Leyden jar

The Leyden jar is an early device for storing electric charge invented in 1745 by Pieter van Musschenbroek (1700–1748). It was the first capacitor. Leyden jars were used to conduct many early experiments in electricity.

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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 (and others) 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 ηλεκτρον ("elektron", amber).

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] By the middle of the 19th century, the Leyden jar had become common enough for writers to assume their readers knew of and understood its basic operation.^[3] By the early 20th century, improved dielectrics and the need to reduce their size for use in the new technology of radio caused the Leyden jar to evolve into the modern compact form of capacitor.

Circuit theory

The scientist James Clerk Maxwell invented the concept of displacement current, dD/dt, to make Ampère'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 Ampère'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.

The "dissectible Leyden jar" urban legend

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. ^[4]

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 an urban legend. 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 at the edges of the plates as they slide along the dielectric during the disassembly. If the experiment were performed in an a highly insulating fluid (such as mineral oil) instead of air, the effect would no longer be present.^[5]

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, the modern meaning of battery (electricity).
3. ^ The Leyden Ball
4. ^ Letter IV: Benjamin Franklin to Peter Collinson, April 29, 1749 (Bigelow vol II p. 237-253) (PDF containing extracts)
5. ^ Capacitor complaints — William J. Beaty, 1996