The mercury arc rectifiers

Rare, weird and wonderful!

Mercury arc rectifier

Kempton is privileged to have a pair of extremely rare working mercury arc rectifiers. They are situated on the turbine floor and their wonderful, eerie flickering violet lights can be seen on steaming weekends. The original rectifiers were scrapped when the engine house was decommissioned in 1980 but, thanks to an enormous stroke of luck and considerable generosity, they have been replaced by the most unlikely donor – the Royal Opera House, Covent Garden.

When the Kempton Great Engines Trust took over the building in 1995, the chances of replacing such rare pieces of equipment seemed hopeless until a member of the Trust, attending a planning meeting at the soon-to-be-refurbished opera house, heard that Covent Garden’s rectifiers were no longer needed. Until 1977, they had been used to power the stage lifts, driven by German U-boat engines obtained in reparation after WW1. Negotiations duly took place and the Royal Opera House agreed to gift them to the Trust.

Because of the extremely fragile nature of the glass vacuum bulbs, which each contains a pool of pure mercury, transporting them safely presented quite a headache. They were brought to Kempton in three separate parts – the two cubicles, the transformer and the two glass bulbs, carefully packed in straw-lined crates. When the rectifiers were inspected, they were found to be in a far worse state than expected, considering they had been in operation until fairly recently. They had probably been in continuous use for more than 50 years and every part of the cubicles and the wiring was encrusted in oily dirt thanks to the forced-air cooling, which ensured that it penetrated every crevice. The wiring was insulated with vulcanised India rubber (VIR) and this had become so brittle that it no longer insulated properly.

The purpose of the rectifiers is to convert incoming 415-volt three-phase alternating current (AC) to the 200-volt direct current (DC) needed to operate the various motors on pumps and compressors in the engine house. In the absence of the original mercury arc rectifiers, Kempton had been using a modern solid-state rectifier. When tested, the no-load voltage was 242 volts, as expected, and dropped to 203 volts with a load of 2 amps as predicted by theory. The output voltage at 48 amps was 200 volts. 

After the arrival of the rectifiers, volunteers David Walker (left) and Tim Moth, with the occasional help of others, arc-rectifierspainstakingly dismantled, cleaned, refurbished or, where necessary, replaced every part with ones as close to contemporaneous as possible. They then reassembled and tested them, and replaced almost all the wiring with equivalent modern cable, sleeved to resemble the original. The power transformer was also refurbished and new controls added. On 29 June 2010, the mercury arc rectifiers were switched on and successfully loaded to 48 amps for the first time since 1997. 

When working on steaming weekends, they are a constant source of fascination to adults and children alike – kids particularly love the Dr Who-like effect of the beautiful, eerie, flickering violet lights, which become deeper and more active under load, as the current flies through the mercury vapour on its way to being converted. These rare and wonderful pieces of equipment are believed to be among the only working ones in the country and they are not to be missed when visiting the museum.