Transducer, device that converts an input energy into an output energy. Usually, the output energy is a different kind of energy than the input energy. An example is a temperature gauge in which a spiral metallic spring converts thermal energy into a mechanical deflection of the dial needle. Because of the ease with which electrical energy may be transmitted and amplified, the most useful transducers are those that convert other forms of energy, such as heat, light, or sound, into electrical energy. Some examples are microphones, which convert sound energy into electrical energy; photoelectric materials, which convert light energy into electrical energy; and pyroelectric energy crystals, which convert heat energy into electrical energy.