David wrote:
On Tue, 04 Apr 2006 00:26:27 GMT, HFguy wrote:

David wrote:

Any bulb will burn for decades if the voltage is low enough.

Also the fact that it's been on continuously. Incandescent light bulbs
seldom burn out while they are on, unless dropped or hit. Failures are
caused by the current surge when it's turned on. However they must have
had power failures during the many years the bulb has been there. I
wonder how they protect it from surges when the power comes back on.


The filament has high resistance. The current never gets high enough
to stress it. I'm surprised it survives earthquakes.

From the 'Great Internet Light Bulb Book':


-WHY BULBS OFTEN BURN OUT WHEN YOU TURN THEM ON-

Many people wonder what goes on when you turn on a light. It is often
annoying that a weak, aging light bulb will not burn out until the next
time you turn it on.
The answer here is with those thin spots in the filament. Since they
have less mass than the less-evaporated parts of the filament, they heat
up more quickly.
Part of the problem is the fact that tungsten, like most metals, has
less resistance when it is cool and more resistance when it is hot. This
explains the current surge that light bulbs draw when they are first
turned on.
When the thin spots have reached the temperature that they would be
running at, the thicker, heavier parts of the filament have not yet
reached their final temperature. This means that the filament's
resistance is still a bit low and excessive current is still flowing.
This causes the thinner parts of the filament to get even hotter while
the rest of the filament is still warming up. This means that the thin
spots, which run too hot anyway, get even hotter when the thicker parts
of the filament have not yet fully warmed up. This is why weak, aging
bulbs can't survive being turned on.