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[Phys-L] Incandescent bulbs (was light bulb transient)



I am grateful to Michael for doing the measurement that revealed the
120 Hz component in AC tungsten incandescent light. I always had a
throwaway topic (meaning I didn't examine on it) in my lectures that
discriminated between fluorescent light and incandescent light,
noting that the former had a 120 Hz flicker which was absent in the
latter. One can easily detect this flicker by noting the stroboscopic
effect when, for example, one waves one's fingers rapidly back and
forth in front of a fluorescent light. I said I didn't see the
slightest hint of that when I did this with an incandescent light
source, but I just tried it again and, knowing from Michael's
experiment that the flicker is there, I am no longer so sure I can't
see it. "The mind sees what's in the mind's eye."

This is just another item to add to a long list of interesting
physical aspects of what has unfortunately become a commonplace
device in our increasingly technically complex modern world. Larry
Woolf is the light bulb expert in this group, and if he didn't know
about the 120 Hz component before, he does now, and will include it
in his next light bulb book. The incandescent lamp was rightly hailed
as a miraculous invention when it was introduced, and there have been
many improvements to it since its invention.

In 1967 Mickey Walsh of the General Electric Research Laboratory
pointed out to me the one that is most stunning. Tungsten is a
refractory metal, yet it can be made ductile by a mechanical process.
This process was invented almost a century ago by William Coolidge,
the same person who invented the successful X-ray tubes that bear his
name, who also worked at GE. Ductile tungsten can be drawn into
extremely fine wires as it must be to make light bulb filaments. The
next time you have a burned out 115 V bulb, break it open and examine
the filament carefully with a 20x magnifier or better. If you have
never done so, you are in for a delightful surprise.

There other aspects of the common incandescent light bulb that are
very interesting and applicable as examples in physics teaching, but
this happens to be the time in history when the niche for choice of
tungsten incandescent lighting is on its way out. I have converted
many of my own incandescent lamps to compact fluorescents because the
local electric utility, BC Hydro, made handsome financial incentives
available to do so, and LED lighting is just around the corner.
Electroluminescent panels are still impractical, I am told.

On 16-Nov-05 Brian Whatcott wrote:

Ahem...I wasn't going to mention it - but a filament fed by AC
does not demonstrate a monotonic temperature change: it goes
uppity-up up, then down down down!
And a uniform function is often taken to be constant in rate -
whereas a filament's temperature changes faster in the middle than
than at the supports.

Well, well well! :-)

For the purposes of the former discussion there is an implicit
integration time determined by persistence of vision, but I suspect
everybody else already knew that. The 120 Hz flicker in light from an
incandescent bulb is imperceptible. Need I also mention that the
detection of light by the eye is a quantum process and it must
"actually" flicker at a much higher rate? I think not, for the same
reason.

Leigh
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