Design of long-scale indicating instruments

FROM the viewpoint of utility, the scale length of an indicating instrument bears somewhat the same relationship to its dimensions as does the output of a motor to its frame size. The end product of an instrument is quantitative information. The longer and more legible the scale, the more definite the resulting scale readings, through reduction of observational errors both in the initial calibration and in subsequent use. Thus, instrument designers have endeavored to produce maximum scale lengths within a given space, examples of which are 5-inch scales of 6-inch rectangular switchboard instruments, and the 3 1/2-inch scales of 4-inch rectangular instruments. In general, however, the movements of such instruments have been restricted to an angular deflection of about 90 degrees, with consequent limitation in scale lengths as compared to the scale lengths of non-electrical instruments such as steam and pressure gauges. A study of these fundamentals leads the authors, who attacked the problem of designing a new line of switchboard instruments, to the conclusion that a definite contribution to measurement technique could best be made by designing such instruments for long-range indication, and the smallest practicable panel space.