Sir,
Having been a projectionist at The Picture House when the mercury arc rectifier was in use, I was interested by the Courier article about it last Friday.
While this apparatus could help to keep the speed of the projectors at a constant speed (24 frames per second) the projector mechanism itself served that purpose. Its function was to power the two projectors’ carbon arc lamps. These wouldn’t work on alternating current, so the rectifier was needed to convert the incoming power from 240 volts a/c to 110 volts d/c and maintain it at that level so there were no fluctuations in screen brightness.