Definitions

arc

[ahrk]
arc, in electricity, highly luminous and intensely hot discharge of electricity between two electrodes. The arc was discovered early in the 19th cent. by the English scientist Sir Humphry Davy, who so named it because of its shape. An arc is characterized by a high current, low voltage, and indefinite duration. It is usually started when two electrodes carrying an electric current are drawn apart. At the instant the electrodes are parted, strong electric forces draw electrons from one electrode to the other, initiating the arc. The discharge consists of a current composed of these electrons and charged gas particles, called ions, that form between the electrodes. The first practical electric light, the arc lamp, made use of the arc formed between two carbon rods (see lighting). Today the use of the arc lamp is limited to special purposes, e.g., in searchlights and in research applications. The principle of the electric arc is employed in welding (as in the hydrogen arc, where hydrogen is introduced between tungsten electrodes) and also in generating heat in the electric furnace. A spark, like an arc, is a discharge of electricity between two points, but it has a high voltage and a short duration. Lightning is an example of a spark.
arc, in geometry, a curved line or any part of it; in particular, a portion of the circumference of a circle. The length s of an arc of a circle of radius r and subtending a central angle of θ radians is s=rθ; if θ is measured in degrees, then the arc is given by s=2πrθ/360.
Arc may refer to:

Engineering

• Arc chute, a protective device in an electrical circuit breaker
• Arc lamp, a device that produces light by an electric arc
• Arc welding, a method of welding that uses an electric arc to melt the metals together
• Electric arc or voltaic arc, an ongoing plasma discharge (an electric current through a gas), producing light and heat
• Arc or arch, a bowed or curved structure capable of spanning a space while supporting significant weight

Optics

• 120° parhelion, relatively rare halo, an optical phenomenon occasionally appearing along with very bright sun dogs
• Infralateral arc, a rare halo, an optical phenomenon appearing similar to a rainbow under a white parhelic circle
• Lower tangent arc, rarely observable halo, an optical phenomenon appearing under and tangent to a 22° halo centred around the sun
• Parry arc, a rare halo, an optical phenomenon which occasionally appears over a 22° halo together with a upper tangent arc
• Subhelic arc, a rare halo that curves upwards from the horizon and touches the tricker arc above the anthelic point
• Supralateral arc, a rare halo, an optical phenomenon often confused with the indeed infrequently appearing 46° halo
• Upper tangent arc, halo, an atmospheric optical phenomenon which appears over and tangent to the 22° halo centred around the sun