Sky & Telescope - April 2025 - 75

make marks upward and downward
every millimeter for about a centimeter
in each direction. That's your
rough gauge to count complete turns of
the screw (1 mm per turn), while the
100-increment disk counts hundredths
of a millimeter.
That's pretty much your spherometer.
So how do you use it? You start
by setting it on something really flat.
A commercial countertop or a thick
piece of window glass will do. Back the
screw off a bit and set the spherometer
on the flat surface, then turn the screw
downward until it just makes contact.
If your screw is loose (ha ha), you can
feel it snug up when it hits bottom. If
your screw is tight, you won't feel it
hit bottom, so the way to tell when it
has is to give the spherometer a little
tangential nudge each time you lower
the screw. When it spins on the central
screw rather than skids on the outer
legs, you've hit bottom.
Once you've evened out on a flat
surface, take note of the dial reading. It
probably won't be zero, but that doesn't
matter. Just note what it reads, then
move to your work piece and do the
same. Count up how many millimeters
you moved the screw. Then plug it into
the formula below to get your radius of
curvature.
And there's your new tool to be used
again and again.
¢ Contributing Editor JERRY OLTION
now knows the radius of curvature of his
barbecue lid, his desk lamp shade, and
the lens he's grinding.
HOW CURVED IS YOUR
SURFACE?
The formula for radius of curvature is
R = +
h
2
a2
6h
where h is the spherometer reading
and a is the average distance
between the three outer legs. So if
your spherometer has a leg spacing
of 69 mm and you measure a depth
of 0.39 mm, you've got a surface with
a radius of curvature of 2,035 mm, or
80 inches.
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    
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Sky & Telescope - April 2025

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