A cat's-whisker detector (sometimes called a crystal detector) is an antique electronic component consisting of a thin wire that lightly touches a crystal of semiconducting mineral (usually galena) to make a crude point-contact rectifier. Phosphor bronze wire of about 30 gauge was commonly used because it had the optimal amount of springiness. It was mounted on an adjustable arm with an insulated handle so that the entire exposed surface of the crystal could be probed from many directions to find the most sensitive spot. Cat's whiskers in simple detectors were straight or curved, but most professional cat's whiskers had a coiled section in the middle that served as a spring. The crystal required just the right gentle pressure by the wire; too much pressure caused the device to conduct in both directions. Precision detectors often used a metal needle instead of a cat's whisker, mounted on a thumbscrew-operated leaf spring to adjust the pressure applied.
Developed around 1904 by early radio researchers Henry H. C. Dunwoody, G. W. Pickard and others, this device was used as the detector in early crystal radios, from the early twentieth century through World War II, and gave this type of radio receiver its name. Crystal radios were the most popular type of radio until the mid 1920s. The cat's whisker detector was the first type of semiconductor diode, and in fact, one of the first semiconductor electronic devices (after photoconductors). Cat's whisker detectors are obsolete and are now only used in antique or antique-reproduction radios, and for educational purposes.
The tip of the wire contacting the surface of the crystal formed a crude but unstable point-contact metal–semiconductor junction, forming a Schottky barrier diode. This junction conducts electric current in only one direction and resists current flowing in the other direction. In a crystal radio, its function was to rectify the radio signal, converting it from alternating current to a pulsing direct current, to extract the audio signal (modulation) from the radio frequency carrier wave. The metal whisker is the anode, and the crystal is the cathode; current flows from the whisker into the crystal but not in the other direction.
Historically, many other minerals and compounds besides galena were used for the crystal, the most important being iron pyrite ("fool's gold", iron disulfide, FeS2), silicon, molybdenite (MoS2), and silicon carbide (carborundum, SiC). Some were used with gold or graphite cat's whiskers. Another type had a crystal-to-crystal junction instead of a cat's whisker, with two crystals mounted facing each other. One crystal was moved forward on an adjustable mount until the crystal faces touched. The most common of these was a zincite-bornite (ZnO-Cu5FeS4) junction trade-named Perikon, but zincite-chalcopyrite, silicon-arsenic and silicon-antimony junctions were also used.