Mercury Arc rectifier

A mercury-arc rectifier in use - Laxey substation on the Manx Electric Railway, Isle of Man. The substation was built in 1935, and as far as I can tell this rectifier, made by the Hackbridge-Hewettic company, was installed at that time. From what I have been able to find out (sources of information appear to be limited!) there are two 200kW rectifiers here. The supply to the substation is 6,600V AC, the output is 550V DC and it supplies the Manx Electric Railway main line, along with other substations at Groudle and Ballaglass. Photos of the rest of the substation can be found at http://lucking.org/i/156793 (pages 2 and 3). At the time of filming in June 2004, another pair of mercury arc rectifiers at Bungalow substation on the Snaefell Mountain Railway had just been taken out of use - there was still an unused spare rectifier bulb in a packing case! They were 150kW examples, and photos of them can be seen at http://lucking.org/i/156579 . As far as I am aware, the mercury arc rectifiers at Laxey are still in daily use. The basic principle behind the operation of a mercury-arc rectifier is that the mercury vapour will only conduct electricity in one direction - the pool of mercury at the bottom is the cathode (connected to the DC output), and the anodes (connected to the transformer windings) are in the 'arms'. More about Mercury Arc rectifiers: http://www.tigercom.dsl.pipex.com/whatis.htm http://en.wikipedia.org/wiki/Mercury_arc_valve The third image on this page is a very good circuit diagram explaining how (and why!)mercury-arc rectifiers work. http://www.electricstuff.co.uk/mercarc.html