Doing a bit of reverse mathematical engineering, I reckon that you'd
have to know in advance that it's designed to work with a circle with a
six inch diameter. Perhaps you're supposed to guess that from the fact
that it's a six inch ruler.
Given two points on that circle, measure the length of the straight line
between them, the "chord", using the CHO scale. The number on the CHO
scale gives you the angle between those points in degrees.
For two points at 90 degrees, we can use Pythagoras to determine the
length to be sqrt(2*r^2). When r=3 inches the 90-degree chord is 4.24264
inches, which matches the length of the CHO scale. (Which is where I got
the "six inch diameter" from.)
Because the CHO scale isn't on the edge of the ruler, you can't measure
the length directly with the ruler. You have to use dividers to transfer
CHO lengths to and from the ruler. Perhaps this rather cumbersome method
of measuring angles might produce slightly more accurate results than
using a small protractor.
25 ส.ค. 50 13:01:16