Mercury rectifier
A mercury arc valve is a type of electrical rectifier which converts alternating current into direct current.
Rectifiers of this type were used in electric motor power supplies for industry, in electric railways, streetcars and electric locomotives. They also found use in static inverter stations and as rectifiers for high-voltage direct current power transmission.
Check Wikipedia for more details.
The mercury arc valve on the left is rated at 200V and 10A, thus handling 2kW of power. It was built in the 1940's and has been used in research projects at E2C, where it today still can be admired.
Below, you can see a detailed view of the evacuated glass bulb, which contains a pool of liquid mercury sitting in the bottom as the cathode. You can also see the arms with graphite rods which serve as anodes.
