Recent posts regarding Mercury-arc rectifiers inspired me to included this example, being a mechanical synchronous rectifier. This was installed at the Royal Naval Cordite Factory, Holton Heath and was part of the kit to supply the Cottrell plant for dust extraction from the fume vents. These days, we would probably use a couple of 1N5408's, but at the time of installation, no such convenient device had been invented!
See:
http://www.mw0sec.co.uk/hhintro.html
For a bit more information about Holton Heath.
Les.