Diffraction: micrometers as variable slits

From: Nicholas Bodley (nbodley@world.std.com)
Date: Wed Mar 28 2001 - 11:18:30 PST

If you know a machinist, and he (or she) has a micrometer [caliper],
and if you're courteous and trustworthy, the machinist might let you
sight between the measuring faces. Hold them quite close to your eye,
and sight between them at a distant scene. If the micrometer is in
good condition, the surfaces are very flat, and polished; they make
good mirrors. You can adjust the gap down to an indicated 0.0001
inch, or with a metric micrometer, maybe 0.001 mm. Note how the
spacing between fringes changes as you change the size of the slit.

An absolute no-no is to use this measuring instrument as a clamp, or
to tighten it at zero or onto any object. This is real abuse, as bad
as dropping it onto a concrete floor from many feet in the air.

There are used-tool shops that sell low-cost used micrometers that
might have measuring faces in decent shape. You might find one for

Secret of an inch-reading micrometer: Its thread pitch is 40 threads
to the inch, which means each revolution of the spindle moves it by
0.025 inches. (1/40 = 0.025)

Starting from zero, one turn separates the measuring faces by 0.025",
next turn by 0.050", next, 0.075", etc. Multiples of 25 ought to be
easy to deal with.

For some reason, the classic descriptions of how to read a micrometer
that I recall keep this a big secret! Tradition and cultural inertia,
combined with only modest intelligence, I guess.

Nicholas Bodley |@| Waltham, Mass.
Please reply to nbodley@alumni.princeton.edu
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Autodidact and polymath to some extent

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