The paper is in three Sections.Section 1 describes a new method of reducing current-transformer errors, using a compensator which may be either an integral part of the current transformer or a separate unit for use with an existing transformer. The compensator employs normal design principles and does not depend on highly critical conditions such as saturated cores.Very high accuracies are obtained, and standard transformers may be constructed having almost unmeasurably small errors over the whole current range, using the most sensitive testing equipment.In Section 2 a method of testing voltage transformers is described in which the Arnold current-transformer equipment may be adapted for this purpose. The set measures the difference in error between a standard and the test transformer.Section 3 deals with the estimation of the errors of threephase star-to-star voltage transformers at their working burdens, from the results of tests at unity-power-factor burdens. Two methods are given: the first uses the six phase-neutral test results usually provided by manufacturers; the second, in which no three phase calculations are involved, uses the results of 12 special tests if three phases are to be loaded or of 6 tests if only two phases are to be loaded.