacitor. It consists of a glass jar coated on the inside and outside with metal foil. The foils stop well short of the mouth of the jar so that the charge can't arc between the foils through the mouth. A brass rod electrode pierces the wooden stopper and extends into the jar, and from it hangs a metal chain which makes contact with the inner foil. The jar is charged by grounding the outside foil and applying electricity from an electrostatic machine to the electrode.