<P> 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, since it "straightens" the direction of current . Physically, rectifiers take a number of forms, including vacuum tube diodes, mercury - arc valves, stacks of copper and selenium oxide plates, 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". </P> <P> Rectifiers have many uses, but are often found serving as components of DC power supplies and high - voltage direct current power transmission systems . Rectification may serve in roles other than to generate direct current for use as a source of power . As noted, detectors of radio signals serve as rectifiers . In gas heating systems flame rectification is used to detect presence of a flame . </P>

Machine which converts ac into dc is known as