Electric Arc

There are many types of arc devices. Some operate at atmospheric pressure and may be open, and others operate at low pressure and are therefore closed in a container, like glass. The property of high current in the arc is used in the mercury arc rectifiers, like the thyratron. An alternate potential difference is applied, and the arc transfers the current in one direction only. The cathode is heated by a filament.

The high temperature created by an electric arc in the gas is used in furnaces. Arc welders are used for welding, where a metal is fused and added in a joint. The arc can supply the heat only, or one of its electrodes can serve as the consumable parent metal. Plasma torches are used for cutting, spraying, and gas heating. Cutting may be done by means of an arc formed between the metal and the electrode.

Arc lamps provide high luminous efficiency and great brightness. The light comes from the highly incandescence (about 7,000°F [3,871°C]) electrodes, as in carbon arcs, or from the heated, ionized gases surrounded the arc, as in flame arcs. The carbon arc, where two carbon rods serve as electrodes, was the first practical commercial electric lighting device, and it is still one of the brightest sources of light. It is used in theater motion-picture projectors, large searchlights, and lighthouses. Flame arcs are used in color photography and in photochemical processes because they closely approximate natural sunshine. The carbon is impregnated with volatile chemicals, which become luminous when evaporated and driven into the arc. The color of the arc depends on the material; the material could be calcium, barium, titanium, or strontium. In some, the wavelength of the radiation is out of the visible spectrum. Mercury arcs produce ultraviolet radiation at high pressure. They can also produce visible light in a low pressure tube, if the internal walls are coated with fluorescence material such as phosphor; the phosphor emits light when illuminated by the ultraviolet radiation from the mercury.

Other uses of arcs include valves (used in the early days of the radio), and as a source of ions in nuclear accelerators and thermonuclear devices. The excitation of electrons in the arc, in particular the direct electron bombardment, leads to narrow spectral lines. The arc, therefore, can provide information on the composition of the electrodes. The spectra of metal alloys are widely studied using arcs; the metals are incorporated with the electrodes material, and when vaporized, they produce distinct spectra.