Tachometer

A tachometer (also called a revolution-counter, rev-counter, or RPM gauge) is an instrument that measures the rotation speed of a shaft or disk, as in a motor or other machine. The device usually displays the revolution per minute (RPM) on a calibrated analogue dial, but digital displays are increasingly common. The term comes from greek Ταχος, tachos, "speed", andmetron, "to measure"

History

The first mechanical tachometers were based on measuring the centrifugal force, similar to the operation of a centrifugal governer. The inventor is assumed to be the German engineer Dietrich Uhlhorn; he used it for measuring the speed of machines in 1817. Since 1840, it has been used to measure the speed of locomotive