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The mercury arc rectifier is an obsolete device for rectifying AC power to DC for high current applications. They were usually found in devices such as elevator and electric train power supplies.

The rectifier is paired with a transformer, which converts the power to the desired voltage, and provides voltage to a high-voltage excitation transformer.

My ASCII art skills are lacking, so please forgive the oddness of this drawing...

         __________
        /          \
       (   Vacuum   )
        \ Chamber  /    
    __   \        /   __
  1 \*\   \      /   /*/ 1
     \ \   \    /   / /
      \ \__/    \__/ /
       \            /
        \_        _/
     2 ====      ==== 2
    3======\__4__/
              |
              -
1 - Anodes
2 - Excitation anodes
3 - Ignition anode
4 - Cathode pool (Mercury)

The main anodes (1) are connected to the ends of the transformer secondary, with the center tap tied to neutral. This allows the tube to act as a full-wave rectifier. There are also three-phase rectifiers; these have three anodes.

The ignition anode (3) is a movable contact, moved by a solenoid from outside the envelope. When the rectifier first receives power, it is not conducting. To start it, a solenoid dips the ignition anode into the cathode pool and lifts it up while applying voltage between it and the pool. As it rises, it draws an arc, ionizing the vapor inside the rectifier.

The excitation anodes, (2), have a high voltage applied between them and the cathode pool. This keeps the mercury ionized, allowing the rectifier to continue operation.

And, last but not least, AC power is applied to the main anodes, and conducts through to the cathode. The rectifier generates lots of heat, visible, and ultraviolet light while operating. The discharge path goes down from the anode into the pool of mercury at the bottom.

The mercury rectifier has mainly been replaced by silicon diodes, but a few can still be found in operation. Since they contain a LARGE amount of mercury (up to several liters!), they can be hazardous if broken or not properly disposed of.