Andrew Gabriel wrote:

In article ,
(John Watson) writes:

Here endeth the Physics lesson . . .

Roger

What an excellent Physics lesson, Roger.
I am an electrical engineer/ sparkie with an inquiring mind, and have
often wondered why this was the case.


Personally, I don't buy that explanation, as the filament temperatures
are not different.



They are. That was the real reson for use of LV halogen bulbs in
projectors years ago - a much better colour temperature due to use of
higher filament operating temperature.

The design problem which causes lower efficiency of
mains voltage lamps is due to the way the surface area can't be kept
optimum due to the need to change the filament resistance for different
operating voltages. There's one optimum surface area for a given power
output and lamp life, and it turns out that this optimum surface area is
driven by a voltage of around 55V for a lamp of the order 100W. As you
design lamps to operate at voltages further away from this ideal, you
have to compromise their efficiency. 240V happens to be rather more
compromised than 12V.


In vacuo, the surface area will be a function of the power input and
filament temperature anyway, since almost all the heat is lost by
radiation, so it sounds like you are merely saying the same thing in a
different way.

You aren't Ivor Catt are you?



I described this effect in more detail in article:
http://groups.google.com/groups?selm...net.uk.sun.com