On Thu, 04 Oct 2012 10:03:21 -0400, Phil Hobbs
wrote:

Yep. As I understand it (possible wrong), AC filaments break in the
middle, mostly from vibration flexing.

I don't think so, because there's no mechanism for that, as I said. The
wire is fully annealed at all times, so there's no possibility of
progressive fatigue failure.

Oscillating filament light bulb:
http://www.youtube.com/watch?v=t_DwwNVA-7Q
Whether the earths magnetic field is strong enough to induce such
oscillations is questionable.

While digging for the apparently mythical lifetime test data on
incandescent light bulbs, I've found numerous theories on why
filaments fail. Tungsten evaporation, causing hot spots, is the most
common. One suggested that thermal cycling hardens the tungsten and
makes it brittle. Another suggested that the inrush current causes a
mechanical shock if it hits at the 60Hz peak, instead of at the zero
crossing. Yet another speculates that the temperature differential
between the hot filament, and the relatively cold mounting structure
may cause cracking.


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Jeff Liebermann
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