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English: A Leyden jar, shown sectioned through its center to reveal construction, from a 1914 radio book. The Leyden jar was an antique electrical component which stored electric charge; the first form of capacitor. It consisted of a glass jar with metal foil cemented to the inside and outside surfaces. A nonconductive stopper covered the mouth, through which a brass electrode passed, ending in a chain which made contact with the inner foil. The jar was charged by touching the central electrode to a source of high voltage static electricity such as an electrostatic machine. The foil stopped short of the jar mouth to prevent the jar from discharging by arcs between the inner and outer foil through the mouth.