Since the issue of B.S.S. 358 in 1929 on the “Measurement of Voltage with Sphere-Gaps,” a number of calibrations of the sphere spark-gap have been put forward in Germany and the United States, but these differ appreciably amongst themselves and, moreover, are usually restricted to voltages of the order of 500 kV(eff.) and to spheres of 75 cm. diameter or less.The present paper is still another contribution to the literature on the subject, its justification being that it contains calibrations for larger spheres and at higher voltages than have hitherto been obtainable in this country.After a brief review of existing information the calibration at 50 cycles (a.c.) of a range of sphere spark-gaps is discussed in detail; methods of absolute measurement of high voltages are described, and calibrations are put forward for spheres up to 200 cm. diameter at voltages up to 1 million volts (effective) with one sphere earthed. These calibrations are based on a method of voltage measurement which is believed to be unique.Some attention has been directed towards the determination of the polarity of the high-voltage electrode when breakdown occurs, in view of the obvious importance of this feature in its relation to the breakdown of asymmetrical gaps under direct and impulsive voltages. A new type of polarity indicator is described.A correlation was observed between the noise of the spark and the polarity of the high-voltage electrode at breakdown.The effect of irradiation of the gap with radium and by other means is described.