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This paper introduces the structure and the working principle of a high-accuracy current comparator. This paper also indicates the primary source of error: the leakage capacitance coupling current between the turns of the secondary winding. In order to reduce this error, the secondary winding is wound with a coaxial cable. With good shielding, the error introduced by the connecting windings is minimized. This paper also presents a new self-calibration method for the 1:1 current comparator and an innovative design of the current comparator. The self-calibration method can also be used to calibrate multiratio current comparators with an uncertainty of less than 5×10-7 at a primary current ranging from 5 A to 2000 A.