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This is an interesting look into old school rectifiers. Mercury Arc Rectifier have now been replaced with solid state bridge rectifiers which are inexpensive and reliable. I wonder how many of these Mercury Arc Rectifiers are still in operation, it seems that lots of this old stuff will be kept in use until it fails.
“Operation of the rectifier relies on an electrical arc discharge between electrodes in a sealed envelope containing mercury vapor. A pool of liquid mercury acts as a self-renewing cathode that does not deteriorate with time. The mercury emits electrons freely, whereas the carbon anodes emit very few electrons even when heated, so the current of electrons can only pass through the tube in one direction, from cathode to anode, which allows the tube to rectify alternating current.
Once an arc is formed, electrons are emitted from the surface of the pool, causing ionization of mercury vapor along the path towards the anodes. The mercury ions are attracted towards the cathode, and the resulting ionic bombardment of the pool maintains the temperature of the emission spot, so long as a current of a few amperes continues.”
