The Leyden jar, or Leiden jar, is a device that "stores" static electricity between two electrodes on the inside and outside of a jar. It was invented independently by German cleric Ewald Georg von Kleist on 11 October 1744 and by Dutch scientist Pieter van Musschenbroek of Leiden (Leyden) in 1745–1746. The invention was named for this city. It was the original form of the capacitor. The Leyden jar was used to conduct many early experiments in electricity, and its discovery was of fundamental importance in the study of electricity. Previously, researchers had to resort to insulated conductors of large dimensions to store a charge. The Leyden jar provided a much more compact alternative.
A typical design consists of a glass jar with conducting metal foil coating the inner and outer surfaces. The foil coatings stop short of the mouth of the jar, to prevent the charge from arcing between the foils. A rod electrode projects through the mouth of the jar, electrically connected by some means (usually a chain) to the inner foil, to allow it to be charged. The jar is charged by an electrostatic generator, or other source of electric charge, connected to the inner electrode while the outer foil is grounded. The inner and outer surfaces of the jar store equal but opposite charges.
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 role of the outer plate was provided by the hand of the experimenter. 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. It was initially believed that the charge was stored in the water. Benjamin Franklin investigated the Leyden jar, and concluded that the charge was stored in the glass, not in the water, as others had assumed. The charge leaks to the surface of the dielectric if contact is imperfect and the electric field is intense enough. Because of this, the fluid inside can be replaced with a metal foil lining. Early experimenters found that the thinner the dielectric, the closer the plates, and the greater the surface, the greater the charge that could be stored at a given voltage.
Further developments in electrostatics revealed that the dielectric material was not essential, but increased the storage capability (capacitance) and prevented arcing between the plates. Two plates separated by a small distance also act as a capacitor, even in vacuum.
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. A typical Leyden jar of one pint size has a capacitance of about 1 nF.
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