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Electric arc
An electric arc or arc discharge is an electrical breakdown of a gas that produces an ongoing plasma discharge, resulting from a current through normally nonconductive media such as air. An arc discharge is characterized by a lower voltage than a glow discharge, and relies on thermionic emission of electrons from the electrodes supporting the arc. An archaic term is voltaic arc, as used in the phrase "voltaic arc lamp".
==History==
The phenomenon is believed to be first described by Sir Humphry Davy, in an 1801 paper published in William Nicholson's Journal of Natural Philosophy, Chemistry and the Arts.〔(The Electric Arc, By Hertha Ayrton, page 94 )〕 However, that was not an electric arc as this phenomenon is considered by the modern science: "This is evidently the description, not of an arc, but of a spark. For the essence of an arc is that it should be continuous, and that the poles should not be in contact after it has once started. The spark produced by Sir Humphry Davy was plainly not continuous; and although the carbons remained red hot for some time after contact, there can have been no arc joining them, or so close an observer would have mentioned it".〔(The Electric Arc, By Hertha Ayrton, page 20 )〕 In the same year Davy publicly demonstrated the effect, before the Royal Society, by transmitting an electric current through two touching carbon rods and then pulling them a short distance apart. The demonstration produced a "feeble" arc, not readily distinguished from a sustained spark, between charcoal points. The Society subscribed for a more powerful battery of 1000 plates and in 1808 he demonstrated the large-scale arc. He is credited with naming the arc. He called it an arc because it assumes the shape of an upward bow when the distance between the electrodes is not small.〔 This is the likely origin of the term ''arc''.〕 This is due to the buoyant force on the hot gas. Independently the phenomenon was discovered in 1802 and described in 1803〔(Tracking down the origin of arc plasma Science-II. Early continuous discharges, by André ANDERS )〕 as a "special fluid with electrical properties", by Vasily V. Petrov, a Russian scientist experimenting with a copper-zinc battery consisting of 4200 discs.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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