Hot wire barretter
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The hot wire barretter was a demodulating detector invented in 1902 by Reginald Fessenden that found limited use in early radio receivers. In effect it was a highly sensitive thermoresistor developed to permit the reception of amplitude modulated signals, something that the coherer (the standard detector of the time) could not do.
The first device used to demodulate audio signals, it was later superseded by the electrolytic detector, also generally attributed to Fessenden.
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[edit] Description and construction
An extremely fine platinum wire, about .003 inch in diameter, is embedded in the middle of a silver wire having a diameter of about one-tenth inch. This compound wire is then drawn until the silver wire had a diameter of about .002 inch; as the platinum wire within it is reduced in the same ratio, it is drawn down to a final diameter of .00006 inch (1.5 μm). The result is called Wollaston wire.
The silver cladding is etched off a short piece of the composite wire, leaving an extremely fine platinum wire; this is supported, on two heavier silver wires, in a loop inside a glass bulb. The leads are taken out through the glass envelope and the whole device is sealed up and put under vacuum.
[edit] Operation
The hot wire Barretter depends upon the variation (usually an increase) of a metal's resistivity as a function of increasing temperature. The device is biased by a direct current adjusted to heat the wire to its most sensitive temperature. When an oscillating current from the antenna flows through the extremely fine platinum wire loop, it rapidly increases and decreases its electrical resistance. Headphones are connected in series with the D.C. circuit and the variations in the flow of current are rendered as sound.
[edit] External links
[edit] Patents
- US patent 706744 , "Current Actuated Wave Responsive Device" – August, 1902 ("barretter" detector)
- US patent 727331 , "Receiver for Electromagnetic Waves" – May, 1903 (improved "barretter")
[edit] Other
- Detectors of electrical oscillations
- Tech Definitions - Radio Concepts
- United States Early Radio History
- Secor, H. Winfield (January, 1917). Radio Detector Development. The Electrical Experimenter, pages 652+, accessed 2007-12-20.
