On 14/01/2019 11:02, Thomas Prufer wrote:
On Mon, 14 Jan 2019 10:31:56 +0100,
(D.M. Procida) wrote:

I assume it would not come to any harm run at a lower voltage, though I
know that some lamps can darken as the result of deposits on the inside
of the glass when run dimmed.

The darkening is metal vaporizing from the filament and settled on the glass.
The bulbs that habe a mechanisnm to avoid this are the halogen bulbs: the glass
is run hot, a halogen scrubs the metal off the glass, and re-deposits it on the
filament.

There was a "fashion" (in the late 80s I think) for cars to have
dim-dipped lights - in fact I think it was a legal requirement at one
time - where, with the ignition on and sidelights on, the headlamp bulbs
were connected in series (for more light in dusk I assume). This was
supposed to have shortened the lives of the halogen bulbs used at the time.

--
Max Demian