A tachometer (revolution-counter, Tach, rev-counter, RPM gauge) is an instrument measuring the rotation speed of a shaft or disk, as in a motor or other machine. The device usually displays the revolutions per minute (RPM) on a calibrated analogue dial, but digital displays are increasingly common. The word comes from the Greek Ταχος, tachos, "speed", and metron, "to measure".
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“Tachometer,” Digital Exhibits at Washington and Lee University Library, accessed September 22, 2017, http://omeka.wlu.edu/items/show/202.