The mercury-arc rectifier is used to convert (or rectify) alternating current into direct current. The basic form is a glass bulb valve featuring one or more arms with steel or carbon anodes, and a cathode consisting of a self-restoring pool of liquid mercury.

Are mercury arc rectifier still in use?

The ignitor draws a small spark to ionise the mercury vapour, initiating the main arc. Mercury arc rectifiers have now been made totally obsolete by semiconductors, although there are a few still in service in old installations.

What is meant by mercury arc?

: an electric discharge through mercury vapor in a glass or quartz tube emitting a blue-green light rich in actinic and ultraviolet rays and used for various purposes (as for water sterilization, in photography, and in a rectifier)