vernier [key], auxiliary scale, either straight or an arc of a circle, designed to slide along a fixed scale. Its unit divisions, usually smaller than those on the fixed scale, permit a far more precise reading. The vernier is attached to the scales of instruments employed for very accurate linear or angular measurements; these include the transit, sextant, barometer, compass, and caliper. It was devised by a French mathematician, Pierre Vernier, who described it in his Construction, usage et propriétés du quadrant nouveau de mathématiques (1631). Certain auxiliary control mechanisms used for fine measurements or adjustments are often called verniers.
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