Description
An incandescent light bulb is a light source in which a filament in an oxygen-poor glass bulb is heated by an electric current from a voltage source, causing it to emit light. The filament consists of the transition metal tungsten, which has a very high melting temperature. Initially, carbon (derived from bamboo, silk, or cellulose) or other metals, such as platinum or osmium, were used for the filament. Tungsten proved to be the best solution, partly due to its high temperature resistance. Lamps with a carbon filament (known as carbon filament lamps) are still produced on a small scale for decorative purposes, but have very poor efficiency. Philips shut down its last carbon filament lamp machine in Weert in 2007. The electrical resistance of the filament depends on its thickness, length, and the type of material. This resistance is chosen to achieve the best balance between light output and lifespan at the recommended operating voltage. The electrical resistance of the tungsten filament is 15 times lower when cold than when hot. Switching on an incandescent lamp is therefore accompanied by a brief current surge, causing the filament to vibrate mechanically due to magnetic induction. This is why the lifespan of many incandescent lamps ends precisely at the moment they are switched on. The filament is protected from burning by a glass bulb containing little or no oxygen. If oxygen from the air reaches the filament due to damage to the bulb, it will burn within a second of being switched on. The first incandescent lamps were vacuum-sealed, but in the early twentieth century, it was discovered that argon-filled lamps produced a brighter light and also lasted longer. Nevertheless, it took many decades for argon lamps to supplant vacuum lamps. Compared to gas-filled lamps, vacuum lamps have a larger diameter bulb and thicker glass. The glass of a gas-filled lamp is a few tenths of a millimeter thick. Even in an incandescent lamp with an undamaged bulb, the filament material evaporates very gradually due to the heating and deposits on the inside of the glass bulb, darkening it. Filling the balloon with gas slows this process. The presence of traces of water vapor actually accelerates the process.
