tachometre

A tachometer (British spelling tachometre) is an instrument that measures the rotational speed of a shaft, typically expressed in revolutions per minute (rpm). It is widely used to monitor engine speed and other rotating machinery; some devices also provide corresponding values in radians per second.

In mechanical tachometers, a gear train couples the instrument to the rotating shaft, translating angular velocity into a pointer movement on an analog dial. Electrical and electronic tachometers use sensors such as magnetic pickups or Hall-effect devices to generate a signal proportional to rotation, which is then scaled to display rpm. Optical tachometers detect timing marks or use light interruptions; some modern models are non-contact and use laser or infrared sensing.

Applications range from vehicles to machinery. Automotive tachometers display engine rpm to aid shifting and diagnostics. In aviation, tachometers monitor engine or propeller speed; in industry, they help control motors and production lines. Non-contact models are common for hazardous or high-speed equipment. Accuracy depends on calibration, gearing ratios, and sensor quality.

Note on terminology: Tachometre is a spelling variant used in some regions. Tachometer is the more common term in American English. A tachymeter, by contrast, is a scale on a chronograph watch used to calculate speed over a fixed distance, not a measurement of rotational speed.