An instrument that converts the physical stimulus to be measured; such as, temperature, vibration, or pressure, into an electrical signal and therefor operates as the actual measuring instrument.

Transducers can take many forms and they can be self-generating or externally energized.

An example of the self-generating type is a vibration sensor based on the use of a piezoelectric material; that is, one that produces an electrical signal when it is mechanically deformed.

Many externally energized transducers operate by producing an electrical signal in response to mechanical deformation. Typical physical inputs producing such deformations are pressure, mechanical stress, and acceleration.

A simple mechanical transducer-sensing device is a strain gauge based on the change in electrical resistance of a wire or a semiconductor material under strain. Another externally energized transducer, called the variable-reluctance type, is one in which the magnetic circuit is broken by an air gap.

The mechanical movement to be measured is used to change this air gap and so it changes the reluctance, or opposition, to the production of a magnetic field in the circuit. The change in reluctance is then translated into an electrical signal.