Four-terminal-pair networks as precision admittance and impedance standards

THE DEVELOPMENT of suitable standards for the maintenance of impedance and admittance has been along two separate lines. One of these lines is based on the concept of 4-terminal measurement, which requires a standard to be provided with two current and two potential terminals. This technique has long been used with low d-c resistances, and may be applied directly to the measurement of small mutual inductances. The technique results in the elimination of uncertainties caused by variations in contact impedances. The other line of approach is applicable to very high impedances in which case even a very low stray admittance between the terminals could change appreciably the apparent impedance of the standard. The effect of stray admittances is eliminated by shielding the standard and making measurements in such a way that stray admittances to the shield are not included in the measurement. The technique is most often applied to the measurement of low capacitances; this standard is called a 3-terminal capacitor. The technique is also feasible for use with very high d-c resistances.