On 2012-10-05, Roderick Stewart wrote:
In article de99517e-e5e1-4f9d-91e0-
, George Herold wrote:
And thatÂ’s it. Repeated on and off means that the thin region has a
higher average temperature than the thick part of the filament. It
evaporates faster and fails sooner.
Won't a thin region of a lamp filament have a higher temperature than
the rest of it all the time, not just when the lamp is turning on?
It will, but due to the thermal coefficient of resistance of tungsten
(most other metals are similar) the heating will be even greater when the
filament heats from cold. The hot spot gains resistance faster, so it
drops a greater voltage while the rest of the filament is still cold.
disclaimer: figures made up to illustrate the point
assume a constant-voltage supply compute the power disipated by he
thin spot (middle resistor) in each case
cold ----[100]---[1]---[100]---
hot ----[1000]-[12]-[1000]----
during warm-up thin spot warms up fastest.
start ---[200]---[3]---[200]---
yeah, it would be a good project for the mythbusters, I'd love to see
a slow motion film of an incandescent lamp failing at turn-on.
but could they affor do dedicate their fast camera for long enough.
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