Electric arc

An electric arc, or arc discharge, is an electrical breakdown of a gas that produces a prolonged electrical discharge. The current through a normally nonconductive medium such as air produces a plasma; the plasma may produce visible light. 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'. An electric arc, or arc discharge, is an electrical breakdown of a gas that produces a prolonged electrical discharge. The current through a normally nonconductive medium such as air produces a plasma; the plasma may produce visible light. 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'. Techniques for arc suppression can be used to reduce the duration or likelihood of arc formation. In the late 1800s, electric arc lighting was in wide use for public lighting.Some low-pressure electric arcs are used in many applications. For example, fluorescent tubes, mercury, sodium, and metal-halide lamps are used for lighting; xenon arc lamps have been used for movie projectors. 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. However, Davy's description was not an electric arc, but spark, 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'. 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 1,000 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 due to the buoyant force on the hot gas. The first continuous arc was discovered independently in 1802 and described in 1803 as a 'special fluid with electrical properties', by Vasily V. Petrov, a Russian scientist experimenting with a copper-zinc battery consisting of 4200 discs. In the late nineteenth century, electric arc lighting was in wide use for public lighting. The tendency of electric arcs to flicker and hiss was a major problem. In 1895, Hertha Marks Ayrton wrote a series of articles for the Electrician, explaining that these phenomena were the result of oxygen coming into contact with the carbon rods used to create the arc. In 1899, she was the first woman ever to read her own paper before the Institution of Electrical Engineers (IEE). Her paper was entitled 'The Hissing of the Electric Arc'. Shortly thereafter, Ayrton was elected the first female member of the IEE; the next woman to be admitted to the IEE was in 1958. She petitioned to present a paper before the Royal Society, but she was not allowed because of her sex, and 'The Mechanism of the Electric Arc' was read by John Perry in her stead in 1901.