Which device is used to measure rotational speed?

• A. Tachometer
• B. Optical encoder
• C. LVDT
• D. Reed switch

Answer: (A)

Rotational speed is best measured by a device that is designed to read how fast a shaft turns and provide a direct speed value in RPM. A tachometer does just that, giving a straightforward readout of how quickly the system is spinning, which is exactly what you need when you want to quantify speed.

Other devices have different primary roles. An optical encoder detects rotation by counting light pulses to determine angle and can derive speed from pulse rate, but its main purpose is encoding position and motion, not just measuring speed. An LVDT measures linear displacement, not rotation. A reed switch acts as a simple magnetic contact that detects the presence of a magnet, not rotation speed.