Calibration of precision d.c. potentiometers by an a.c. method

The possibility of calibrating precision d.c. potentiometers on alternating current, so that an inductive divider may be used as the reference standard, is examined. It is found that the change in ratio with frequency of potentiometers is partly due to the a.c. properties of the resistance coils and partly due to e.m.f.s induced in the voltage circuit of the potentiometer, the latter effect predominating at low dial settings. It is shown that measurements made at a frequency of about 80Hz on various types of modern precision potentiometers do not differ from the d.c. values by more than about 0.2 parts in 106. A self-contained double-circuit ratio bridge has been developed and is described. The design concept and constructional details of the special transformer, which renders the necessary two supplies to the bridge independent of the stability of the source voltage, is given.