What instrument is used to measure rotational speed in revolutions per minute (rpm)?

A tachometer is the instrument specifically designed to measure the rotational speed of an object, typically expressed in revolutions per minute (rpm). This device is crucial in various applications, including motor vehicles and machinery, as it provides real-time feedback on engine performance, allowing operators to understand how effectively the vehicle or machine is performing.

The tachometer achieves this by using a sensor that detects the rotation of the engine's crankshaft or similar component and translates that into a readable figure. Knowing the engine's rpm helps operators optimize performance and prevent damage caused by over-revving.

In contrast, while a speedometer measures the linear speed of a vehicle, typically in miles per hour or kilometers per hour, it does not provide information on how fast the engine or another rotating component is spinning. An RPM gauge, while it might sound accurate, is essentially another name for the tachometer and does not represent a distinct instrument. A chronometer is typically used to measure time intervals with a high degree of accuracy, which does not relate to rotational speed at all. Therefore, the tachometer is the correct instrument for measuring rotational speed in rpm.