Tachometer

A tachometer, also called a rev-counter or RPM gauge, is an instrument that measures the rotation speed of a shaft or disk, most familiarly the engine speed of a motor vehicle displayed in revolutions per minute; the name comes from the Greek tachos, speed, and metron, measure, the same roots that give speedometer its meaning, though automotive practice keeps tachometer for engine speed and speedometer for vehicle speed. Bryan Donkin presented the first tachometer design to the Royal Society of Arts in 1810 and won the Society's Gold Medal for it, using a mercury-filled bowl whose liquid level shifted under centrifugal force to indicate rotation speed, and the German engineer Dietrich Uhlhorn is credited with inventing a mechanical tachometer working on the same centrifugal principle around 1817; tachometers have been used to measure locomotive speeds since 1840.