The paper describes some of the common methods and the author's new null method of testing instrument transformers.The technical points of the method are discussed and examples are given illustrating the simple derivation of ratio and phase-angle error of current and potential transformers.The accuracy of the method is dealt with, tests on a holetype current transformer are described, and the number of standard transformers required is discussed.In conclusion the author sums up the advantages of the method and describes some interesting applications of it in connection with the testing of power transformers etc.In the Appendix is given the proof of the method.