The paper describes a high-speed continuously-evacuated cathode-ray oscillograph for the recording of high-voltage transients; the distinctive feature of the instrument is that impulsive voltages of 100 kV to earth may be applied directly to either deflection plate singly, or +100 kV to one and − 100 kV simultaneously to the other, without the use of a potential divider. The reasons for. and advantages accruing from, the use of such an oscillograph are discussed, with particular reference to the distortion inherent in the use of potential dividers. Precautions to be observed in the operation of the high-voltage plates are described, reference being made to the means of producing the high vacuum required in the tube.A circuit producing a delayed high-voltage impulse is shown, and the possibilities of the instrument are demonstrated by a brief investigation of the characteristics of a sphere-gap when sparking over on the front of a steep-fronted wave.