QUESTION:
Why does the
surface of the incandescent light bulbs become progressively Greyer over time?
ANSWER:
The greying of the inner surfaces of incandescent bulbs is the result of
gradual evaporation of tungsten from the filament while the light is on. This
evaporation eventually makes the filament so thin it burns out.
Various
methods have been developed to reduce greying. Filaments of the first
incandescent lamps burnt in a vacuum, but it was soon found that introducing
inert gas to the bulb reduced the rate of greying.
A
mixture of nitrogen and argon is used today. In addition, ``getters'' reactive
metal such as tantalum and titanium can be placed near the filament to attract
the tungsten so that it is not deposited on the glass.
Alternatively,
a small amount of abrasive tungsten powder can be placed in the bulb. Shaking
it occasionally will remove the grey coating from the surface of the glass.
Greying
can almost eliminated by introducing a small amount of the halogens iodine and
bromine. As tungsten evaporates from the filament, it reacts with the halogens
which then redeposit the tungsten on the filament. This keeps the bulb wall
clean.
To
prevent the tungsten halides from condensing on the bulb and breaking the
cycle, the temperature of the bulb wall must be at least 500 -C. This is too
hot for glass bulbs, which normally operate at about 150 -C, so fused quartz
(silicon dioxide) must be used instead.
Compared
with ordinary incandescent lamps, quartz-halogen lamps have longer lives and
maintain their light output over time. For example, a quartz-halogen lamp with
a 2000-hour life.
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