High Quality Content by WIKIPEDIA articles! A mercury-arc valve or mercury-vapor rectifier or (UK) mercury-arc rectifier is a type of electrical rectifier used for converting high-voltage or high-current alternating current (AC) into direct current (DC). It is a type of cold cathode gas discharge tube but is unusual in that the cathode, instead of being solid, is made from liquid mercury and is therefore self-restoring. As a result, mercury-arc valves were much more rugged, long-lasting and capable of carrying much higher currents than most other types of gas discharge tube.