GUSTOIMAGES / SCIENCE PHOTO LIBRARY GUSTOIMAGES / SCIENCE PHOTO LIBRARY
Light bulbs. Coloured X-ray of an assortment of electric light bulbs. Some have screw fittings and some bayonet fittings. Each has a finely coiled filament inside a glass envelope. A fine tungsten wire (about 0.01mm thick) is wound into a helical coil, and then wound into a larger diameter helical coil. Tungsten is ideal for a filament as it is strong and has a high melting point (3410 degrees Celsius). The glass bulb is filled with an inert nitrogen/argon mixture to prevent oxidation. As the bulb runs, however, small amounts of tungsten deposit onto the glass. Eventually, this results in the filament breaking; a domestic light bulb lasts about 1,000 hours.
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