The voltage to be measured is applied across the primary of a standard potential transformer and the secondary voltage, at approximately 110 V, is balanced against a direct voltage, using a cathode-ray tube as a null indicator. The direct voltage is measured accurately using a suitable potentiometer and volt-ratio box. Using a cathode-ray tube specially developed for this particular application, an accuracy of better than 0.15% may be readily obtained. An alternative arrangement, in which the potential transformer is replaced by a suitable high voltage resistor divider, is described.