vernier

n. [ So named after the inventor, Pierre Vernier. ] A short scale made to slide along the divisions of a graduated instrument, as the limb of a sextant, or the scale of a barometer, for indicating parts of divisions. It is so graduated that a certain convenient number of its divisions are just equal to a certain number, either one less or one more, of the divisions of the instrument, so that parts of a division are determined by observing what line on the vernier coincides with a line on the instrument. [ 1913 Webster ]

Vernier calipers,
Vernier gauge, a gauge with a graduated bar and a sliding jaw bearing a vernier, used for accurate measurements. --
Vernier compass, a surveyor's compass with a vernier for the accurate adjustment of the zero point in accordance with magnetic variation. --
Vernier transit, a surveyor's transit instrument with a vernier compass. [ 1913 Webster ]