They've been a mainstay of Sci-Fi props for years but you'll be hard pressed to find someone on the street that can tell you what they actually do. They're Mercury Arc Rectifiers and our friends at Oobject have collected nine of the best.
A three phase, six anode mercury arc rectifier, formerly used at a 600V DC traction substation of the Athens-Piraeus railway
Mercury rectifier on display in Beromunster, Switzerland
Mercury Vapour Arc Rectifier from University of Aberdeen Collection
Rectifier at the Cengelhan Koc Museum in Turkey
Rectifier from Type 80 Early Warning Radar System