Mercury Arc Valve A mercury arc valve or mercury vapour rectifier or (UK) mercury arc rectifier is an electrical rectifier used to convert high voltage or high current alternating current (AC) to direct current (DC). This is a gas-filled cold cathode tube but is unusual in that the cathode is not solid, but consists of a pool of liquid mercury and is therefore self-regenerating. Mercury arc valves are therefore more rugged and can carry higher currents than most other types of gas discharge tube