Introduction DURING the development of the high voltage technic for nuclear disintegration purposes in the Radiation Laboratory of the University of California, under the direction of Professor Ernest O. Lawrence, one of the authors (D. H. S.) devised and constructed an apparatus for producing high voltages by the use of a radio frequency resonance transformer in vacuum. This method was announced in the “Physical Review” (1) and was amplified in another article, soon to appear. Tests showed that peak voltages of from 700 to 1,000 kilovolts could be obtained and, when a suitable filament was installed, x-rays with this range of peak energies were obtained. When it became evident that such an apparatus could be made sufficiently practical, arrangements were made to install one in the University of California Hospital, in San Francisco, for the treatment of patients. This particular installation is described in the present article. Part I-Apparatus Principle.-A very high power short wave radio oscillator sends its power into the high voltage resonant circuit instead of into the usual transmitting antenna. This high voltage resonance transformer is a helical spiral of about twelve turns of copper pipe, without insulation, supported by the upper end which is grounded to the roof of the surrounding copper-walled vacuum tank. The coil, with its capacity to the walls, forms a quarter wave length resonant circuit with a voltage node at the top support and a voltage maximum at the bottom end which supports the x-ray target. The high voltage thus is produced entirely within the vacuum, exactly where it is to be used, and is free from all insulating material. Figure 1 shows the high voltage coil in the vacuum tank. Description.-The vacuum tank which serves both as container for the transformer and as the x-ray tube, is cylindrical in shape, 42 inches in diameter and 40 inches high, with walls of three-eighths of an inch steel and end plates of 2-inch steel, the lower one of which is removable (Fig. 2). The tank is supported by having the top plate bolted to channel beams crossing the room with the ends securely mounted in the walls. A steel ring forms a ledge around the bottom of the tank and serves the double purpose of supporting the necessary lead for shielding and furnishing a machined face for a rubber-gasket tongue-and-groove vacuum seal for the bottom plate. This is supported by counterweights and can be lowered, to give access to the interior. A large pipe for a pumping outlet and smaller ones for mounting the x-ray filament, primary coil insulators, main coil supports, windows, etc., are welded into the tank. Electric welding is used throughout.