A large Leyden jar has about 1 nF of capacitance. Originally, the amount of capacitance was measured in number of 'jars' of a given size, or through the total coated area, assuming reasonably standard thickness and composition of the glass. Two plates separated by a small distance also act as a capacitor, even in vacuum. Early experimenters found without difficulty that the thinner the dielectric, the closer the plates, and the greater the surface, the greater the amount of charge that could be stored at a given voltage.įurther developments in electrostatics revealed that the dielectric material was not essential, but increased the storage capability (capacitance) and prevented arcing between the plates. Because of this, the fluid inside can be replaced with a metal foil lining. We now know that the charge is actually stored not in the conductors, but only in a thin layer along the facing surfaces that touch the glass, or dielectric, maybe leaking to the surface of the dielectric if contact is imperfect and the electric field is intense enough. Benjamin Franklin investigated the Leyden jar, and concluded that the charge was stored in the glass, not in the water, as others had assumed. It was initially believed that the charge was stored in the water. Soon it was found that it was better to coat the exterior of the jar with metal foil (Watson, 1746), leaving the (accidentally) impure water inside acting as a conductor, connected by a chain or wire to an external terminal, a sphere to avoid losses by corona discharge. The role of the outer plate was provided by the hand of the experimenter. The original form of the device was just a glass bottle partially filled with water, with a metal wire passing through a cork closing it. The inner and outer surfaces of the jar store equal but opposite charges. The jar is charged by an electrostatic generator connected to the inner electrode while the outer plate is grounded. A conducting foil is wrapped around the outside of the jar, matching the internal coated area. 4 The "dissectible Leyden jar" urban legendĪ typical design consists of a top electrode electrically connected by some means (usually a chain) to a metal foil coating part of the inner surface of a glass jar.
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