Thermal Instrument for Measurement of Voltage, Current, Power, and Energy at Power Frequencies

The design of an instrument for precision measurement of ac voltage, current, power, and energy is described. It has been developed as a standard for evaluating the performance of instrumentation used in the power frequency range (45-65 Hz). Its accuracy relies on a continuous ac/dc transfer which is achieved by automatically balancing the alternating current derived from the quantity to be measured against the equivalent direct current, both passing alternately through the heater of a thermal converter. Since the instrument can be calibrated with direct voltage and current, its total systematic error is limited by the ac/dc transfer to ±0.0005 percent of full scale for voltage and current, and ±0.001 percent of full scale for power and energy measurements.