The Leyden jar was the original capacitor.
Description
The device was a glass jar coated inside and out with metal. The inner coating was connected to a rod that passed through the lid and ended in a metal ball. Typical designs consist of an electrode and a plate, each of which stores an opposite charge. These two elements are conductive and are separated by an insulator (e.g., the glass dielectric). The charge is stored at the surface of the elements, at the boundary with the dielectric.
History
Leyden jars, Museum Boerhave, Leyden
The ancient Greeks used balls of amber on spindles that they rubbed to generate sparks. This is the triboelectric effect, mechanical separation of charge in a dielectric. Their work was a precursor to the development of the leyden jar.
Around 1650, Otto von Guericke built a crude friction generator - a sulphur ball that rotated at high speed on a shaft. When Guericke held his hand against the ball and turned the shaft quickly, a static electric charge built up. In 1745, another German, Ewald Jürgen Georg von Kleist, found a method of storing this charge. He lined a glass jar with silver foil, and charged the foil with a friction machine. Kleist was convinced that a substantial charge could be collected when he received a significant shock from the device.
In 1746, Pieter van Musschenbroek of Leiden, Netherlands, independently made the same discovery. The device is known as a "Leyden jar" because Musschenbroek made the storage jar known to the scientific world. The jar was invented at Leiden University in the Netherlands.
The physicist James Clerk Maxwell invented the concept of displacement current, dD/dt, to make Ampere's law consistent with conservation of charge in cases where charge is accumulating, for example in a Leyden jar. He interpreted this as a real motion of charges, even in vacuum, where he supposed that it corresponded to motion of dipole charges in the ether. Although this interpretation has been abandoned, Maxwell's correction to Ampere's law remains valid (a changing electric field produces a magnetic field). The displacement current must be included, for example, to apply Kirchhoff's current law to a Leyden jar.
