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A high-sensitivity EMF comparator, suitable for automatic comparison of standard voltage or current thermal converters is described. One of the two compared EMF's and their difference are simultaneously balanced to null with two potentiometric voltages produced on low-value resistors by controlled currents. Any direct switching or resistance adjustment in the circuit involving the EMF's is avoided. Also the sensitivity control of the two light-beam-coupled nanovolt preamplifiers, utilized as null detectors, is performed only by feedback on the potentiometric voltages. Tracking between these voltages and one of the EMF's reduces the balance instability due to random source variations and drifts. Additional amplitude stabilization is obtained directly on the ac and dc sources by deriving a feedback signal from the comparator. A first implementation of the system allows to obtain balances below the nanovolt level and exhibits a proportional standard deviation of 5 Ã 10-8 in a series of ten successive intercomparison measures of two thermal voltage converters with output EMF's in the 10-mV range.