Cat's-whisker detector

Galena cat's whisker detector

Precision cat's whisker detector using iron pyrite crystal, early 1900s. The crystal is inside the metal capsule under the vertical needle "whisker" (right).

A cat’s whisker detector is an early electronic component consisting of a thin wire that lightly touches a semiconducting crystal to make a crude contact-junction rectifier. This device was used as the detector in early crystal radios, and gave this type of radio receiver its name. They were the first type of semiconductor diode. The term was also sometimes used to describe the crystal receiver itself. Cat's whisker detectors are obsolete and are now only used in antique or antique-reproduction radios.

Description

A small portable crystal radio, with a cat's whisker detector visible at top

The wire touching the surface of the crystal formed a primitive and unstable metal–semiconductor point-contact junction, forming a Schottky barrier diode. This junction conducts electric current in only one direction and resists current flowing in the other direction.

Only certain sites on the crystal surface functioned as rectifying junctions, and the device was very sensitive to the exact geometry and pressure of contact between wire and crystal. Therefore it was made adjustable, and a usable point of contact was found by trial and error. 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, and then adjust the cat's whisker until the station sounded loudest 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.

Crystal

Modern galena cat's whisker detector, showing parts

A natural mineral crystal forms the semiconductor side of the junction. Today the most common crystal used is galena (lead sulfide), a naturally occurring ore of lead and a semiconductor with a small bandgap of about 0.4 eV, used without treatment directly as it is mined. Galena with good detecting properties is rare and has 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. Because the relatively high temperature of tin-lead solder can damage many crystals, the crystal was often mounted in a low melting point (well under 200°F) metal such as Wood's metal. One surface was left exposed to allow contact with the cat's whisker wire.

Whisker

The "cat's 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 try and find the most sensitive working junction. The crystal required just the right gentle pressure by the wire; too much pressure caused the device to conduct in both directions. Professional detectors often had thumbscrew operated leaf springs to adjust the pressure applied.

Different types

Historically, many other minerals and compounds besides galena were used for the crystal, the most important being iron pyrite or "fool's gold" (iron disulfide), vitreous silicon, silicon carbide (carborundum), and a zincite-chalcopyrite crystal-to-crystal junction trade named Perikon. The goal of researchers was to find junctions that were not as sensitive to vibration and unreliable as galena and pyrite. Many of these other junctions were stable enough that they used a more permanent spring-loaded point contact rather than a "cat's whisker". These were preferred in large installations like commercial, military and shipboard wireless stations, while "cat's whisker" devices were used in consumer and hobbyist radios. To increase sensitivity, some of these junctions such as silicon carbide were "biased" by connecting a battery and potentiometer across them to provide a small constant forward voltage across the junction.^[1]

History

The "point rectifier effect", as it was then called, was discovered by Ferdinand Braun, a German physicist and radio pioneer, in 1874 at the University of Würzburg. Based on this work G.W. Pickard developed the cat's whisker diode using a silicon crystal, which was patented in 1906. However, Bengali scientist Jagadish Chandra Bose may have been the first to use a semiconductor to rectify radio waves, applying for a patent on a galena detector in 1901.

When these devices were in common use, more advanced proprietary versions of "permanent" detector were developed, many of them by G. W. Pickard, who tested more than 30,000 combinations of crystal and wire contacts.^[2] One consisted of various combinations of pairs of different crystals such as Zincite touching Bornite or Chalcopyrite, in fairly heavily spring-loaded contact. Pickard named this the Perikon detector, from "PERfect pIcKard cONtact". Other detectors patented by Pickard included the common crystal iron pyrite. Pickard has the distinction of having brought silicon into use as a detector, patenting it in 1906. At nearly the same time, General Henry Harrison Chase Dunwoody patented the use of the silicon carbide (carborundum) detector, an artificial substance created accidentally during attempts by Edward Acheson to create diamonds.

Unamplified radio receivers, most of which were crystal radios, were the only way to receive radio signals during most of the wireless telegraphy era, which ended around 1920. Mineral detectors were largely superseded by vacuum tubes, invented in 1906, although the expense of tube receivers meant that full replacement took several decades. By the 1920s crystal radios were relegated to use by hobbyists and youth groups.

A crystal detector in commercial form from the 1960's

The point-contact Si detector was subsequently resurrected around WW2 because of the military requirement for microwave radar. Vacuum tube detectors do not work at microwave frequencies. Even the semiconductor p-n junction detectors may not be fast enough compared to semiconductor point contact detectors because of minority carrier storage problem, and large capacitance if the area is not small enough. Although cat-whisker detectors are obsolete, modern point-contact Si detectors are still commercially available. For example, Advanced Semiconductor Inc. (North Hollywood, California, USA) is selling Si point contact detectors which will cover from UHF (ultra high frequency) to 16 GHz. Thus the approach pioneered by Ferdinand Braun and Greenleaf Whittier Pickard is still very much alive today.

Copies of original cat's-whisker detectors are still manufactured and sold, for antique radio hobbyists.

See also

• List of historic technological nomenclature
• Reginald Fessenden
• barretter detector
• electrolytic detector

References

External links

Patents

• U.S. Patent 836,531 - Means for receiving intelligence communicated by electric waves (silicon detector), Greenleaf Whittier Pickard, 1906
• U.S. Patent 837,616 - Wireless telegraph system (silicon carbide detector), Henry H.C. Dunwoody, 1906
• U.S. Patent 906,991 - Oscillation detector (multiple metallic sulfide detectors), Clifford D. Babcock, 1908
• U.S. Patent 912,613 - Oscillation detector and rectifier ("plated" silicon carbide detector with DC bias), G.W. Pickard, 1909
• U.S. Patent 912,726 - Oscillation receiver (fractured surface red zinc oxide (zincite) detector), G.W. Pickard, 1909
• U.S. Patent 933,263 - Oscillation device (iron pyrite detector), G.W. Pickard, 1909
• U.S. Patent 1,118,228 - Oscillation detectors (paired dissimilar minerals), G.W. Pickard, 1914

General

• Crystal and Solid Contact Rectifiers 1909 publication describes experiments to determine the means of rectification (PDF file)
• Radio Detector Development from 1917 The Electrical Experimenter
• The Crystal Experimenters Handbook 1922 London publication devoted to point contact diode detectors. (PDF file courtesy of Lorne Clark via http://www.earlywireless.com)