A High-Resolution Electrodynamic AC-to-DC Power Transfer Instrument

The design of an electrodynamic instrument for measurement of ac power at line frequency is described. The deflection of its moving coil is kept at zero by passing alternating and direct currents simultaneously through both coils. An automatic control circuit generates the balancing direct current related to the ac power to be measured. The high resolution of the instrument, 2 × 10-7 of rated power, makes a detailed investigation of its performance in the error range below 1 × 10-5 possible.