Metal rectifier

A metal rectifier is an early type of semiconductor rectifier in which the semiconductor is copper oxide or selenium. They were used to convert alternating current to direct current. They were often used in devices such as radios and battery chargers. Westinghouse Electric was a major manufacturer of these rectifiers since the late 1920s.
In some countries the term "metal rectifier" is applied to all such devices. In other countries the term "metal rectifier" normally refers to copper-oxide types, and "selenium rectifier" to selenium-iron types.