mechanical tachometer

A tachometer is an instrument that indicates the angular speed of a rotating shaft, typically in revolutions per minute (rpm). The term usually refers to devices showing instantaneous speed, rather than those that only count total revolutions over time.12

Mechanical tachometers operate on principles like centrifugal force, where the force on a rotating mass, dependent on speed, interacts with a spring. Another type is the vibrating-reed tachometer, which uses tuned reeds to indicate speed by their vibration frequency.1

Electrical tachometers include the eddy-current type, common in car speedometers, where a rotating magnet generates eddy currents proportional to speed. Generator tachometers produce either alternating or direct current based on rotation. A stroboscope can also function as a tachometer by illuminating rotating objects to make them appear stationary. The Greek root tach- means "speed," which is reflected in the word's meaning.12