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The Arc Lamp


An electric discharge can arc through air between two electrodes. In an arc lamp two carbon electrodes are first brought together and an electric current of at least 55 volts is passed through them. The electrodes are then moved apart about one eighth of an inch. As the tips are separated resistance causes the tips to glow and the emission of electrons heats the air to over 20,000 degrees Celsius. The thermal plasma glows and is a source light. Most of the light comes, however, from the glowing tips of the carbon electrodes which are incandescent. As the tips burn they must be moved closer together to keep the gap about 1/8 of an inch.



The Incandescent Lamp



In the incandescent lamp a conductor has a current passed through it which causes the wire to glow to white heat due to resistance. The conductor is usually a thin wire filament mounted on a glass support and attached to thicker wires through which the current is supplied. To prevent the filament from burning in the air, it is sealed in a glass bulb which has been evacuated to form a vacuum. The first filaments used in light bulbs were made of carbon. It was not possible to raise the temperature of these filaments to white heat so the light that they gave out was quite dim. Carbon filaments were abandoned in favor of tungsten metal. Tungsten can be heated to 3,000 degrees without melting. Special techniques are used in the manufacture of these filaments. Tungsten is ultimately drawn into wire that is only 0.0004 inches in diameter. To give you an idea of how thin this is, 200 miles of wire is produced from only one pound of tungsten metal. The length of filament used in a modern light bulb is roughly 30 inches. The wire is very tightly coiled.