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A micrometer is a measurement device incorporating a calibrated screw used widely for precise measurement of small distances in mechanical engineering and machining as well as most mechanical trades. The first micrometer was developed by Jean Laurent Palmer of Paris in 1848.
A micrometer may have its measurements marked in an inch scale, a metric scale or a vernier scale. Inch scales have 40 threads for each inch. Each turn moves axially 0.025 of an inch. A metric scale contains 2 threads for every millimeter so that one turn is equal to 0.5 of a millimeter.