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were used in power supplies for electronic equipment and in high-current battery-charger applications until they were superseded by silicon diode rectifiers in the late 1960s.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.