Oct 6 2007
This article was updated on the 11th September 2019.
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The structure of incandescent light bulbs is very simple. They have two metal contacts at the base that connect to the ends of an electrical circuit in, for example, a lamp socket. The metal contacts are connected to a thin metal filament in the middle of the bulb with two stiff wires. The metal filament is supported by a glass mount or stem. The metal filament, wires and glass mount are contained within the glass bulb.
An electric current flows from one metal contact, through the thin filament and to the other metal contact when the light bulb is connected to a power supply. The thin metal filament must be heated to an elevated temperature before a useful amount of visible light (incandescence) can be emitted.
Many metals would melt and become liquid before reaching a high enough temperature. Therefore, the metal filament in an incandescent light bulb is constructed from tungsten, which has the highest melting point of any metal in pure form (3422 °C, 3695 K).
The Tungsten Filament Inside an Incandescent Light Bulb
The tungsten filament is thin and long. For example, a typical 60-watt bulb has a tungsten filament that is approximately 2000 mm long and 0.25 mm thick. The filament is wound to make one coil, and this coil wound again to produce a larger coil. This double coil configuration is required in order to fit the long filament into a small space. In a typical 60-watt bulb, the length of the coil is less than 25 mm.
One problem with the incandescent light bulb is evaporation of the tungsten filament. At operating temperatures, vibrating tungsten atoms eventually detach from each other and fly into the air or gas around them. Hotter and more efficient filaments will evaporate faster. As more and more tungsten atoms evaporate, the filament will disintegrate and the bulb will no longer work. Therefore, the lifetime of an incandescent bulb is a balance between longevity and efficiency.
In 1913, General Electric researchers Irving Langmuir and Lewi Tonks discovered that if a light bulb is filled with an inert gas such as argon, filament evaporation is slowed. When the evaporation of a tungsten atom occurs, it collides with an argon atom, bounces back toward the filament and rejoins the solid structure. The addition of an inert gas also reduced the strength required of the glass.
Power Consumption of Incandescent Light Bulb
The power consumed by an incandescent light bulb is used to generate heat rather than light. In fact, approximately 95% of the electricity consumed is given off as heat. Because of this inefficient use of energy, incandescent light bulbs are becoming less common, while more efficient light sources such as energy-efficient light bulbs, fluorescent bulbs and LEDs are gaining popularity.
Applications for Incandescent Light Bulb
The power consumed by an incandescent light bulb is used to generate heat rather than light. In fact, approximately 95% of the electricity consumed is given off as heat. Because of this inefficient use of energy, incandescent light bulbs are becoming less common, while more efficient light sources such as energy-efficient light bulbs, fluorescent bulbs and LEDs are gaining popularity.
Source: AZoOptics