A transducer is a device that converts a signal from one physical form to a corresponding signal having a different physical form. Therefore, it is an energy converter. This means that the input signal always has energy or power, i.e. signals consist of two component quantities whose product has energy or power dimension. But in measurement systems, one of the two components of the measured signal is usually so small that it is negligible, and thus only the remaining component is measured.
Since there are six different kinds of signals—mechanical, thermal, magnetic, electric, chemical and radiation (corpuscular and electromagnetic, including light)—any device converting signals of one kind to signals of a different kind is a transducer. The resulting signals can be of any useful physical form. Devices offering an electric output are called sensors. Most measurement systems use electric signals, and hence rely on sensors.
Sensor and transducer are sometimes used as synonymous terms. However, sensor suggests the extension of our capacity to acquire information about physical quantities not perceived by human senses because of their subliminal nature or minuteness. Transducer implies that input and output quantities are not the same. A sensor may not be a transducer.
The distinction between input-transducer (physical signal/electric signal) and output-transducer (electric signal/display or actuation) is seldom used at present. Nowadays, input transducers are termed sensors, or detectors for radiation, and output transducers are termed actuators or effectors. Sensors are intended to acquire information. Actuators are designed mainly for power conversion.
This means that whereas in sensors uncertainty is essential and ideally no energy is drawn from the system being measured, in actuators efficiency in the energy conversion is essential.
Nevertheless, the use of these terms is far from uniform and their use in some industry sectors can be quite different. For example, some current transducers are based on transformers that only involve electrical signals.