Micrometer Gauge: Pratt, 1878

A micrometer clamps the work between a fixed abutment and the tip of a fine screw. Both faces must be smooth and very hard, but tempering a finished screw changes it: the heat expands the metal, part of the expansion stays, and the cooling is uneven, so a screw that was accurate before hardening often is not after. George M. Pratt of Middletown, Connecticut, patented the remedy in 1878, US 202,866: leave the screw untempered and give it a socket at its end for a separate tip of steel tempered as hard as possible. A worn tip is simply replaced. He also shaped the horseshoe frame widest at the middle and tapering towards the ends, so it is stiff where it most needs to resist springing but lighter overall.