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Cat's-whisker detector
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. 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.
==Description==
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.
Only certain sites on the crystal surface functioned as rectifying junctions. The device was very sensitive to the exact geometry and pressure of contact between wire and crystal. It was therefore made adjustable, and a usable point of contact was found by trial and error before each use. The wire was suspended from a moveable arm and was dragged across the crystal face by the operator until the device began functioning. In a crystal radio, the operator would tune the radio to a strong local station if possible and then adjust the cat's whisker until the station or static was heard in the radio's earphones. This required some skill and a great deal of patience; even then, a good contact could easily be lost by the slightest vibration. An alternative method of adjustment was to use a battery-operated buzzer to generate a test signal. The spark produced by the buzzer's contacts functioned as a weak radio transmitter whose emissions could be detected when a diode in the crystal had been found and the buzz could be heard in the earphones, at which time the buzzer was turned off. The temperamental, unreliable action of the crystal detector was a barrier to its acceptance as a standard component in commercial radio equipment and was one reason for its rapid replacement by vacuum tubes after 1920. Frederick Seitz, a later semiconductor researcher, wrote:
Such variability, bordering on what seemed the mystical, plagued the early history of crystal detectors and caused many of the vacuum tube experts of a later generation to regard the art of crystal rectification as being close to disreputable.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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