If you heat a metal wire in the air, it will quickly oxidize and crumble. But this does not happen in a light bulb. Why? It's not about the spiral itself, but about what surrounds it.
1. Vacuum or inert gas.
There is no ordinary air inside the bulb. There is either a vacuum or an inert gas (most often argon or a mixture of argon and nitrogen). The vacuum prevents oxygen from reaching the incandescent filament, which means there is no oxidation, and the metal does not "burn".
The inert gas additionally slows down the evaporation of tungsten, allowing the spiral to retain its shape longer and not lose mass. Modern incandescent lamps almost always use a gaseous medium — it distributes heat more efficiently and prolongs the service life.
2. the metal is at its limit.
The spiral is made of tungsten, the most refractory metal available (melts at 3422 C).
It is able to glow at 2500-2700 C without collapsing immediately.
At the same time, the lamp does not turn on instantly:
The spiral gradually heats up under the current — and only then begins to glow.
3. Thin, but rolled up/The thread is thin to warm up quickly. But it is coiled into a spiral, or even a double one, in order to:
- compactly fit a large length of wire;
- enhance radiation;
- reduce heat loss.
Over time, the tungsten evaporates anyway. Dark spots appear on the glass — this is deposited metal.
Sooner or later, the spiral gets thinner and burns out.
The first lamps almost did not burn out.
At the beginning of the 20th century, incandescent lamps were almost eternal. Some have been burning for decades — and still continue to glow. One such example is a lamp from Livermore in the USA, which has been operating since 1901.
This has become a problem for the business. In 1924, major manufacturers (including Philips, Osram, and General Electric) secretly joined the Phobos Cartel and agreed to:
- limit lamp life to 1,000 hours;
- monitor compliance with the standard;
- penalize companies for "too reliable" lamps.
Thus began the era of planned obsolescence. You can view the centennial light bulb here: https://www.centennialbulb.org/pic-favs.htm The lamp was installed in the fire department back in 1901.
