The technique of measuring impulse voltages with the aid of the high-voltage cathode-ray oscillograph and the resistor voltage-divider is discussed, and applied to the determination of the minimum impulse flashover characteristics of gaps between spheres ranging in diameters from 125 mm. to 1000 mm. Flashover measurements on sphere-gaps have also been made at power frequencies and compared with the corresponding figures for impulse voltages.The results of some previous workers are considered, and compared with those obtained by the authors.