Editing Leyden jar (section)

== Design ==
[[file:Leyden jar showing construction.png|thumb|upright=0.7|left|Leyden jar construction]]
[[file:Leyden unit jar.png|thumb|upright|Measuring Leyden jar]]

A typical design consists of a [[glass]] jar with conducting tin 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 metal rod [[electrode]] projects through the nonconductive stopper at the mouth of the jar, electrically connected by some means (usually a hanging chain) to the inner foil, to allow it to be charged. The jar is charged by an [[electrostatic generator]], or other source of electric charge, connected to the inner electrode while the outer foil is [[ground (electricity)|grounded]]. The inner and outer surfaces of the jar store equal but opposite charges.<ref>{{cite web| url=http://electronics.howstuffworks.com/capacitor3.htm| title=How Capacitors Work| date=2007-09-17| access-date=2014-02-15| archive-url=https://web.archive.org/web/20180103184652/https://electronics.howstuffworks.com/capacitor3.htm| archive-date=2018-01-03| url-status=dead}}</ref>

The original form of the device is just a glass bottle partially filled with water, with a metal wire passing through a cork closing it. The role of the outer plate is provided by the hand of the experimenter. Soon [[John Bevis]] found (in 1747) that it was possible to coat the exterior of the jar with metal foil, and he also found that he could achieve the same effect by using a plate of glass with metal foil on both sides.<ref name="George Allen & Unwin">{{cite book|last1=Wolf|first1=A|last2=McKie|first2=D.|title=A history of science, technology and philosophy in the 18th century|date=1962|publisher=George Allen & Unwin|location=London|page=224|edition=2nd|url=https://archive.org/stream/in.ernet.dli.2015.460063/2015.460063.A-History#page/n261/mode/2up}}</ref> These developments inspired [[William Watson (scientist)|William Watson]] in the same year to have a jar made with a metal foil lining both inside and outside, dropping the use of water.<ref name="George Allen & Unwin"/><ref>{{cite journal|last1=Watson|first1=W.|title=III. A collection of the electrical experiments communicated to the Royal Society by Wm. Watson, F. R. S. read at several meetings between October 29.1747. and Jan. 21. following|journal=Philosophical Transactions|date=1748|volume=45|pages=92 ff|url=http://rstl.royalsocietypublishing.org/content/45/485/49.full.pdf+html|access-date=30 April 2018}}</ref>

Early experimenters (such as [[Benjamin Wilson (painter)|Benjamin Wilson]] in 1746) reported that the thinner the [[dielectric]] and the greater the surface, the greater the charge that could be accumulated.<ref>{{cite book |last1=Anders |first1=André |title=Cathodic Arcs |volume=50 |date=2008 |publisher=Springer |location=New York |isbn=978-0-387-79108-1 |chapter-url=https://books.google.com/books?id=rwIUhsbBHQYC&pg=PA9|page=9 |doi=10.1007/978-0-387-79108-1_2|chapter=A Brief History of Cathodic Arc Coating |series=Springer Series on Atomic, Optical, and Plasma Physics }}</ref>

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 a [[vacuum]].