threaded posts at each end to which connections were made. Selenium rectifiers were once widely used as high-tension rectifiers in transformerless radio and TV sets, before cheaper silicon diodes became available. Although they were reasonably efficient in this application, (at least compared to vacuum-tube rectifiers), their internal resistance tended to increase as they aged. Apart from reducing the available high voltage, this tends to make them run hotter, producing an unpleasant smell as the selenium starts to evaporate. TV sets and photocopiers Specially designed selenium rectifiers were once widely used as EHT rectifiers in television sets and photocopiers. A layer of selenium was applied to a sheet of soft iron foil, and thousands of tiny discs (typically 2mm diameter) were punched out of this and assembled as "stacks" inside ceramic tubes. Rectifiers capable of supplying tens of thousands of volts could be made this way. Their internal resistance was extremely high, but most EHT applications only required a few hundred microamps at most, so this was not normally an issue. With the development of inexpensive high voltage silicon rectifiers, this technology has fallen into disuse. Electrolysis Metal rectifiers have been replaced by silicon diodes in most devices, however there are certain applications where the replacement of metal rectifiers with silicon units has proven impractical. These are mostly in electroplating, aluminium smelting and similar high-current low-voltage industrial applications, where the lower forward voltage drop of metal rectifiers is more important than their reverse breakdown voltage. References External links Rectifier used as an AM-demodulator Cougar Electronics - example of "legacy" selenium rectifier manufacturer The Copper Oxide Rectifier This document discusses the history of the copper oxide rectifier, and, in particular, the use of Chilean copper in its manufacture; the thallium catastrophe that befell the manufacture of selenium rectifiers is also discussed. Semiconductor devices Rectifiers