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Light bulb science and technology



Far from being boring, I think there is lots of, um, enlightening
discussion which can emerge from contemplation of the engineering
marvel that has evolved since its invention by the Wizard of
Menlo Park.

to which Chuck Britton added:

And the engineering involved in actually CONSTRUCTING the Tungsten filiment
is quite interesting also. Tungsten is a notoriously difficult metal to
machine or otherwise shape. It is extremely unmalleable. It is no wonder
that Edison et al. didn't have the opportunity to try early experiments
with our now 'standard' filaments.

In 1967 I attended a solid state summer school at which Mickey Walsh,
a GE physicist who specialized in metals, was a principal speaker. He
emphasized the remarkable feat Chuck is talking about. Ordinary
tungsten is a brittle refractory metal, one which has many properties
which suit it ideally to service in light bulb filaments, but one
which would not normally be described by anyone as ductile. In order
to appreciate just how amazing the tungsten light bulb filament is, I
urge every physics teacher to make the small effort required to look
closely at the filament from a 60W or 100W light bulb. Break a burned
out bulb to get at it; you can't see it though the envelope without a
good long distance microscope. A good hand magnifier will serve to
reveal the structure, but a stereo microscope is better if you have
access to one. If you get really adventurous you might try looking at
the burning filament through a solar filter on a long distance
microscope (a Questar telescope serves well for this purpose) and you
might even hook your bulb up to a Variac and watch it burn out
through the telescope. I haven't done this last experiment myself,
but it shouldn't be too difficult to accomplish. An ambitious student
might want to try to videotape this phenomenon for a science fair
project...

It is clear that there is much to learn and much to marvel at in this
boring topic. Next, perhaps you'd like to speculate about the black
coating that appears inside the envelopes of tungsten filament bulbs
after they have burned for some time. Where does it appear? What can
one infer about the technology of the light bulb by making these
observations? Your students will better appreciate just what it is
that engineers do (and have done) as they probe this mundane, boring
topic just a bit more deeply.

Leigh