Cat's-whisker detector
From Wikipedia, the free encyclopedia
Cat’s whisker refers to a thin wire that lightly touches a semiconducting crystal to make an imperfect contact-junction detector in a crystal radio. While originally a figurative description of a mechanical part, the term grew to encompass the entire detector assembly and also in some English speaking communities to describe the receiver itself.
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[edit] Description
As a detector it is simply a relatively primitive and unstable metal-semiconductor point-contact junction forming a Schottky barrier diode. It is based on the discovery of the semiconductor or "point rectifier effect" by Karl Ferdinand Braun, a German physicist and radio pioneer, in 1874 at the University of Wurzburg. Based on this work G.W. Pickard developed the cat's whisker diode using a silicon crystal, which was patented in 1906.
[edit] Crystal
A natural mineral crystal forms the semiconductor side of the junction. Today the most common crystal used is galena, a naturally occurring sulphide ore of lead, used without treatment directly as it is mined. A specially selected rough pebble of this material 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 degrees F) metal such as Wood's metal. One surface was left exposed to allow contact with the cat's whisker wire. Good detector quality galena is rather rare and not reliably identifiable by visual inspection. Other minerals were also used; significant among them was iron disulfide (pyrite or "fools gold"). Many of the earlier noteworthy mineral detectors did not use the cat's whisker configuration, such as vitreous silicon, silicon carbide (carborundum) and a zincite-bornite rock-to-rock junction.
[edit] Whisker
The metal cat's whisker forms the metal side of the junction, which is merely a springy piece of thin metal wire (phosphor bronze was common), mounted in a suitable holder so that the entire exposed surface of the crystal can be probed from many directions to try and find the most sensitive working junction. This requires some skill and a great deal of patience; even then a good contact can easily be lost by the slightest vibration.
[edit] Developments and eventual replacement
When these devices were in common use, more advanced proprietary versions of "permanent" detector were developed, many of them by G. W. Pickard. One consisted of various combinations of pairs of different crystals such as Zincite touching Bornite or Chalcopyrite, in fairly heavily spring-loaded contact. This variation was known as the Perikon detector, a pseudo-rebus derivation from "perfect Pickard contact" that also sounds like "pair of contact detectors". Other detectors patented by Pickard included the common crystal iron pyrite. Major Dunworthy persistently pursued experiments with silicon compounds in a systematic search of a good detector material and made a major discovery in 1906 of a new compound he called "Carborundum" which we now know as silicon carbide.
Use of mineral detectors was largely superseded by the development of vacuum tube detectors, although the expense of the latter meant that full replacement took several decades.
[edit] See also
[edit] External links
[edit] Patents
- U.S. Patent 836531 - Means for receiving intelligence communicated by electric waves (silicon detector), Greenleaf Whittier Pickard, 1906
- U.S. Patent 837616 - Wireless telegraph system (silicon carbide detector), Henry H.C. Dunwoody, 1906
- U.S. Patent 906991 - Oscillation detector (multiple metallic sulfide detectors), Clifford D. Babcock, 1908
- U.S. Patent 912613 - Oscillation detector and rectifier ("plated" silicon carbide detector with DC bias), G.W. Pickard, 1909
- U.S. Patent 912726 - Oscillation receiver (fractured surface red zinc oxide (zincite) detector), G.W. Pickard, 1909
- U.S. Patent 933263 - Oscillation device (iron pyrite detector), G.W. Pickard, 1909
- U.S. Patent 1118228 - Oscillation detectors (paired dissimilar minerals), G.W. Pickard, 1914
[edit] 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)
