What is "selenium rectifier"

A selenium rectifier is a type of metal rectifier, invented in 1933. They were used in power supplies for electronic equipment, and in high current battery charger applications until they were superseded by silicon diode rectifiers in the late 1960s. The arrival of the alternator in some automobiles was the result of compact, low cost, high current silicon rectifiers. These units were small enough to be inside the alternator case, unlike the selenium units that preceded silicon devices.

The photoelectric and rectifying properties of selenium were observed by C. E. Fitts around 1886 but practical rectifier devices were not manufactured routinely until the 1930s. Compared with the earlier copper oxide rectifier, the selenium cell could withstand higher voltage but at a lower current capacity per unit area.