Mercury-Arc Rectifiers

By: Joao Martins

Have you ever wondered how high voltage AC (Alternating Current) was converted into DC (Direct Current) before the 1970's? Well, probably not, but this is how it was done: using Mercury-Arc Rectifiers.
In the 20th Century, scientists and engineers needed a new and more efficient way of converting Alternating Current, which changes from negative to positive current very quickly, to Direct Current, in which the current always flows in the same direction. The old technology that performed this task was incredibly inefficient and it was also prone to failure, so in 1902, Peter Cooper Hewitt invented the Mercury-Arc Rectifier. This is how it works: A pool of liquid Mercury sits at the bottom of a closed glass bulb. The Mercury acts as a Cathode, while at the top of the bulb carbon rods act as the Anode. When electricity and heat is applied, the Mercury pool emits electrons which are attracted to the carbon anode. This "electron arc" makes the current only flow in one direction only, effectively converting AC into DC. This process is a lot more efficient than older methods, and quickly Mercury-Arc Rectifiers began to be used to power trains, streetcars, and industrial motors. These rectifiers do have some downsides, however. The mercury in it makes it really dangerous for the people that work around it if one breaks, and they take up a lot of space. After 1970, most of them were replaced after the invention of semiconductors, which are more efficient and not toxic, but some Mercury-Arc Rectifiers are still in use to this day powering older parts of the NYC subway system among other things.

Source:

Cory, B.J.; Adamson, C.; Ainsworth, J.D.; Freris, L.L.; Funke, B.; Harris, L.A.; Sykes, J.H.M. (1965). "Chapter 2". High voltage direct current converters and systems. Macdonald & Co. Ltd