CHAPTER Transducers and valves 4 A transducer is a device, usually electrical, electronic, electro-mechanical, electromagnetic, photonic or photovoltaic, that converts input physical energy in one form into output physical energy of another form for various purposes, including measurement or information transfer. A typical example of a transducer is a recorder, which converts sound (mechanical) energy into electrical (electromagnetic) energy. In such devices an electrical switch opens and closes under the control of another electrical or electronic circuit. Earlier devices had a switch that is operated by an electromagnet to open or close one, or many sets of contacts to form a high-voltage circuit controlled by a low-voltage signal, as in some types of modems or audio amplifiers. However, as a field device in an industrial control system, such a switch will be a device of two states, On and Off, the value of which being dependent on some controls or conditions. A valve is a device that regulates the flow of materials (gases, fluidized solids, slurries, or liquids) by opening, closing, or partially obstructing various passageways. Valves are technically pipe fittings,