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A current comparator technique, applied to several auxiliary instruments, enables accurate power measurements to be made for measuring losses in medium and large transformers. The instruments include high-voltage active dividers with nominal outputs of 120 Volts, a current transformer with nominal output of 1 A and precision wattmeters. Two stage compensated current transformer technology is used in each instrument to achieve uncertainties of < 10 ppm in magnitude and phase respectively. In the high-voltage divider, the current comparator is used in a feedback loop to correct the magnitude and phase errors of the associated outputs. This paper discusses the technology used and the associated uncertainties of the instruments to achieve a full-scale system uncertainty of < 50 ppm at all power factors.