Incandescent Light

In 1802, Sir Humphry Davy showed that electricity running through thin strips of metal could heat them to temperatures high enough that they would give off light; this is the basic principle by which all incandescent lamps work. In 1820, De La Rue demonstrated a lamp made of a coiled platinum wire in a glass tube with brass endcaps. When the current was switched on, electricity ran through the endcaps and through the wire (the filament). The wire was heated by its resistance to the current until it glowed white-hot, producing light. Between this time and the 1870s, the delicate lamps were unreliable, short-lived, and expensive to operate. The lifetime was short because the filament would burn up in air. To combat the short lifetime, early developers used thick low-resistance filaments, but heating them to incandescence required large currents—and generating large currents was costly.

Thomas Edison is well-known as "the inventor of the light bulb," but he was, in fact, only one of several researchers that created early electrical incandescent lamps in the 1870s. These researchers include Joseph Swan, Frederick DeMoyleyns, and St. George Lane-Fox in England, as well as Moses Farmer, Hiram Maxim, and William Sawyer in the United States.

Edison's contribution was an understanding of the necessary electrical properties for lamps. He knew that a system for delivering electricity was needed to make lamps practical; that it should be designed so that the lamps are run in parallel, rather than in series; and that the lamp filament should have high, rather than low, resistance. Because voltage in a circuit equals the current times the resistance, one can reduce the amount of current by increasing the resistance of the load. Increasing the resistance also reduces the amount of energy required to heat the filament to incandescence.

Edison replaced low-resistance carbon or platinum filaments with a high-resistance carbon filament. This lamp had electrical contacts connected to a cotton thread that had been burned to char (carbonized) and placed in a glass container with all the air pumped out. The vacuum, produced by a pump developed only a decade earlier by Herman Sprengel, dramatically increased the lifetime of the filament. The first practical version of the electric light bulb was lit on October 19, 1879, which burned for 40 hours, and produced 1.4 lumens per watt of electricity.

An incandescent non-electric lamp still in use is the Welsbach burner, commonly seen in camping lanterns. This burner, invented in 1886 by Karl Auer, Baron von Welsbach, consists of a mantle made of knit cotton soaked in oxides (originally nitrates were used), that is burned to ash the first time it is lit. The ash holds its shape and becomes incandescent when placed over a flame—and is much brighter than the flame itself.