Gauge Block

A gauge block, sometimes called a Johansson gauge after its inventor, is a precisely ground and lapped block of hardened steel, ceramic, or carbide used as a reference standard for length in industrial and metrology settings. Swedish machinist Carl Edvard Johansson developed the design in the 1890s while working to give Swedish rifle manufacturing more accurate and interchangeable measuring tools, and patented it in 1901. A set of blocks with graduated thicknesses can be stacked and wrung together, a bonding effect created by their extremely flat, polished faces, to build up almost any length to within millionths of an inch, and machine shops use the resulting stack to calibrate micrometers, calipers, and other measuring instruments. Johansson's own 25.4 millimeter conversion factor between the metric and imperial systems was later adopted as the international standard industrial inch.