An x‐ray tube which operates successfully up to 650,000 volts has been constructed from a conical porcelain transformer bushing, rated at 750,000 volts external flashover. Although the bushing had previously been filled with transformer oil, no difficulties were encountered in outgassing the inner surface. With a pumping system of about 300 liters per second capacity, a vacuum of 10−5mm can be maintained, and this does not change appreciably during operation. Cold emission from the anode, rather than gas current or breakdown of the porcelain, limits the voltage which can be applied. Filament emission currents up to 4 milliamperes can be employed without undue heating of the target. For a given voltage, a porcelain tube of this type is much more compact than a glass tube, the strength of its walls against puncture makes elaborate internal shielding unnecessary, and there is no more difficulty in maintaining a good vacuum than in a glass tube of comparable volume. The source of high potential is the 1,000,000 volt transformer set in the High Tension Laboratory.