On Fri, 05 Oct 2012 19:00:56 -0400, Phil Hobbs
wrote:

George Herold wrote:

On Oct 5, 2:51 pm, Jeff Liebermann wrote:
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.

Grin, the internet as a 'fire hose' of information. I went searching
for something that contained "Philips tech. rev." and found a
reference to the following article,

H. Horster, E. Kauer and W. Lechner — The Burn-out Mechanism of
Incandescent Lamps Philips Technical Review 32,155-164, 1971.

It was referenced in "Illuminating Engineering - Page 32 - Google
Books"

But nothing about turn on failure... sigh.

Here is a patent by some of the same guys at Philips... lots of stuff
about the filament getting hottest in the middle.

http://www.freepatentsonline.com/3868159.html
(Actually a decently written patent.)

George H.

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.


I can believe that the filament is hottest in the middle. It's
furtherst from the support, so whatever conductive heat sinking there is
will be less, but more than that, it sees the radiative input from the
rest of the filament on both sides instead of just one.

Aren't the filaments welded to the elements at the ends? It would seem that
this would cause a narrowing. ISTR most filaments broken near the supports,
which would be counter to the hotter-in-the-middle theory.

My theory is that bulbs tend to fail when turned on because of the thermal
shock but only because they were about to fail anyway. Cycling, itself,
doesn't have a huge effect on longevity, certainly not a factor of two.

Sort of similar to the case of a long solenoid, whose B field at the
ends is half what it is in the middle.

Do they only burn out when energized? ;-)