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Electrophorus from 1800s.

An electrophorus is a capacitive generator used to produce electrostatic charge via the process of electrostatic induction. A first version of it was invented in 1762 by Swedish professor Johan Carl Wilcke,[1] [2] but Italian scientist Alessandro Volta improved and popularized the device in 1775,[3] and is sometimes erroneously credited with its invention.[4] [5] The word electrophorus was coined by Volta from the Greek ήλεκτρον ('elektron'), ϕέρω ('phero'), meaning 'electricity bearer'.[6]

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. [7] First, the dielectric plate is charged through the triboelectric effect by rubbing it with fur or cloth. For this discussion, imagine the dielectric gains negative charge by rubbing, as in the illustration below. Then, the metal plate is placed onto the dielectric plate. The dielectric does not transfer a significant fraction of its surface charge to the metal because the microscopic contact is poor. Instead the electrostatic field of the charged dielectric causes the charges in the metal plate to separate. It develops two regions of charge — the positive charges in the plate are attracted to the side facing down toward the dielectric, charging it positively, while the negative charges are repelled to the side facing up, charging it negatively, with the plate remaining electrically neutral as a whole. Then, the side facing up is momentarily grounded (which can be done by touching it with a finger), draining off the negative charge. Finally, the metal plate, now carrying only one sign of charge (positive in our example), is lifted. [2]

Showing induced charge on plate before grounding.

Since the charge on the dielectric is not depleted in this process, the charge on the metal plate can be used for experiments, for example by touching it to metal conductors allowing the charge to drain away, and the uncharged metal plate can be placed back on the dielectric and the process repeated to get another charge. This can be repeated as often as desired, so in principle an unlimited amount of induced charge can be obtained from a single charge on the dielectric. For this reason Volta called it elettroforo perpetuo (the perpetual electricity bearer).[8] In actual use the charge on the dielectric will eventually (within a few days at most) leak off through the surface of the cake or the atmosphere to recombine with opposite charges around to restore neutrality.

One of the largest examples of an electrophorus was built in 1777 by German scientist Georg Christoph Lichtenberg.[6] 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.

The source of the charge

Charge in the universe is conserved. This is simply a process for separating positive and negative charges. One sign of charge ends up on the metal plate (or other storage conductor), and other sign of charge is stored in other object after grounding (in the earth or the person touching the metal plate). This separation takes work since the lowest energy state is to have neutral objects. Work is done raising the charged metal plate away from the oppositely charged resinous plate. This additional energy put into the system is converted to potential energy in the form of charge separation (opposite charges that were originally on the plate), so raising the metal plate actually increases its voltage relative to the dielectic plate.

The electrophorus is thus actually a manually operated electrostatic generator, using the same principle of electrostatic induction as electrostatic machines such as the Wimshurst machine and the Van de Graaf generator.[2]

Notes

  1. ^ Pancaldi, Giuliano (2003). Volta, Science and Culture in the Age of Enlightenment. Princeton Univ. Press. ISBN 0691122261. http://books.google.com/books?id=hGoYB1Twx4sC&pg=PA73.  , p.73
  2. ^ a b c Jones, Thomas B. (July 2007). "Electrophorus and accessories". Thomas B. Jones website. Univ. of Rochester. http://www.ece.rochester.edu/~jones/demos/electrophorus.html. Retrieved 2007-12-27.  
  3. ^ Pancaldi 2003, p.75-105
  4. ^ Lewis, Nancy D.. "Alesandro Volta, The Perpetual Electrophorus". Electricity:A Summary of Scientists and their Discoveries. http://itp.nyu.edu/~nql3186/electricity/pages/volta1.html. Retrieved 2007-12-27.  
  5. ^ "Alessandro Volta". World Of Biography. http://www.worldofbiography.com/9148-Alessandro%20Volta/life.htm. Retrieved 2007-12-27.  
  6. ^ a b Harris, William Snow (1867). A Treatise on Frictional Electricity in Theory and Practice. London: Virtue & Co.. pp. 86. http://books.google.com/books?id=tehLAAAAMAAJ&pg=PA86.  
  7. ^ "Electrophorus". Encyclopaedia Britannica, 11th Ed.. 9. The Encyclopaedia Britannica Publishing Co.. 1910. http://books.google.com/books?id=t_0tAAAAIAAJ&pg=PA237. Retrieved 2007-12-27.  , p.237
  8. ^ Schiffer, Michael Brian (2003). Draw the Lightning Down:Benjamin Franklin and electrical technology in the Age of Enlightenment. Univ. of California Press. ISBN 0520238028. http://books.google.com/books?id=9TuH6Lg8IasC&dq=electrophorus+volta.   p.55-57

References


1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

ELECTROPHORUS, an instrument invented by Alessandro Volta in 1775, by which mechanical work is transformed into electrostatic charge by the aid of a small initial charge of electricity. The operation depends on the facts of electrostatic induction discovered by John Canton in 175 3, and, independently, by J. K. Wilcke in 1762 (see Electricity). Volta, in a letter to J. Priestley on the 10th of June 1775 (see Collezione dell' opere, ed. 1816, vol. i. p. 118), described the invention of a device he called an elettroforo perpetuo, based on the fact that a conductor held near an electrified body and touched by the finger was found, when withdrawn, to possess an electric charge of opposite sign to that of the electrified body. His electrophorus in one form consisted of a disk of non-conducting material, such as pitch or resin, placed between two metal sheets, one being provided with an insulating handle. For the pitch or resin may be substituted a sheet of glass, ebonite, indiarubber or any other good dielectric placed upon a metallic sheet, called the sole-plate. To use the apparatus the surface of the dielectric is rubbed with a piece of warm flannel, silk or catskin, so as to electrify it, and the upper metal plate is then placed upon it. Owing to the irregularities in the surfaces of the dielectric and upper plate the two are only in contact at a few points, and owing 1 See J. A. Fleming, Handbook for the Electrical Laboratory and Testing Room, vol. i. p. 448 (London, 1901).

to the insulating quality of the dielectric its surface electrical charge cannot move over it. It therefore acts inductively upon the upper plate and induces on the adjacent surface an electric charge of opposite sign. Suppose, for instance, that the dielectric is a plate of resin rubbed with catskin, it will then be negatively electrified and will act by induction on the upper plate across the film of air separating the upper resin surface and lower surface of the upper metal plate. If the upper plate is touched with the finger or connected to earth for a moment, a negative charge will escape from the metal plate to earth at that moment. The arrangement thus constitutes a condenser; the upper plate on its under surface carries a charge of positive electricity and the resin plate a charge of negative electricity on its upper surface, the air film between them being the dielectric of the condenser. If, therefore, the upper plate is elevated, mechanical work has to be done to separate the two electric charges. Accordingly on raising the upper plate, the charge on it, in oldfashioned nomenclature, becomes free and can be communicated to any other insulated conductor at a lower potential, the upper plate thereby becoming more or less discharged. On placing the upper plate again on the resin and touching it for a moment, the process can be repeated, and so at the expense of mechanical work done in lifting the upper plate against the mutual attraction of two electric charges of opposite sign, an indefinitely large electric charge can be accumulated and given to any other suitable conductor. In course of time, however, the surface charge of the resin becomes dissipated and it then has to be again excited. To avoid the necessity for touching the upper plate every time it is put down on the resin, a metal pin may be brought through the insulator from the sole-plate so that each time that the upper plate is put down on the resin it is automatically connected to earth. We are thus able by a process of merely lifting the upper plate repeatedly to convey a large electrical charge to some conductor starting from the small charge produced by friction on the resin. The above explanation does not take into account the function of the sole-plate, which is important. The sole-plate serves to increase the electrical capacity of the upper plate when placed down upon the resin or excited insulator. Hence when so placed it takes a larger charge. When touched by the finger the upper plate is brought to zero potential. If then the upper plate is lifted by its insulating handle its capacity becomes diminished. Since, however, it carries with it the charge it had when resting on the resin, its potential becomes increased as its capacity becomes less, and it therefore rises to a high potential, and will give a spark if the knuckle is approached to it when it is lifted after having been touched and raised.

The study of Volta's electrophorus at once suggested the performance of these cyclical operations by some form of rotation instead of elevation, and led to the invention of various forms of doubler or multiplier. The instrument was thus the first of a long series of machines for converting mechanical work into electrostatic energy, and the predecessor of the modern type of influence machine (see Electrical Machine). Volta himself devised a double and reciprocal electrophorus and also made mention of the subject of multiplying condensers in a paper published in the Phil. Trans. for 1782 (p. 237, and appendix, p. vii.). He states, however, that the use of a condenser in connexion with an electrophorus to make evident and multiply weak charges was due to T. Cavallo (Phil. Trans., 1788).

For further information see S. P. Thompson, "The Influence Machine from 1788 to 1888," Journ. Inst. Tel. Eng., 1888, 17, p. 569. Many references to original papers connected with the electrophorus will be found in A. Winkelmann's Handbuch der Physik (Breslau, 1905), vol. iv. p. 48. (J. A. F.)


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Wikispecies

Up to date as of January 23, 2010

From Wikispecies

Electrophorus electricus

Taxonavigation

Main Page
Cladus: Eukaryota
Supergroup: Unikonta
Cladus: Opisthokonta
Regnum: Animalia
Subregnum: Eumetazoa
Cladus: Bilateria
Cladus: Nephrozoa
Cladus: Deuterostomia
Phylum: Chordata
Subphylum: Vertebrata
Infraphylum: Gnathostomata
Superclassis: Osteichthyes
Classis: Actinopterygii
Subclassis: Neopterygii
Infraclassis: Teleostei
Superordo: Ostariophysi
Ordo: Gymnotiformes
Familia: Gymnotidae
Genus: Electrophorus
Species: E. electricus

Name

Electrophorus Gill, 1864

References








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