Four-Terminal Equal-Power Transformer-Ratio-Arm Bridge

A transfonner-ratio-arm bridge is described that combines the high sensitivity of equal-power bridges with the immunity from lead errors of ac bridges analogous to the Kelvin double bridge. The latter apply either equal current or voltage to both impedances being compared and suffer a reduction in sensitivity when the impedance ratio is high, while the sensitivity of bridges that apply equal power (or voltamperes) to both impedances is independent of this ratio. The new bridge has two transformers in addition to the two of the usual equal-power bridge to reduce lead-impedance errors. Two current loops are formed around each of the two impedances being compared making four-terminal connections to both. The auxiliary transformers couple between loops on the two sides of the bridge and act to maintain accurate current and voltage ratios in spite of series impedance. The expression for the error due to lead impedance is given and is compared with those for other four-terminal bridges and with measured results. A lead-impedance-balance procedure is given that reduces lead errors still further.