To control mechanical processes one needs to obtain information about the state of the system, to process the information, and then to act on the results. Originally, the simplest controls were purely mechanical feedback systems; more complex systems required human intervention. At present, most controls are provided by purely electromechanical systems, but there are also many situations in which one needs sophisticated measurements for later analysis. Developments in electronic information processing have over the past few decades far outpaced the developments in the sensors and actuators that provide the input and output of the control systems; as a result, the transducers account for the major part of the expense, mass, and failure points of many modern machines. Improvements in control systems thus depend on a clear and thorough understanding of the principles of transducers. It is the aim of this book to help meet that need. Unlike other treatments of sensors or actuators, this book approaches the devices from the point of view of the fundamental coupling mechanism between the electrical and mechanical behavior. Thus, for example, a solenoid can b used either to detect horizontal motion and position (i.e. as a sensor), or to impose a motion on a piston (i.e. as an actuator); the principles of operation of the solenoid are the same in both cases, and this book thus treats them together. The book begins with a discussion of systems analysis as a tool for modeling transducers. It then turns to a detailed discussion of transduction mechanisms (electric and magnetic fields, piezo- and pyro -electric effects, electromagnetic induction, and so forth). The book concludes with an input/output analysis of transducers.