A bulb filament when exposed to the atmosphere causes it to burn out, however the heater works with its coil exposed to atmosphere. How is this possible?

Why does the filament of a bulb burn out when exposed

The incandescent light bulb and heater work on the principle that when a current passes through a conductor having a finite resistance, light and heat is generated. Oxidation reaction, that is, chemical reaction of an element with oxygen to form new compounds, is responsible for the bulb’s filament burning out.

The main important factors influencing their working are the temperature and the environment of operation, as well as the chemical nature of the conducting wire.

In the case of bulb, formerly known as `incandescent bulb,’ the light (electromagnetic radiation) is radiated out, the filament is heated to very high temperatures (around 3,000 degrees Celsius).

The filament in a bulb is made of a tungsten wire of very small diameter (less than that of a human hair), made into long spiral or spring-like structure (when fully extended, the length can be up to a few feet).

Due to this coiled nature of the wire, it offers high resistance to the electrical current, thereby heating up the filament to a very high temperature. Under these conditions, if it is exposed to atmosphere, tungsten filament will react with atmospheric oxygen to form tungsten-oxides. So as to prevent this deleterious process, which can limit the lifetime of the filament, the filament is sealed in vacuum or in inert gas environment.

The filament of tungsten burns out due to oxidation when exposed to atmosphere because tungsten has high susceptibility to oxidation.Tungsten is chosen as the material of the filament because of its high melting point (3,410 degrees Celsius) so that it can be heated to very high temperatures (about 3,000 degrees Celsius) when the bulb can provide us with the necessary light.

In the case of heater, the filament is made of Nicrome alloy (an alloy of nickel and chromium). Since the operating temperatures are in the range of a few hundred degrees Celsius, the required electrical resistance is relatively small and hence, the wire diameter is typically in millimetres and the length is relatively small.

Firstly, the temperature of operation of a heater is rather significantly less compared to the bulb and secondly, the alloy is more resistant to chemical oxidation compared to tungsten. The alloy being oxidation resistant at the temperature of the application, it does not get oxidised while the tungsten filament of the bulb gets readily oxidised if exposed to atmospheric oxygen at the high temperature.