Electrophorus

An electrophorus is a capacitive generator used to produce electrostatic charge via the process of electrostatic induction. It was invented in 1764 by Swedish professor Johan Carl Wilcke, [cite book
last=Pancaldi
first=Giuliano
year=2003
title=Volta, Science and Culture in the Age of Enlightenment
publisher=Princeton Univ. Press
isbn=0691122261
url=http://books.google.com/books?id=hGoYB1Twx4sC&pg=PA73, p.73] cite web
last=Jones
first=Thomas B.
month=July | year=2007
title=Electrophorus and accessories
work=Thomas B. Jones website
publisher=Univ. of Rochester
url=http://www.ece.rochester.edu/~jones/demos/electrophorus.html
accessdate=2007-12-27] but Italian scientist Alessandro Volta improved and popularized the device in 1775, [ [http://books.google.com/books?id=hGoYB1Twx4sC&pg=PA73 Pancaldi 2003, p.75-105] ] and is sometimes erroneously credited with its invention. [cite web
last=Lewis
first=Nancy D.
title=Alesandro Volta, The Perpetual Electrophorus
work= [http://itp.nyu.edu/~nql3186/electricity/ Electricity:A Summary of Scientists and their Discoveries]
url=http://itp.nyu.edu/~nql3186/electricity/pages/volta1.html
accessdate=2007-12-27] [cite web
title=Alessandro Volta
publisher= [http://www.worldofbiography.com/ World Of Biography]
url=http://www.worldofbiography.com/9148-Alessandro%20Volta/life.htm
accessdate=2007-12-27] The word "electrophorus" was coined by Volta from the Greek ήλεκτρον ('elektron'), ϕέρω ('phero'), meaning 'electricity bearer'.cite book
last=Harris
first=William Snow
title=A Treatise on Frictional Electricity in Theory and Practice
year=1867
publisher=Virtue & Co.
location=London
pages=p.86
url=http://books.google.com/books?id=tehLAAAAMAAJ&pg=PA86]

The electrophorus consists of a dielectric plate (originally a 'cake' of resinous material like pitch or wax, but in modern versions plastic is used) and a metal plate with an insulating handle. [cite encyclopedia
title=Electrophorus
encyclopedia=Encyclopaedia Britannica, 11th Ed.
volume=9
publisher=The Encyclopaedia Britannica Publishing Co.
url=http://books.google.com/books?id=t_0tAAAAIAAJ&pg=PA237
accessdate=2007-12-27, p.237] First, the dielectric plate is charged through the triboelectric effect by rubbing it with fur or cloth. Then, the metal plate is placed onto the dielectric plate. The electrostatic field of the dielectric causes the charges in the metal plate to separate. The metal develops two regions of charge — the side facing the charged dielectric plate charges opposite to the charge of plate, while the side facing away from the dielectric charge attains the same sign of charge as the dielectric plate, with the metal plate remaining electrically neutral as a whole. Then, the side facing away from the dielectric plate is momentarily grounded (which can be done by touching it with a finger), draining off the alike charge. Finally, the metal plate, now carrying only one sign of charge, is lifted.

The charge on the plate can be discharged and the process can be repeated, replacing the plate on the dielectric and grounding the top to get a new charge on the plate. This can be repeated as often as desired without depleting the dielectric's charge, and in this way an unlimited amount of charge can be obtained from the device (although in actual use the charge on the dielectric will eventually leak away through the atmosphere). For this reason Volta called it "elettroforo perpetuo" (the perpetual electrophorus). [cite book
last=Schiffer
first=Michael Brian
year=2003
title=Draw the Lightning Down:Benjamin Franklin and electrical technology in the Age of Enlightenment
publisher=Univ. of California Press
url=http://books.google.com/books?id=9TuH6Lg8IasC&dq=electrophorus+volta
isbn=0520238028 p.55-57]

One of the largest examples of an electrophorus was built in 1777 by German scientist Georg Christoph Lichtenberg. It was 6 feet (2 m) in diameter, with the metal plate raised and lowered using a pulley system. It could reportedly produce 15 inch (38 cm) sparks. Lichtenberg used its discharges to create the strange treelike marks known as Lichtenberg figures.

Where does the charge come from?

It is sometimes asked, how can an unlimited amount of charge be gotten from the limited initial charge on the device? The answer is that the charge on the dielectric isn't consumed in the process. Its role is just to induce charge in the plate. Although the plate is set on the dielectric, it only makes contact with the surface in a few places, and little or no charge is transferred since charge can't move through the dielectric; in fact the electrophorus can function without the two parts touching.

Where does the energy for all this electricity come from? The energy to accumulate each charge comes from the work done in lifting the charged plate away from the dielectric surface, against the electrostatic force between them. [cite book
last=Millikan
first=Robert A.
coauthors=Bishop, E.F.
year=1917
title=Elements of Electricity
publisher=American Technical Society
location=Chicago
url=http://books.google.com/books?id=dZM3AAAAMAAJ&pg=PA35, p.35-36] The electrophorus is actually a manually operated electrostatic generator, using the same induction principle as electrostatic machines such as the Wimshurst machine and the Van de Graaf generator.

References

*cite book|last=Pancaldi|first=Giuliano|year=2003|title=Volta, Science and Culture in the Age of Enlightenment|publisher=Princeton Univ. Press|url=http://books.google.com/books?id=hGoYB1Twx4sC&pg=PA73|isbn=0691122261, p.73-105 Volta's 'invention' of the electrophorus
*cite web
last=Jones
first=Thomas B.
month=July | year=2007
title=Electrophorus and accessories
work=Thomas B. Jones website
publisher=Univ. of Rochester
url=http://www.ece.rochester.edu/~jones/demos/electrophorus.html
accessdate=2007-12-27
*cite book
last=Schiffer
first=Michael Brian
year=2003
title=Draw the Lightning Down:Benjamin Franklin and electrical technology in the Age of Enlightenment
publisher=Univ. of California Press
url=http://books.google.com/books?id=9TuH6Lg8IasC&dq=electrophorus+volta
isbn=0520238028 p.55-57. Place of electrophorus in history of electrostatics, although doesn't mention Wilcke's contribution.
*cite encyclopedia
title=Electrophorus
encyclopedia=Encyclopaedia Britannica, 11th Ed.
volume=9
publisher=The Encyclopaedia Britannica Publishing Co.
pages=237
url=http://books.google.com/books?id=t_0tAAAAIAAJ&pg=PA237
accessdate=2007-12-27

Notes