11.9 Potentiometers and bridges
11.9.1 D.c. potentiometers
D.c. potentiometers may be used to measure a direct voltage by comparison with the known ratio of the e.m.f. of a Weston standard cell or electronic solid state reference device. The methods of achieving the linear ratio between the unknown p.d. and the standard-cell e.m.f. are by successive approximation to the balance condition (a) using groups of matched resistors, (b) using opposed m.m.f.s between matched windings and current-ratio resistors (comparator potentiometer), and (c) pulse-width modulation (ratio of time interval). The d.c. potentiometer is a ratio device of high precision with the uncertainty of the standard voltage source limiting the ultimate accuracy.