electric discharge lamp, cooler and more efficient than incandescent lamps, that produces light by the fluorescence of a phosphor coating. A fluorescent lamp consists of a glass tube filled with a mixture of argon and mercury vapour. Metal electrodes at each end are coated with an alkaline-earth oxide that gives off electrons easily. When current flows through the ionized gas between the electrodes, it emits ultraviolet radiation. The inside of the tube is coated with phosphors (phosphor), substances that absorb ultraviolet radiation and fluoresce (reradiate the energy as visible light). Two common phosphors are zinc silicate and magnesium tungstate. A starter and ballast provide the extra voltage, up to four times of the operating voltage, needed to ionize the gas when starting.