Leyden jar

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Original capacitor

The Leyden jar was the original capacitor, invented in 1745 by Pieter van Muschenbroek (1700–1748) and used to conduct many early experiments in electricity.

[edit] Description

Typical designs consist of a top electrode connected by some means (usually a chain) to the inner surface of the jar and an external conducting foil wrapping. The inner and outer surfaces of the jar store equal but opposite charge.

The original form of the device was a glass jar coated on the outside with metal foil and containing (accidentally) impure water that acts as a conductor, connected by a chain or wire to an external sphere. It was initially believed that the charge was stored in the water. Benjamin Franklin investigated the Leyden jar, and proved that the charge was stored on the glass, not in the water as others had assumed. Since the charge is actually stored on facing surfaces of the conducting elements separated by the glass, which forms a dielectric, the fluid inside can be replaced with a foil lining, this also connected to a rod that passes through the lid and ends in a metal ball. The charge is stored at the surface of the elements, at the boundary with the dielectric. The thinner the dielectric, and so the closer the plates, the greater the amount of charge that can be stored at a given voltage.

Further developments in capacitors 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 will also act as a capacitor, even in a vacuum.

Originally, the units of capacitance were in 'jars' and a jar is equivalent to about 1 nF.

[edit] History

A battery of four Leyden jars, Museum Boerhaave, Leiden [1]. Invented by Benjamin Franklin who also coined the name, derived from a battery of cannon

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. This invention went on to be known as the Leyden jar because in 1746, Pieter van Musschenbroek of the University of Leiden, Netherlands, independently made the same discovery. Musschenbroek made the storage jar known to the scientific world, hence the jar was named after Leiden, the home town of the university. Daniel Gralath was the first to combine several jars in parallel into a "battery" to increase the total possible stored charge.^[1][2]

[edit] Circuit theory

The scientist 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 aether. 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.

[edit] The "dissectible Leyden jar" myth

A popular, but misleading, demonstration of the Leyden jar involves taking one apart after it has been charged and showing that the energy is stored on the dielectric, not the plates. The first documented instance of this demonstration is in a letter by Benjamin Franklin. ^[3]

A Leyden jar is constructed out of a plastic cup nested between two snugly fitting metal cups. When the jar is charged with a high voltage and carefully dismantled, it is discovered that all the parts may be freely handled without discharging the jar. If the pieces are re-assembled, a large spark may still be obtained.

This demonstration shows that the charge has been transferred to the surface of the dielectric, and is not on the metal conductors. When the jar is taken apart, simply touching the cup does not give enough contact area to remove all the charge. The conductors normally provide this surface area.

When not properly explained, however, this is a myth. This behaviour is not typical of capacitors, and does not happen at lower voltages. In a typical capacitor, the charge is on the surface of the conductors. The transfer of charge to the dielectric in the above experiment results from the high voltages present when the conductors are separated from the dielectric, which redeposits charge onto the surface of the dielectric by means of a corona discharge. If the experiment were performed in an insulating fluid (such as mineral oil) instead of air, the effect would no longer be present.^[4]

[edit] References

1. ^ The Leyden Jar Discovered — World Wide School
2. ^ The term "battery" was coined by Benjamin Franklin, who likened it to a battery of cannon (cannons grouped in a common place). The term was later used for arrangements of multiple electrochemical cells, forming the device in common use today (see battery (electricity)).
3. ^ Letter IV: Benjamin Franklin to Peter Collinson, April 29, 1749 (Bigelow vol II p. 237-253) (PDF containing extracts)
4. ^ Capacitor complaints — William J. Beaty, 1996

[edit] See also

• Capacitor
• Electricity