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A vertical water-jacketed pyrex cylinder having an anode at the upper end, a mercury-pool cathode in the lower end, and a high vapor pressure in the region of the cathode constitutes a new type of controlled high-voltage rectifier or switch. A boiler and deflector or nozzle system maintains a relatively high vapor pressure and corresponding low sparking potential in the region of the cathode while the water jacket maintains a low potential gradient and corresponding high sparking potential in the region of the anode. When the water jacket is electrically connected to the cathode, its shielding action prevents the establishment by the anode potential of significant potential gradients through the high-pressure region resulting in high sparking potential. When a moderate alternating voltage is impressed between the water jacket and cathode, the high-pressure vapor is ionized periodically and the rectifier passes current during the positive half cycle of anode voltage. The value of the average current is limited by the voltage, circuit resistance, and duration of the conducting period. By varying the phase relation between the voltages impressed upon the water jacket and the anode, the instantaneous value of the anode voltage at which the arc is initiated for each cycle can be varied at will over approximately the entire positive half cycle of anode voltage.