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An automatic system for calibrating high-accuracy ac voltmeters and calibrators is described. The system is based on traditional coaxial, thermal voltage converters to provide ac voltage measurement uncertainties of 5–20 ppm in the audiofrequency range and 5–250 ppm over the full range from 10 Hz to 1 MHz at voltages between 0.5–600 V. Lower levels (0.01–0.5 V) are realized using wideband inductive dividers. Specialized hardware and measurement techniques make it possible to achieve these uncertainties in test periods of approximately 1 min. Random errors introduced by the measurement system are typically less than 2 ppm (one standard deviation).