English: A galena cat's-whisker detector, an early radio wave detector used in crystal radio receivers from about 1905 to the 1940s. The fine metal wire attached to the adjustable arm touches the face of a natural semiconducting crystal of galena (lead sulfide) in the capsule at right. The contact forms a PN junction making a crude semiconductor diode. In a crystal radio its function is to rectify the alternating current radio signal, extracting the audio (sound) signal from the radio frequency carrier wave. Only certain sites on the crystal surface function as rectifying junctions, so the device had to be adjusted before each use by dragging the wire across the crystal to find a "sweet spot".