Which gasses are used in electric bulbs? Or vacuum is left inside?

There is no vacuum in the electric bulb. On the contrary, argon gas or argon gas mixed with a little nitrogen is filled in the bulb under some pressure. Years ago there was a practice to leave vacuum inside the bulb so as to prevent the burning of tungsten filament in the temperature of 2000° Celsius inside the bulb. But at such a high temperature tungsten filament starts losing its atoms. Or, to be precise twisted strands of tungsten filament start ‘evaporating’ slowly. As the escaping particles of burnt tungsten settle on the transparent glass of the bulb, accumulating carbon becomes an obstruction to the light.

Ultimately a situation comes about in which although the bulb is lit it does not throw much light in the room. Also, continuously ‘evaporating’ filament of tungsten may not last for a long time. A well-known American chemist named Erwin Lang-Moore invented two methods in 1913 to bring an end to both these problems: (1) He twisted the strands of tungsten filament weaving them into a plait which till now were in the form of spring like coils. Plaiting made the tungsten filament more durable. (2) He replaced the practice of leaving vacuum in the bulbs with that of filling them up with nitrogen or argon. Argon is an inert gas which does not give rise to any chemical reaction and so would not affect the working of tungsten filament. At the same time the pressure of Argon gas would almost prevent ‘evaporation’ of the filament.

It is because of the above two inventions that a present day electric bulb has useful life of about 1,000 hours.

More reading:
Incandescent light bulb (Wikipedia)

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