A device or system which performs the function of transforming one form of energy (e.g. thermal) into another (e.g. electrical) is called a transducer (from the Latin transducere: to transfer, display, expose). Transducers which are intended to measure a physical quantity and convert it to an electrical signal are called sensors. Sometimes a single component can perform the transformation, other times it is necessary to combine a physical sensing element with additional circuitry to produce a complete sensor system.
We can also have transducers which convert from electrical to physical signals. We saw one example in Experiment 1.1: the incandescent light bulb which converts electrical power into optical power. Transducers which convert electrical signals to mechanical signals (i.e. motion) are called actuators.