Fused electric bulb


A filament based electric bulb generates light through the process of incandescence which is the glow associated with a hot-body. Hence this type of lamps are called incandescent lamps.

The electric bulb we normally use consists of a coiled filament made of tungsten wire (essentially an electrical resistor) of different thicknesses depending on the power ratings of the lamp. When the lamp is switched on, an electric current passes through the resistor producing a lot of heating called Joule heating. When the filament is heated to near white-hot, its body temperature rises to a few thousand (s) degrees centigrade which in turn take the tungsten filament near its melting/vaporization point.

One of the problems of the standard electric light bulb is evaporation of the filament. Small variations in resistivity along the filament cause “hot spots” to form at points of higher resistivity; a variation of diameter of only 1% will cause a 25% reduction in service life. The hot spots evaporate faster than the rest of the filament, increasing resistance at that point

On continued usage, this vaporization leads to loss of the tungsten material making it thinner and thinner at some points along the length of the filament coil. In order to minimize the vaporization loss, the whole filament system is enclosed in an evacuated (or filled with inert gases) glass envelope (bulb).

When the electric bulb is switched on and off several times, its filament is taken through several cycles of heating and cooling leading to the filament getting worn-out at some points that would result in the filament getting cut at these point(s). This would eventually make flow of current along the filament and hence the incandescent-glow from the bulb impossible.

Filament notching describes another phenomenon that limits the life of lamps. Lamps operated on direct current develop random stair-step irregularities on the filament surface, reducing the cross section and further increasing heat and evaporation of tungsten at these points. In small lamps operated on direct current, lifespan may be cut in half compared to AC operation. Different alloys of tungsten and rhenium can be used to counteract the effect.

If a light bulb envelope leaks, the hot tungsten filament reacts with air, yielding an aerosol of brown tungsten nitride, brown tungsten dioxide, violet-blue tungsten pentoxide, and yellow tungsten trioxide that then deposits on the nearby surfaces or the bulb interior. It could also burn out the bulb filament.