A comparator is a metrological instrument used for the comparison type of measurement: it does not read the absolute size of a part directly, but shows how much a dimension deviates from a master (slip gauge or reference) to which the instrument is first zeroed. Comparators are classified by the physics of the amplifying/sensing system they use.

The LVDT (Linear Variable Differential Transformer) is an electromagnetic transducer. It has one primary coil and two identically wound secondary coils, with a soft-iron core that moves with the measuring plunger. When the core is centred, the two secondaries induce equal and opposite voltages, giving zero net output; when the plunger moves by a small amount, the core shifts and unbalances the coupling, so the differential output AC voltage becomes directly proportional to the linear displacement. This voltage is amplified and shown on a dial or digital readout.

Because the sensing element is a transformer producing an electrical signal, the LVDT is the heart of an electrical comparator. This gives very high magnification, remote/electronic readout, and easy signal processing.

• A comparator that relies on gear-and-rack, lever, or twisted-strip mechanisms uses purely mechanical amplification and contains no electrical transducer, so it cannot house an LVDT.
• A comparator using lenses and a projected light beam or reflected scale is an optical type; its magnification comes from light and mirrors, not from an electromagnetic core.
• A comparator that senses the change in air pressure or flow through a small nozzle as the gap to the part varies is a pneumatic type; again there is no LVDT.

Hence LVDT → Electrical comparator.