On Tue, 11 Dec 2012 09:18:37 -0800, "William Sommerwerck"
wrote:

We run a halogen (over our dinner table) from a dimmer.
We seldom use it at full brightness. It's about 10 years old
and still on the original bulb.

The envelope is probably black by now. Incandescent life goes with
something like the 12th power of, er, power, so it's not surprising.
You'll probably find that it won't last all that long, now, at full
power. Its filament is now on the inside of the envelope.

Actually, it's the 12th power of voltage.

Of course.

Even a slight drop in voltage causes a big increase in life. For example, a
5% reduction almost doubles the lamp's life (1.85 times). At a 10%
reduction, the lamp lasts 3.5 times as long.