Arc

Arc, type of continuous electric discharge, giving intense light and heat, formed between two electrodes in a gas at low pressure or in open air. It was first discovered and demonstrated by the British chemist Sir Humphry Davy in 1800.

To start an arc, the ends of two pencil-like electrodes, usually made of carbon, are brought into contact and a large current (about 10 amp) is passed through them. This current causes intense heating at the point of contact, and if the electrodes are then separated, a flame-like arc is formed between them. The discharge is carried largely by electrons travelling from the negative to the positive electrode, but also in part by positive ions travelling in the opposite direction. The impact of the ions produces great heat in the electrodes, but the positive electrode is hotter, because the electrons impinging on it have greater total energy. In an arc in air at normal pressure, the positive electrode reaches a temperature of 3500° C (6332° F).

The intense heat of the electric arc is often used in special furnaces to melt refractory materials (materials that have very high melting points). Temperatures of about 2800° C (5072° F) can easily be obtained with such a furnace. Arcs are also used as a high-intensity light source. Arc lights have the advantage of being concentrated sources of light, because some 85 per cent of the light intensity comes from a small area of the tip of the positive carbon electrode. Such lamps were formerly much used for street lighting, but are now chiefly employed in film projectors. Mercury vapour lamps and sodium-vapour lamps are enclosed arc lamps in which the arc is maintained in an atmosphere of mercury or sodium vapour at reduced pressure.