Chapter 8 Measurement of Length P. H. Sydenham 8.1 IntroductIon Length is probably the most measured physical parameter. This parameter is known under many alternative names-- displacement, movement, motion. Length is often the intermediate stage of systems used to measure other parameters. For example, a common method of measuring fluid pressure is to use the force of the pressure to elongate a metal element, a length sensor then being used to give an electrical output related to pressure. older methods were largely mechanical, giving readout suited to an observer's eyes. The possibility of using elec- trical and radiation techniques to give electronic outputs can be covered with a specific geometrical scaling of a sensor's configuration. This chapter introduces the reader to the common methods that are used in the micrometer-to-subkilometer range. For further reading, it may be noted that most instrumen- tation books contain one chapter or more on length mea- surement of the modern forms, examples being Mansfield (1973), Norton (1969), oliver (1971), and Sydenham (1983, 1984). Mechanical methodology is more generally reported in the earlier literature on the subjects of mechanical mea- surements, toolroom gauging, and optical tooling. Some such books are Batson and Hyde (1931), Hume (1970),