Arc

—1. In electric lighting, a stream of incandescent vapor connecting the terminals of a lamp when they have been drawn apart and having sufficient E. M. F. maintained between them. The luminosity of the arc is due partly to the vapor which contains volatilized particles from the terminals, and partly to the incandescence of the terminals themselves: 2. Any flashing occurring between the terminals of an elect rice circuit when the circuit has been interrupted. ‘ 3. Any portion of the circumference of a circle, the curved boundary of a segment, or that part of the circumference cut off by any angle.