Re: stroboscopic fillament

[palmer@sfu.ca (Leigh Palmer)]

> Very surprising. The time interval between the peaks of I^2 (at 60 Hz) is
> only 1/120 s. And this is sufficient to overcome the "thermal inertia" of
> a tungsten fillament. Amazing. How thin must the fillament (a black body
> source) be to make this possible?

In anticipation of sometime doing something like this again I purchased
a 500 watt tungsten halogen worklight when I saw one on sale for C$15
(less than US$12). The filament in that is a tungsten helix about 7 cm
long and just under 1 mm in diameter. The wire itself looks to be about
50 um indiameter (all of these are estimated; I didn't open it up) and
I suspect that it is not of the doubly helical sort that is found in
ordinary 100 W incandescents, so the answer to your question is about
50 um (estimated).

This brings up another topic. If you have never done so, save the next
light bulb that burns out at home. There are a couple of neat things to
do with it. First, take a pair of Channel-locks or gas pliers and gently
grasp the base of the bulb. Fill a sink with water and immerse the bulb
and the base completely, and then crush the base. If you expect the bulb
to fill up as the tip breaks the water rushes into the vacuum you will
likely be disappointed. This experiment shows just how much gas is left
inside a light bulb; It is not nearly evacuated. You probably knew that
in advance, however. The black spot which is formed on the envelope
above the filament after many hours of use is tungsten, and it is
deposited there by convection.

Next, being careful not to cut yourself on the shards, break the
remainder of the glass and recover the filament. Handle it gently, and
dry it on a paper towel. When it is completely dry examine it closely
with a microscope or a powerful hand magnifier. What you will see is a
tiny helix wound from wire which is itself wound in a tiny helix from
that even thinner wire. It is astonishing to realize that a friable,
refractory metal like tungsten can be prepared in a state that is so
ductile that a wire of such exquisite thinness can be drawn!

This filament will not cool as rapidly as the more solid filament of
the tungsten halogen lamp, however, because of it intricate shape and
its much lower operating temperature. The filament cools at a rate
proportional to the fourth power of its absolute temperature.

Leigh