This unique micrometer was patented in 1941 by A. G. Hagstrom and sold since 1955 by Starrett. Unlike other micrometers, the 221 has 2 rotating thimbles. The coarse thimble is like a normal micrometer with increments of .001 inches. The fine thimble amplifies the .001" increments 10x with .0001" increments. They can be read midway to 50 millionths of an inch. The fine thimble is spring loaded to exert the exact same pressure on the spinble for each measurement.
Drawn at 100:1 scale due to very small dimensions which cause problems in Sketchup at 1:1 scale when drawing.