Incandescent light is given off when an object is heated until it glows. To emit white light, an object must be heated to at least 1,341°F (727°C). White-hot iron in a forge, red lava flowing down a volcano, and the red burners on an electric stove are all examples of incandescence. The most common example of incandescence is the white-hot filament in the light-bulb of an incandescent lamp.
History of incandescent lamps
In 1802, English chemist Humphry Davy (1778–1829) demonstrated that by running electricity through a thin strip of metal, that strip could be heated to temperatures high enough so they would give off light. The strip of metal, called a filament, is resistant to the electricity flowing through it (the thinner the metal, the higher the resistance). The resistance turns the electrical energy into heat, and when the filament becomes white-hot, it gives off light. It incandesces because of the heat. This is the basic principle by which all incandescent lamps work.
In the decades following Davy's demonstration, other scientists and inventors tried to develop workable incandescent lamps. But these lamps were delicate, unreliable, short-lived, and expensive to operate. The lifetime was short because the filaments used would burn up in air. To combat the short lifetime, early developers used thick, low-resistance filaments, but heating them to incandescence required large electrical currents—and generating large currents was costly.
In 1860, English chemist and physicist Joseph Wilson Swan (1828–1914) invented a primitive electric lamp using a filament of carbonized paper in a vacuum glass bulb. In Swan's time, however, it was impossible to make a good enough vacuum. As a result, a wire might be brought to incandescence and produce light for a short time, but it quickly burned up and the light went out. Although the lack of a good vacuum prevented the lamp from working very well, Swan's design helped American inventor Thomas Alva Edison (1847–1931) produce the first practical incandescent lightbulb almost twenty years later.
Electricity: A form of energy caused by the presence of electrical charges in matter.
Filament: The light source or part of an incandescent lightbulb that is heated until it becomes incandescent.
Incandescence: Glowing due to heat.
Resistance: Anything that causes an opposition to the flow of electricity through a circuit.
An important key to Edison's success was that much better vacuums were available by the late 1870s. In addition, Edison knew that the lamp filament should have high, rather than low, resistance. By increasing the resistance, one can reduce the amount of current needed. Increasing the resistance also reduces the amount of energy required to heat the filament to incandescence. After spending fifty thousand dollars in one year's worth of experiments in search of some sort of wire that could be heated to incandescence by an electric current, Edison finally abandoned metal altogether. He then discovered a material that warmed to white heat in a vacuum without melting, evaporating, or breaking—a simple piece of charred cotton thread. On October 21, 1879, Edison first demonstrated in public an incandescent lightbulb—made with his charred cotton thread—that burned continuously for forty hours.