Cat's whisker diode
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A cat's whisker is the tiny wire that connects to the 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.
Description
As a detector it is simply a relatively primitive and unstable point-contact semiconductor 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.
Crystal
The crystal normally employed is galena, the common sulphide ore of lead, used without treatment directly as it is mined. A rough pebble of this material about the size of a pea, was mounted in a brass cup usually potted with tin or silver-tin solder. One surface was left exposed to receive the contact. Other minerals could also be used among them were cadmium sulfide, iron disulfide (pyrite), zincite, bornite, and silicon carbide (carborundum).
Whisker
The contact, (the cat's whisker itself) is merely a springy piece of thin phosphor bronze wire, 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.
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 and Bornite or Chalcopyrite, in fairly heavily spring-loaded contact. This variation was known as the Perikon detector, a derivation from "perfect Pickard contact" that also sounds like "pair of contact detectors". Other detectors patented by Pickard included the common crystal iron pyrite (also known as fool's gold for its coloration) and a detector based on silicon carbide.
Their general commercial use was superseded by the development of vacuum tube detectors, although the expense of the latter meant that full replacement took several decades.
External links
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 (siicon 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
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)
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