Screw gauge

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A screw gauge, also known as a pitch gauge or micrometer screw gauge, is an instrument used to measure the internal and external diameters of small objects, such as pins, wires, and rods. It consists of a thin spiral strip called the stem, which has evenly spaced marks along its length, and a thumbscrew that can be turned to bring the stem into contact with the object being measured. The measurement is determined by counting the number of divisions on the stem that have been compressed.