Which instrument is used to measure the speed of a rotating shaft?

The tachometer is the instrument specifically designed to measure the speed of a rotating shaft. It provides a direct readout of rotational speed, typically in revolutions per minute (RPM). This measurement is crucial in various applications, including monitoring the performance of motors, engines, and other machinery that involves rotating components. A tachometer can help ensure that machinery operates within optimal speed ranges, which is vital for efficiency and safety.

In contrast, the other options serve different purposes: a multimeter is used for measuring electrical properties such as voltage, current, and resistance; a voltmeter specifically measures electrical potential difference between two points; and a ruler is a measuring tool used for linear measurements, not applicable for rotational speed. Understanding the specific functions of these instruments is key for effective operation and maintenance in sewage treatment and other environmental settings.