A method of testing current transformers by comparison with standard current transformers is described. The apparatus has three ranges. The most sensitive range is capable of measuring ratio differences up to 5 parts in 10000 with an accuracy of approximately 1 part in 1000000, and phase angles up to 2.5 minutes with an accuracy of approximately 0.01 minute. The other two ranges are respectively 10 and 100 times as great, and the sensitivity is correspondingly reduced. The design and construction of suitable standard current-transformers to cover ratios from 5/5 up to 12000/5 are described, and the methods of calibrating these transformers to an accuracy of approximately 2 parts in 100000 for ratio and 0.1 minute for phase angle are outlined.