page_title: Crystal detector
section_title: Cat whisker detector
hierarchical_section_title: Crystal detector / Types / Cat whisker detector
caption_attribution_description: English: Crystals of galena sold for use in Polish crystal radio receivers probably in the 1930s. They were used in cat's whisker detectors, an antique electronic component used in crystal radios, which remained in use in Eastern Europe into the 1950s. In a cat's whisker detector, the face of the crystal was touched by a fine wire, forming a point-contact semiconductor diode, which rectified the radio waves in the receiver to extract the audio signal from the radio frequency carrier wave. The crystals varied in their effectiveness, so several crystals often had to be tried to find a sensitive one. The text on the box says: "'Ramona' Gold" "World's most powerful radio crystal" "Polish Radio Industry S.F.W."
context_page_description: A crystal detector is an obsolete electronic component used in some early 20th century radio receivers that consists of a piece of crystalline mineral which rectifies the alternating current radio signal and was employed as a detector to extract the audio modulation to produce the sound in the earphones. It was the first type of semiconductor diode, and one of the first semiconductor electronic devices. The most common type was the so-called cat whisker detector, which consisted of a piece of crystalline mineral, usually galena, with a fine wire touching its surface. The "asymmetric conduction" of electric current across electrical contacts between a crystal and a metal was discovered in 1874 by Karl Ferdinand Braun. Crystals were first used as radio wave detectors in 1894 by Jagadish Chandra Bose in his microwave experiments. Bose first patented a crystal detector in 1901. The crystal detector was developed into a practical radio component mainly by G. W. Pickard, who began research on detector materials in 1902 and found hundreds of substances that could be used in forming rectifying junctions.
context_section_description: Patented by Karl Ferdinand Braun and Greenleaf Whittier Pickard in 1906,  this was the most common type of crystal detector, mainly used with galena but also other crystals.  It consisted of a pea-size piece of crystalline mineral in a metal holder, with its surface touched by a fine metal wire or needle (the "cat whisker").  The contact between the tip of the wire and the surface of the crystal formed a crude unstable point-contact metal–semiconductor junction, forming a Schottky barrier diode.  The wire whisker is the anode, and the crystal is the cathode; current can flow from the wire 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, and the contact could be disrupted by the slightest vibration.  Therefore, a usable point of contact had to be 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 user until the device began functioning.  In a crystal radio, the user would tune the radio to a strong local station if possible and then adjust the cat whisker until the station or radio noise (a static hissing noise) was heard in the radio's earphones.  This required some skill and a lot of patience.  An alternative method of adjustment was to use a battery-operated buzzer connected to the radio's ground wire or inductively coupled to the tuning coil, to generate a test signal.  The spark produced by the buzzer's contacts functioned as a weak radio transmitter whose radio waves could be received by the detector, so when a rectifying spot had been found on the crystal the buzz could be heard in the earphones, at which time the buzzer was turned off.
  The detector consisted of two parts mounted next to each other on a flat nonconductive base:
  Crystal
  A crystalline mineral formed the semiconductor side of the junction.  The most common crystal used was galena (lead sulfide, PbS, varieties were sold under the names "Lenzite" and "Hertzite"), a widely occurring ore of lead, although other crystalline minerals were also used, the more common ones were iron pyrite (iron sulfide, FeS₂, "fool's gold", also sold under the trade names "Pyron" and "Ferron"), molybdenite (molybdenum disulfide, MoS₂), and cerussite (lead carbonate, PbCO₃)  Not all specimens of a crystal would function in a detector, often several crystal pieces had to be tried to find an active one.  Galena with good detecting properties was rare and had no reliable visual characteristics distinguishing it from galena samples with poor detecting properties. A rough pebble of detecting mineral about the size of a pea was mounted in a metal cup, which formed one side of the circuit.  The electrical contact between the cup and the crystal had to be good, because this contact must not act as a second rectifying junction, creating two back-to-back diodes which would prevent the device from conducting at all.  To make good contact with the crystal, it was either clamped with setscrews or embedded in solder.  Because the relatively high melting temperature of tin-lead solder can damage many crystals, a fusible alloy with a low melting point, well under 200 °F (93 °C), such as Wood's metal was used.  One surface was left exposed to allow contact with the cat-whisker wire.
  Cat whisker
  The "cat whisker", a springy piece of thin metal wire, formed the metal side of the junction.  Phosphor bronze wire of about 30 gauge was commonly used because it had the right 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 whiskers in simple detectors were straight or curved, but most professional cat 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 b
caption: Galena crystals sold for use in crystal detectors, Poland, 1930s
exif: {"Image XResolution": "72", "Image YResolution": "72", "Image ResolutionUnit": "Pixels/Inch", "Image ExifOffset": "78", "EXIF ExifVersion": "0210", "EXIF FlashPixVersion": "0100", "EXIF ColorSpace": "Uncalibrated"}