A "battery" of four Leyden jars, Museum Boerhaave, Leiden [1]. Invented by Benjamin Franklin , derived from a "battery of cannon"

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Jars

The Leyden jar is an early device for storing electric charge invented in 1745 by Pieter van Musschenbroek (1692â€“1761). It was the first capacitor. Leyden jars were used to conduct many early experiments in electricity. more...

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Description

A 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. A conducting foil is wrapped around the outside of the jar, matching the internal coated area. The jar is charged by an electrostatic generator connected to the inner electrode while the outer plate 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. 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. Because of this, the fluid inside can be replaced with a metal foil lining. 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.

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 large Leyden jar has about 1 nF of capacitance.

History

The ancient Greeks already knew that pieces of amber could be rubbed, becoming electrified and attracting light particles. This is the triboelectric effect, mechanical separation of charge in a dielectric. It is why the word "electricity" was made from the Greek word Î·Î»ÎµÎºÏ„ÏÎ¿Î½ ("elektron", amber).