Leyden jar

Leyden jar

Overview
A Leyden jar, or Leiden jar, is a device that "stores" static electricity
Static electricity
Static electricity refers to the build-up of electric charge on the surface of objects. The static charges remain on an object until they either bleed off to ground or are quickly neutralized by a discharge. Static electricity can be contrasted with current electricity, which can be delivered...

 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 1745 and by Dutch scientist Pieter van Musschenbroek
Pieter van Musschenbroek
Pieter van Musschenbroek was a Dutch scientist. He was a professor in Duisburg, Utrecht, and Leiden, where he held positions in mathematics, philosophy, medicine, and astrology. He is credited with the invention of the first capacitor in 1746: the Leyden jar. He performed pioneering work on the...

 of Leiden (Leyden) in 1745–1746. The invention was named for this city. It was the original form of the capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...

.
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Encyclopedia
A Leyden jar, or Leiden jar, is a device that "stores" static electricity
Static electricity
Static electricity refers to the build-up of electric charge on the surface of objects. The static charges remain on an object until they either bleed off to ground or are quickly neutralized by a discharge. Static electricity can be contrasted with current electricity, which can be delivered...

 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 1745 and by Dutch scientist Pieter van Musschenbroek
Pieter van Musschenbroek
Pieter van Musschenbroek was a Dutch scientist. He was a professor in Duisburg, Utrecht, and Leiden, where he held positions in mathematics, philosophy, medicine, and astrology. He is credited with the invention of the first capacitor in 1746: the Leyden jar. He performed pioneering work on the...

 of Leiden (Leyden) in 1745–1746. The invention was named for this city. It was the original form of the capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...

. 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.

Description


A typical design consists of a glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

 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
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...

 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
Electrostatic generator
An electrostatic generator, or electrostatic machine, is a mechanical device that produces static electricity, or electricity at high voltage and low continuous current...

, or other source of electric charge, connected to the inner electrode while the outer foil is grounded
Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....

. 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
Wire
A wire is a single, usually cylindrical, flexible strand or rod of metal. Wires are used to bear mechanical loads and to carry electricity and telecommunications signals. Wire is commonly formed by drawing the metal through a hole in a die or draw plate. Standard sizes are determined by various...

 to an external terminal, a sphere to avoid losses by corona discharge
Corona discharge
In electricity, a corona discharge is an electrical discharge brought on by the ionization of a fluid surrounding a conductor that is electrically energized...

. It was initially believed that the charge was stored in the water. Benjamin Franklin
Benjamin Franklin
Dr. Benjamin Franklin was one of the Founding Fathers of the United States. A noted polymath, Franklin was a leading author, printer, political theorist, politician, postmaster, scientist, musician, inventor, satirist, civic activist, statesman, and diplomat...

 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
Dielectric
A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...

, the closer the plates, and the greater the surface, the greater the charge that could be stored at a given voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...

.

Further developments in electrostatics revealed that the dielectric material was not essential, but increased the storage capability (capacitance
Capacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...

) and prevented arcing between the plates. Two plates separated by a small distance also act as a capacitor, even in vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...

.

Originally, the amount of capacitance
Capacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...

 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
Pint
The pint is a unit of volume or capacity that was once used across much of Europe with values varying from state to state from less than half a litre to over one litre. Within continental Europe, the pint was replaced with the metric system during the nineteenth century...

 size has a capacitance of about 1 nF
Farad
The farad is the SI unit of capacitance. The unit is named after the English physicist Michael Faraday.- Definition :A farad is the charge in coulombs which a capacitor will accept for the potential across it to change 1 volt. A coulomb is 1 ampere second...

.

History


The ancient Greeks
Greeks
The Greeks, also known as the Hellenes , are a nation and ethnic group native to Greece, Cyprus and neighboring regions. They also form a significant diaspora, with Greek communities established around the world....

 already knew that pieces of amber
Amber
Amber is fossilized tree resin , which has been appreciated for its color and natural beauty since Neolithic times. Amber is used as an ingredient in perfumes, as a healing agent in folk medicine, and as jewelry. There are five classes of amber, defined on the basis of their chemical constituents...

 could attract lightweight particles after being rubbed. The amber becomes electrified by triboelectric effect
Triboelectric effect
The triboelectric effect is a type of contact electrification in which certain materials become electrically charged after they come into contact with another different material and are then separated...

, mechanical separation of charge in a dielectric
Dielectric
A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...

. The Greek word for amber is ηλεκτρον ("elektron") and is the origin of the word "electricity".

Around 1650, Otto von Guericke
Otto von Guericke
Otto von Guericke was a German scientist, inventor, and politician...

 built a crude electrostatic generator
Electrostatic generator
An electrostatic generator, or electrostatic machine, is a mechanical device that produces static electricity, or electricity at high voltage and low continuous current...

 — a sulphur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...

 ball that rotated on a shaft. When Guericke held his hand against the ball and turned the shaft quickly, a static electric charge
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...

 built up. This experiment inspired the development of several forms of "friction machines", that greatly helped in the study of electricity.

The idea for the Leyden jar was discovered by both German scientist and jurist Ewald Georg von Kleist, and by two Dutchmen – Musschenbroek and Cunaeus. It was in 1744 when von Kleist lined a glass jar with silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...

 foil, and charged the foil with a friction machine. Kleist was convinced that a substantial electric charge could be collected when he received a significant shock from the device. The effects of this Kleistian jar were independently discovered around the same time by Dutch scientists Pieter van Musschenbroek
Pieter van Musschenbroek
Pieter van Musschenbroek was a Dutch scientist. He was a professor in Duisburg, Utrecht, and Leiden, where he held positions in mathematics, philosophy, medicine, and astrology. He is credited with the invention of the first capacitor in 1746: the Leyden jar. He performed pioneering work on the...

 and Cunaeus at the University of Leiden. Musschenbroek communicated on it with the French scientific community where it was called the Leyden jar.

Daniel Gralath
Daniel Gralath
Daniel Gralath was a German physicist and Bürgermeister of Danzig.Gralath was born in Danzig in Poland of a well-to-do trade family. He had studied law and philosophy in Halle, then in Leyden and Marburg from 1728 to 1734. Later he became Ratsherr and, in 1763, Bürgermeister of Danzig...

 was the first to combine several jars in parallel into a "battery
Battery (electricity)
An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...

" to increase the total possible stored charge.
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. Around the turn of the century it began to be widely used in spark-gap radio transmitters
Spark-gap transmitter
A spark-gap transmitter is a device for generating radio frequency electromagnetic waves using a spark gap.These devices served as the transmitters for most wireless telegraphy systems for the first three decades of radio and the first demonstrations of practical radio were carried out using them...

 and medical electrotherapy
Electrotherapy
Electrotherapy is the use of electrical energy as a medical treatment In medicine, the term electrotherapy can apply to a variety of treatments, including the use of electrical devices such as deep brain stimulators for neurological disease. The term has also been applied specifically to the use of...

 equipment. By the early 20th century, improved dielectrics and the need to reduce their size and inductance
Inductance
In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...

 for use in the new technology of radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...

 caused the Leyden jar to evolve into the modern compact form of capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...

.

The "dissectible Leyden jar" myth



A popular but misleading demonstration with a Leyden jar involves taking one apart after it has been charged and showing that the charge is stored on the dielectric
Dielectric
A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...

, not the plates. The first documented instance of this demonstration is in a 1749 letter by Benjamin Franklin. Franklin designed a "dissectible" leyden jar, shown at left, which was widely used in demonstrations.

The jar in the demonstration is constructed out of a glass cup nested between two fairly 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
Electric spark
An electric spark is a type of electrostatic discharge that occurs when an electric field creates an ionized electrically conductive channel in air producing a brief emission of light and sound. A spark is formed when the electric field strength exceeds the dielectric field strength of air...

 may still be obtained.

When not properly explained, this demonstration promotes the myth that capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...

s store their charge inside their dielectric. This erroneous theory, due to Franklin, was taught throughout the 1800s, and is still sometimes encountered. However this phenomenon is a special effect caused by the high voltage on the Leyden jar.In the dissectible Leyden jar, charge is transferred to the surface of the glass cup by corona discharge
Corona discharge
In electricity, a corona discharge is an electrical discharge brought on by the ionization of a fluid surrounding a conductor that is electrically energized...

 when the jar is disassembled; this is the source of the residual charge after the jar is reassembled. Handling the cup while disassembled does not provide enough contact to remove all the surface charge. Soda glass is hygroscopic and forms a partially conductive coating on its surface, which holds the charge. Addenbrook (1922) found that in a dissectible jar made of paraffin wax, or glass baked to remove moisture, the charge remained on the metal plates. Zeleny (1944) confirmed these results and observed the corona charge transfer.In capacitors generally, the charge is not stored in the dielectric, but on the inside surfaces of the plates, as can be seen from the fact that capacitors can function with a vacuum between their plates.

Residual charge


If a charged Leyden jar is discharged by shorting the inner and outer coatings and left to sit for a few minutes, the jar will recover some of its previous charge, and a second spark can be obtained from it. Often this can be repeated, and a series of 4 or 5 sparks, decreasing in length can be obtained at intervals. This effect is caused by dielectric absorption
Dielectric absorption
Dielectric absorption is the name given to the effect by which a capacitor that has been charged for a long time discharges only incompletely when briefly discharged. Although an ideal capacitor would remain at zero volts after being discharged, real capacitors will develop a small voltage, a...

.

See also

  • Franklin bells
    Franklin bells
    Franklin bells are an early demonstration of electric charge designed to work with a Leyden jar. The bells consist of a metal stand with a crossbar, from which hang three bells. The outer two bells hang from conductive metal chains, while the central bell hangs from a nonconductive thread...

  • Leyden ball
    Leyden ball
    A Leyden ball is a fictional bullet used in a nineteenth century science fiction novel. It contains a capacitance charge of electrical energy, which discharges instantaneously upon the bullet's impact...

    -a fictional weapon employing the physics of Leyden jars

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