Arc discharge

(electricity) A direct-current electrical current between electrodes in a gas or vapor, having high current density and relatively low voltage drop.

A type of electrical conduction in gases characterized by high current density and low potential drop. The electric arc was discovered by Humphry Davy in 1808, when he connected a piece of carbon to each side of an electric battery, touched the two pieces of carbon together, then drew them slightly apart. The result is a dazzling steam of ionized air, or plasma, at a temperature of 6000°C (10,800°F), the surface temperature of the Sun. A typical arc runs at a voltage drop of 100 V with a current drain of 10 A. The arc has negative resistance—the voltage drop decreases as the current increase—so a stabilizing resistor or inductor in series is required to maintain it. The high-temperature gas rises like a hot-air balloon while it remains anchored to the current-feeding electrodes at its ends. It thereby acquires an upward-curving shape, which accounts for its being called an arc.

There are many applications of such an intensely hot object. The brilliant arc and the incandescent carbon adjacent to it form the standard light source for movie theater projectors. The electronic flashgun in a camera uses an intense pulsed arc in xenon gas, simulating sunlight. Since no solid-state material can withstand this temperature for long, the arc is used industrially for welding steel and other metals. Alternatively, it can be used for cutting metal very rapidly. Electric arcs form automatically when the contacts in electrical switches in power networks are opened, and much effort goes into controlling and extinguishing them. Lightning is an example of a naturally occurring electric arc. See also Arc heating; Arc lamp; Arc welding; Circuit breaker; Lightning; Optical projection systems; Stroboscopic photography; Welding and cutting of materials.

The arc has been pushed to extremely high temperatures in the search for thermonuclear fusion, the record temperature being 4 × 10⁵ °C in a long pulse in helium. The arc temperature appears to be limited by the energy lost in intense radiation from the interface region between the intensely hot, fully ionized plasma core and the surrounding cooler, un-ionized gas.

An electric discharge characterized by the production of light, high cathode-current densities, and a low voltage drop at the cathode.