Incandescent lamps have changed little since they were invented by Thomas Edison around 1878. The filament is encapsulated in a glass bulb filled with an inert gas to prevent the filament from burning up quickly. The glass bulb is set into a metal screw-in base with one electrical contact being the threads and the other being a small protrusion on the bottom. As the tungsten filament glows, it slowly evaporates and is deposited on the inside wall of the glass bulb. You can see this phenomenon as light bulbs slowly acquire a black tint over time.
Incandescent lamps fail when the filament burns all the way through and breaks.
Such lamps use different combinations of gases to fill the bulb, and long-life models prolong the typical lifetime to about 2500 hours but sacrifice lamp efficacy slightly. Last updated December 31, 1995.