"Phil Allison" wrote in message ...
"William Sommerwerck""

If you'd been paying attention, you'd have seen that his most-recent
remark (above), though correct /out of context/, has nothing to do with
the issue at hand.

** What "issue" is that ??

There are actually two issues.

One... A halogen lamp has to run at or above the temperature at which the
tungsten is redeposited on the filament more rapidly than it evaporates.
This temperature is presumably well-above the temperature of a conventional
incandescent lamp. It's reasonable to assume that reducing the filament
voltage some unstated amount will lower the temperature below the critical
recycling temperature, but still high enough to cause the filament to
rapidly burn out. No one here seems to have any information about this.

Two... "Obviously", if the filament voltage is "low enough", the rate of
tungsten evaporation will be so low, that it doesn't matter whether the lamp
is conventional or halogen.

These have nothing to do with each other, as the temperature for One is
almost certainly well above the temperature for Two.

This is what Wikipedia has to say. (The following is 100% accurate and
unimpeachable, of course.)

"Halogen lamps are manufactured with enough halogen to match the rate of
tungsten evaporation at their design voltage. Increasing the applied voltage
increases the rate of evaporation, so at some point there may be
insufficient halogen and the lamp goes black. Over-voltage operation is not
generally recommended. With a reduced voltage the evaporation is lower and
there may be too much halogen, which can lead to abnormal failure. At much
lower voltages, the bulb temperature may be too low to support the halogen
cycle, but by this time the evaporation rate is too low for the bulb to
blacken significantly. There are many situations where halogen lamps are
dimmed successfully. However, lamp life may not be extended as much as
predicted. The life span on dimming depends on lamp construction, the
halogen additive used and whether dimming is normally expected for this
type."

The article says that the first halogen lamps (for a carbon filament) were
patented in 1882.