The calibration of four-terminal resistance standards with alternating current at power frequencies

A substitution method of comparing two 4-terminal resistance standards is described. The known and the unknown standards are connected in series with each other and with the primary winding of a precision current transformer. A resistance subdivided into 10 equal sections is connected in the secondary circuit of the transformer. By means of a change-over switch the voltage-drop across each standard is opposed in turn through a vibration galvanometer to the voltage-drop across the whole, or a part. of the divider. The ratio of the transformer is such that the voltage-drop across each standard is nearly equal to the voltage-drop in the secondary circuit to which it is opposed. Exact equality is obtained by means of capacitance and resistance shunts across the divider. The difference of resistance and time-constant of the two standards is deduced from the values of the shunts. Resistance standards the nominal values of which are in any simple ratio from 1/1 to 1/10 may be compared by this method, which is extremely simple and accurate. Very little precision apparatus is required.