What instrument measures the rate at which a shaft is turning?

A tachometer is the instrument specifically designed to measure the rotational speed of a shaft, typically expressed in revolutions per minute (RPM). It functions by detecting the number of turns the shaft makes over a given period, allowing operators to monitor how quickly machinery is operating. This is critical in various applications where engine performance, synchronization of parts, and system efficiency rely heavily on maintaining optimal RPM levels.

In contrast, a voltmeter measures electrical potential difference between two points in an electric circuit, while an oscilloscope provides a visual representation of electrical signals over time, and an ammeter measures the current flowing through a circuit. Neither of these instruments is designed to assess rotational speed, which underscores why the tachometer is the correct choice for measuring the rotation rate of a shaft.