Which device converts the rotational motion of a shaft into a reading on a dial?

The handheld mechanical tachometer is specifically designed to convert the rotational motion of a shaft into a reading displayed on a dial. This device works by utilizing a mechanical system, often with gears, that translates the speed of rotation into an angular position that can be read off a calibrated dial.

When a shaft rotates, the tachometer detects this motion, usually via a contact or non-contact mechanism, and translates it into a value representing the shaft's speed. The dial provides a visual indication of this speed, usually in revolutions per minute (RPM). This allows operators to easily monitor the performance of machines and ensure they are operating within desired parameters.

In contrast, the other options serve different functions. A digital voltmeter measures electrical potential difference, a pressure switch monitors pressure levels without providing rotational speed, and a flow meter measures fluid flow rates, not rotational motion. Each of these devices operates on principles specific to their applications, rather than converting rotational motion into a dial reading. This distinction affirms why the handheld mechanical tachometer is the correct choice for this question.