"William Sommerwerck" wrote in message
...
One of the reasons that a car bulb is slow to reach full output, is that
the
filament has to be made thick and robust for enhanced reliability and
failure proofing from vibration. That makes it have substantial thermal
inertia, so much so that you can indeed see it hot up. On the other hand,
high voltage domestic lightbulbs have no such constraints on their
filaments, as they are not going on and off all the time like a brake
light
or indicator, and are not subject to vibration in normal use. This
results
in the manufacturers being able to design them with a much more delicate
filament, with the result that, to all intents and purposes, the ramp up
to
full output is 'instantaneous'.

I don't like disagreeing with you, but this is incorrect. You can easily
see
the "ramp up" of household incandescent lights. It's about 1/10 of a
second,
and is plainly visible. Some CFLs are (at least visibly) "instantaneous".



Well, maybe the driving voltage does have something to do with it then. With
double the line voltage here that you have there, I can honestly say that I
cannot see any perceivable delay between flicking the switch, and having
light. I have asked a number of people the question, and not one of them has
said that they can either. Not that that is much of a scientific test, of
course. Interestingly though, the ones that I said that I could see ramp up,
are low voltage halogens. So I wonder if the fact that low voltage bulbs
need amps through them, to get the same filament power as a line voltage
bulb does with miliamps, dictates how robust the filament needs to be, and
hence how much theremal inertia it has? I know that halogens run a hotter
filament to get slightly brighter light, but you can also see the low
voltage ones visibly cool as well, which would lend creedence to the theory
that the filament is more robust.

It is not a function of the fact that the bulbs are of a halogen type per
se, as the fittings that I have just bought also use halogen bulbs, but
unlike the fittings that they are replacing, which *were* low voltage (12v
AC nominal), these are line voltage types. They definitely appear to come on
pretty much 'instantaneously'.

So, perhaps a US 110v bulb, does take twice as long to reach 'full' output,
as a 240v one, and that is indeed enough to be able to see ?

Arfa