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Sometimes when an incandescent light bulb is just about to burn out, the tungsten filament begins ringing.

What causes this?

For people who unfamilar with this effect, what happens is that every once in a while, when a bulb is near the end of its life, it begins ringing or sometimes buzzing. The ringing lasts typically for several hours before the bulb finally burns out, but sometimes can last for days. It only happens in a relatively small percentage of bulbs, maybe 1 in 15 or something like that.

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    $\begingroup$ You sure that's not just the sound of it burning? $\endgroup$ – DKNguyen Jun 3 at 4:42
  • $\begingroup$ @DKNguyen incandescents are filled with neutral gas. The filaments vaporize but don't burn in the sense of oxidation. $\endgroup$ – Carl Witthoft Jun 3 at 14:45
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If you take a "healthy" bulb and drive it with low-voltage AC (60 or 50 Hz range), you will hear some ringing/humming due to the temperature of the filament changing at some harmonic of the driving frequency. At the full (USA) 120 VAC, the filament rapidly reaches thermal stability, no ringing.

I have observed ringing in bulbs whose filament has broken but the two parts are in contact with each other; I'm guessing there's some make/break going on in this case. Perhaps a near-death filament has a section where most of the material has vaporized, so the resistance/unitlength there is much greater than the rest, and thermal mismatches occur.

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