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ELECTROPHORUS

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Originally appearing in Volume V09, Page 237 of the 1911 Encyclopedia Britannica.
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ELECTROPHORUS , an See also:

instrument invented by Alessandro See also:Volta in 1775, by which See also:mechanical See also:work is transformed into electrostatic See also:charge by the aid of a small initial charge of See also:electricity. The operation depends on the facts of electrostatic See also:induction discovered by See also:John See also:Canton in 1753, and, independently, by J. K. Wilcke in 1762 (see ELECTRICITY). Volta, in a See also:letter to J. See also:Priestley on the loth of See also:June 1775 (see Collezione dell' opere, ed. 1816, vol. i. p. 118), described the invention of a See also:device he called an elettrofore per petuo, based on the fact that a conductor held near an electrified See also:body and touched by the See also:finger was found, when withdrawn, to possess an electric charge of opposite sign to that of the electrified body. His electrophorus in one See also:form consisted of a disk of non-conducting material, such as See also:pitch or See also:resin, placed between two See also:metal sheets, one being provided with an insulating handle. For the pitch or resin may be substituted a See also:sheet of See also:glass, ebonite, indiarubber or any other See also:good See also:dielectric placed upon a metallic sheet, called the See also:sole-See also:plate. To use the apparatus the See also:surface of the dielectric is rubbed with a piece of warm See also:flannel, See also: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 tapper plate the two are only in contact at a few points, and owing See J.

A. See also:

Fleming, Handbook for the See also:Electrical Laboratory and Testing See also:Room, vol. i. p. 448 (See also:London, 19oi).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 See also:act by induction on the upper plate across the film of See also:air separating the upper resin surface and See also:lower surface of the upper metal plate. If the upper plate is touched with the finger or connected to See also:earth for a moment, a negative charge will See also:escape from the metal plate to earth at that moment. The arrangement thus constitutes a See also:condenser; the upper plate on its under surface carries a charge of See also: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 See also:separate the two electric charges. Accordingly on raising the upper plate, the charge on it, in old-fashioned nomenclature, becomes See also: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 See also: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 See also:time, however, the surface charge of the resin becomes dissipated and it then has to be again excited. To avoid the See also:necessity for touching the upper plate every time it is put down on the resin, a metal See also: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 See also:

friction on the resin. The above explanation does not take into See also:account the See also: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 See also: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 See also:elevation, and led to the invention of various forms of doubler or multiplier. The instrument was thus the first of a See also:long See also:series of See also:machines for converting mechanical work into electrostatic See also:energy, and the predecessor of the See also:modern type of See also:influence See also:machine (see ELECTRICAL MACHINE). Volta himself devised a See also:double and reciprocal electrophorus and also made mention of the subject of multiplying condensers in a See also: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. See also:

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

48. (J. A.

End of Article: ELECTROPHORUS

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