AC Bridge Methods for the Measurement of Three-Terminal Amittances

To avoid uncertainties due to finite lead impedances, a direct admittance is best defined as the transfer admittance of a two-terminal-pair network. The network may contain both voltage and current transformers. Unwanted loop currents may be suppressed by coaxial chokes and the equivalent circuit reduced to that of a three-terminal admittance. Two direct admittances may be compared with a voltage or current ratio, and two such ratios may be combined to give a general form of four-arm bridge. This general bridge network may be balanced in a way which leads to simple relations independent of the ground admittances between the four direct admittances. At least two separate balance conditions must be satisfied, but it is usually best to make three balances since the error in the main balance is dependent on the product of the errors in the other two. There is no restriction on the type of network which may be used to couple the source and the detector to the bridge, provided the ground balance conditions are satisfied. Networks may be chosen to provide for filtering, impedance matching, and the elimination of transformers. Examples are given of four-arm bridge networks which use adjustable decade ratio-transformers as the main balance controls and fixed ratio-transformers for multiplication or inversion.