Why does a lightbulb get suddenly much brighter shortly before it blows?
In: Physics
As an incandescent bulb is used, it is slowly getting thinner and thinner. It is literally evaporating atoms due to the high heat. (This is one of the reasons that a coil shape is used. It allows some of the boiled-off atoms to re-deposit.)
As the filament gets thinner, its electrical resistance increases, which increases how hot/bright it is. This effect is not really noticeable at first, but at the end of life it becomes important.
Some spots are going to thin out faster than others due to random variation in manufacturing, grain size, etc. Those areas are going to get a bit hotter and thus thin out a bit faster. This creates a “positive feedback” effect. A spot gets thinner so it gets hotter so it gets thinner faster so it gets hotter etc. etc.
Eventually some spot gets very thin and things come to a rapid conclusion. The feedback effect runs very fast, and the very thin spot doesn’t just evaporate atoms, it boils. It turns to a small spot of liquid, and then a plasma in a brief flash of light as current arcs through the plasma. And then it’s gone, leaving a gap in the filament and the bulb no longer conducts electricity.
There can also be a secondary effect in tungsten filaments where the tungsten atoms migrate within the filament, causing it to take on an odd, chunky shape with thick and thin areas as some of the grains grow at the expense of others. But I can’t for the life of me remember what it’s called right now.
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