Tachometers weren't very popular in automobiles when our friend Webster 1913 was around, so let me just say this about that:

A tachometer is now a gauge in the dashboard of a car that shows you the number of revolutions per minute the camshaft in your engine makes as the pistons fire. The more RPMs, the faster it's turning.

Ta*chom"e*ter (?), n. [Gr. swiftness, speed (fr. quick) + -meter: cf. F. tachometre.]

An instrument for measuring the velocity, or indicating changes in the velocity, of a moving body or substance.

Specifically: --

(a)

An instrument for measuring the velocity of running water in a river or canal, consisting of a wheel with inclined vanes, which is turned by the current. The rotations of the wheel are recorded by clockwork.

(b)

An instrument for showing at any moment the speed of a revolving shaft, consisting of a delicate revolving conical pendulum which is driven by the shaft, and the action of which by change of speed moves a pointer which indicates the speed on a graduated dial.

(c) Physiol.

An instrument for measuring the velocity of the blood; a haematachometer.

 

© Webster 1913.

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