In 1901 American electrician Peter Cooper Hewitt invented a mercury vapour lamp.
It consists of a long narrow vacuumed glass tube partly filled with mercury with platinum electrodes or wires. It is designed to be hung in a horizontal position with a chain off center. The lamp is lighted by pulling down the chain which causes the tube to tilt and the mercury to flow along the tube to the other platinum electrode. When it reaches this a conduction path for the current is formed form end to end of the tube, after which the lamp is allowed to return to its original angle so that the mercury runs back into the bulb. As there is now no metallic connection between the electrodes, the current continues to pass through the tube on vaporized mercury. Average length of the tube is 30 inches and it produced bluish-green light of 500 to 300,000 candle-power depending on current.