A
rectifier is an electrical device that
converts alternating current (AC), which periodically reverses direction, to
direct current (DC), which flows in only one direction. The process is known as
rectification. Physically, rectifiers take a number of forms, including
vacuum tube diodes,
mercury-arc valves, copper and selenium oxide rectifiers,
semiconductor diodes,
silicon-controlled rectifiers and other silicon-based semiconductor switches. Historically, even synchronous electromechanical switches and motors have been used. Early radio receivers, called
crystal radios, used a "
cat's whisker" of fine wire pressing on a crystal of
galena (lead sulfide) to serve as a point-contact rectifier or "crystal detector".