Measurements by direct comparison with a reference standard having the same characteristics as those of the quantity measured are called direct measurements, but most measurements yield results in a more indirect way. They are based on knowledge of the relationship between the quantity to be measured and the response of the measuring instrument or system influenced by it. Moving-coil meters are an example: under the influence of the applied voltage or current (or both) a mechanical torque is generated. The pointer attached to the moving coil acquires its final position indicative of the electrical quantity measured, the mechanical torque being balanced by a spring. Thus an electrical quantity is first translated into a torque, which in turn is translated into a position on a scale calibrated in units of the quantity measured.

In this example, the electrical quantities are translated into mechanical ones. In contrast, transducers translate primarily nonelectrical quantities into electric signals.

The term transducer has been applied to a variety of devices, including measuring instruments, acousticenergy transmitters, signal converters, and phonograph cartidges. With the vast increase in the development and use of electronic measuring systems, however, instrumentation engineers found it necessary to devise a more limited definition of transducer as a device used for measurement purposes.

Transducers covered in the first four chapters of...