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Bulb burn-out

At 2:35 PM -0400 4/9/98, David Bowman wrote:

<major snip>

I believe the reason why light bulbs often "burn out" when they are
switched on is that at the instant of being turned on their resistance is
*much* lower than when at operating temperature (for how much lower, see
Brian W.'s posts). This short circuit condition results in a huge initial
current pulse being sent through the filament. The mutual magnetic forces
acting on different parts of the (coiled) filament due to the current pulse
causes a mechanical shock to the filament, flexing it.

I'd like to add a personal anecdote that supports the MAGNETIC part of this
epistle.
During a thunderstorm in the fifty's a lightning bolt struck very near our
house. The overhead light in the back hall had been on, but it FLASHED and
'burned out' as the thunderclap and lightning did their thing. As an
inquisitive youngster, I promptly replaced the bulb and carefully examined
the 'burned-out' bulb. I couldn't see any break in the filament, so I
carefully broke the glass and indeed found the filament to be intact! BUT,
one of the heavy, vertical supporting electrical lead wires had vaporized
for a length of about 1/4 inch (pre-metric days!). I failed to notice WHICH
of the leads vaporized but would now expect it to be the HOT one.

My hypothesis is that a current pulse entered the house (no fuse was blown)
and traveled along the hot wire to the bulb but was blocked by the
INDUCTANCE of the filament, so, the 'stored up' charge vaporized the wire.
(I hope Bill Beatty doesn't have a cow, now, but I can only explain what I
saw in these terms, with capacitance of the wiring and the inductance of
the filament coil playing major roles.)

This bulb was, in fact, the electrical element closest to the service
entrance /fuse box. No, I've never seen ball lightning and wasn't sniffing
glue.

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