Definition of tachometer:

part of speech: noun

An instrument for measuring velocity; especially, ( a) a contrivance for the purpose of indicating small variations in the velocity of machines, one form of which consists of a cup and a tube opening into its centre, both being partly filled with mercury or a colored fluid, and attached to a spindle. This apparatus is whirled round by the machine, and the centrifugal force produced by this whirling causes the mercury to recede from the centre and rise upon the sides of the cup. The mercury in the tube descends at the same time, and the degree of this descent is measured by a scale attached to the tube. On the velocity of the machine being lessened the mercury rises in the centre, causing a proportionate rise in the tube; ( b) an instrument for measuring the velocity of running water in rivers, etc., as by means of its action on a flat surface connected with a lever above the surface carrying a movable counterpoise, or by its action on the vanes of a wheel, whose revolutions are registered by a train of wheelwork.

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