two matching oversized american industrial factory complex "metalarc" 1000 watt arc lamps fabricated by or for sylvania electric products, u.s.a. high-intensity discharge lamp produce light by means of an electric arc between tungsten electrodes housed inside a translucent or transparent fused quartz or fused alumina arc tube. this tube is filled with both gas and metal salts. the gas facilitates the arc's initial strike. once the arc is started, it heats and evaporates the metal salts forming a plasma, which greatly increases the intensity of light produced by the arc and reduces its power consumption. high-intensity discharge lamps are a type of arc lamp. the metal halide discharge lamps contain yellow brass mogul screw bases. priced for the pair.