Monday, August 23, 2010

Why does an Electric Bulb Give off Light?

A light bulb gives off light because an electric current is passed through its filament, a thread of tungsten metal thinner than a human hair, which then becomes white hot.

Sir Joseph Swan (1828-1914) in England and Thomas Alva Edison (1847-1931) in the United States constructed the first incandescent electric lamps in 1879. They succeeded in preventing the rapid burning up of the filament by oxidation, but their problem lay in the choice of a suitable material for the filament.

Edison sat in his laboratory watching a filament of charred cotton thread glow in a glass bulb, which had been exhausted of air, for 40 hours. But the thread was too fragile to sustain the heat provided by the electric current. A material was needed that would stand great heat, for the hotter the filament, the brighter is the light given by the lamp.

But even with the present day use of tungsten, the problem still remains in that the more a filament is heated, the sooner it will burn away. To solve this problem, gas discharge lamps were invented.

These consist of glass tubes filled with sodium or mercury vapor, or neon gas. At each end of the tubes are electrodes, or contacts. When an electric current is applied to one of the contacts it passes through the gas to the other contact, causing the gas to glow and give out light.

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