English: One of the first mercury vapor lamps created by Peter Cooper Hewitt , which he invented and patented in 1903. This was the forerunner of the fluorescent lamp . It consisted of a sealed partially evacuated glass tube with electrodes at either end, with a small amount of liquid mercury inside. When a pulse of high voltage was applied an arc was started between the electrodes which evaporated the mercury. The electrons struck the mercury atoms, knocking off electrons, ionizing them, so the gas in the tube became electrically conductive, and a current flowed through the tube. When the electrons rejoined the atoms the atoms emitted light. The circular device below the lamp is the ballast , an inductor which produced a pulse of high voltage when the lamp was turned on, to start it. The mercury-vapor lamp was a lot more efficient than the incandescent lamp which was the dominant type of electric light at the time, but it produced a greenish light which was unpleasant, so its use was limited to office buildings and hallways and photographic uses. Later a fluorescent coating (a phosphor ) was applied to the inside of the tube, which produced a pleasing white light when struck by ultraviolet light from the mercury vapor. This was the fluorescent lamp which has become most widely used light source today.