Two metal rectifiers, c 1940s.
© Science Museum / Science & Society Picture Library
A rectifier is an electrical device that converts an alternating current into a direct current by allowing it to flow in one direction only. Stacks of oxidised copper plates were used to 'rectify' the alternating voltage of household electricity, creating the steady voltage needed by valves. The cat's whisker detector used in crystal sets also depended on a contact between metal and semiconductor to form a rectifying diode.