Comparison Measurement

the general name of measurement methods in which the quantity measured is compared with a quantity reproduced by a standard.

Comparison measurement includes (1) the opposition method, in which two quantities—the measured quantity and the quantity reproduced by the standard—act simultaneously on a comparison device, as in measuring a weight on a balance, (2) the difference method, in which the difference between quantities acts on a comparison device, for example, in comparing the lengths gage blocks on an interferometer, (3) the null method, in which the resultant effect of the comparison is zero, for example, in measuring resistance by means of a DC bridge with complete balancing, (4) the substitution method, in which the measured quantity is replaced by a quantity reproduced by the standard, for example, in weighing a body by alternately placing the body and weights on the same pan of a balance, and (5) the coincidence method, in which the difference between quantities is measured by the coincidence of marks on scales or of signals, as indicated, for example, by a vernier or stroboscope.

Comparison measurement is applicable to quantities that can be reproduced by standards. This method usually affords greater precision than indirect measurement because the error of the result is chiefly determined by the insignificant error of the standard, and the residual errors can be kept to low values.