An improved four‐window demountable rotating anode x‐ray tube fitted with rapidly interchangeable targets has been constructed. Compactness equal to that of a standard sealed‐off tube has been achieved by the use of an improved re‐entrant vacuum seal and bearing assembly. By employing peripherally directed water cooling in the anode behind the focal spot and a full wave rectified high tension supply, it has been possible to obtain greatly improved performance. With a copper target five inches in diameter rotating at 1000 rpm and with a focal spot 8×0.5 mm, the continuous load at 50 kv was 120 ma without any signs of target erosion.