An electric arc gets hungrier for current the more current it takes
Most circuits resist harder as current rises. An electric arc does the opposite: once lit, more current means a lower voltage across it, so from a fixed supply it keeps drawing more until something breaks. That quirk explains why stray arcs wreck equipment and why welders and furnaces need careful control.
An arc forms when a gas that normally blocks electricity breaks down and carries a sustained current, turning into a glowing plasma. It can be started by a high-voltage glow discharge, or by touching two electrodes and pulling them apart, which is exactly how a welder strikes an arc against a workpiece. Once running, electrons boil off the hot electrodes, and at the cathode the current can crowd into a spot carrying around a million amperes per square centimetre. In air, arc temperatures range from 3500K to 19,600K.
Because of that falling voltage, engineers must add a ballast, some positive impedance, to keep an arc stable. The same behaviour makes the arc useful for protection: a spark gap placed beside a vulnerable unit on a high-voltage line ignites when voltage spikes and shorts the danger past it. Arcing horns, two wires spreading into a narrow V, let the hot arc climb until the gap is too wide and it snaps, the principle behind the Jacob's ladder toy. Modern switchgear often smothers arcs with pressurised sulphur hexafluoride.
Sir Humphry Davy met the phenomenon in 1800 and named it for its upward bow, which happens because hot gas rises. His 1801 Royal Society demonstration between charcoal points was feeble, and modern science reads his first description as a spark; with a 1,000-plate battery he showed a large arc in 1808. The Russian Vasily Petrov independently found a continuous arc in 1802 using a battery of 4200 discs.
Arc lamps lit 19th-century streets but flickered and hissed. Hertha Marks Ayrton traced the noise to oxygen reaching the carbon rods, and in 1899 became the first woman to read her own paper to the Institution of Electrical Engineers, then its first female member; the next was admitted in 1958. The Royal Society refused to let her present in 1901, so John Perry read her work. Arcs now drive steel recycling furnaces, plasma cutters, xenon cinema projectors and even arcjet spacecraft thrusters.
Source: Electric arc
