The paper opens with a description of a simple form of co-ordinate a.c. potentiometer which requires little power to operate and is very light and portable. Its application to the measurement of the ratio and phase errors of a voltage-transformer follows. It may also be used to measure the errors of current-transformers, and several methods are described which avoid the difficulties inherent in the more usual “spill current” methods. Brief notes of the construction of an oil- cooled, shielded resistor for 33 kV, and of two standard current-transformers, are given.