Leyden jar (lĪˈdən) [key], form of capacitor invented at the Univ. of Leiden in the 18th cent. It consists of a narrow-necked glass jar coated over part of its inner and outer surfaces with conductive metal foil; a conducting rod or wire passes through an insulating stopper in the neck of the jar and contacts the inner foil layer, which is separated from the outer layer by the glass wall. By modern standards, the Leyden jar is cumbersome and inefficient. It is rarely used except in laboratory demonstrations of capacitance.
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