Which device would be used to measure linear displacement along a straight line?

• A. LVDT
• B. Tachometer
• C. Reed switch
• D. Photocell

Answer: (A)

Measuring linear displacement along a straight path requires a sensor that converts the movement of a core into a continuous electrical signal with a stable, linear response. The LVDT fits this need because it uses an AC-excited core inside a transformer assembly. As the ferromagnetic core moves, the magnetic coupling to two output coils changes, producing a differential voltage that is proportional to position. This gives high-resolution, repeatable, non-contact measurement of linear displacement over a useful range, with good long-term stability and low wear.

Tachometers are meant for rotational speed and RPM measurements, not straight-line movement. Reed switches act as magnetic switches that close or open in response to a nearby magnet, yielding a discrete signal rather than a continuous position measure. Photocells sense light levels and aren’t inherently suited to precise linear displacement unless part of a more complex, less direct arrangement.

So the best choice for measuring linear displacement along a straight line is the LVDT.