The development of the current comparator, a three-winding current ratio transformer, is reviewed and its characteristics as an alternating current ratio standard are analyzed. Particular attention is given to the use of magnetic shielding and its effect on the accuracy and usefulness of the device. Some error characteristics of three types of audio-frequency current comparators are given and possible applications discussed. These include the calibration of current transformers and impedance comparisons. The adaptation of this device to dc operation is made possible by modulation techniques. A 20 000-ampere self-balancing direct current comparator, designed specifically for the calibration in situ of transductors or direct current transformers, is described. The application of this comparator to the calibration of shunts at high currents also is discussed and some results presented.