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Measurements of spark-over voltage were made at 700 and 1800 kilocycles using needle points, blunt electrodes, and spheres. By impressing the voltage across a shielded capacitance of known value, then measuring the current which flows and the frequency, the voltage was calculated. Ultraviolet irradiation of the spark gap was required for consistent results. When spheres were used, the radio-frequency spark-over voltage was 17 to 20 per cent below the 60-cycle voltage. The air-density correction factor at 700 kilocycles was found to be nearly proportional to the air density. Measurements of radio-frequency spark-over voltage were made on a number of air condensers and a plate thickness of one half to one third the air gap was found to be the most satisfactory.