Which instrument is specifically designed to measure rotational speed?

The tachometer is specifically designed to measure rotational speed, typically of an engine or a rotating component. It provides the user with real-time information about how fast a shaft or engine is rotating, usually expressed in revolutions per minute (RPM).

In the context of diesel engines and automotive applications, a tachometer is essential for monitoring engine performance and ensuring that it operates within its optimal RPM range, which can help in maximizing fuel efficiency and preventing engine damage. This makes the tachometer an invaluable tool for diesel technicians who need to diagnose and maintain engine performance effectively.

In contrast, the other options serve different purposes. A voltmeter measures electrical potential difference between two points; a manometer measures the pressure of gases or liquids; and an odometer tracks the distance traveled by a vehicle. None of these instruments provide information regarding rotational speed, which highlights the specific function of the tachometer as the correct choice.