Hot wire barretter

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The first device used to demodulate audio signals, it was later superseded by the electrolytic detector, also generally attributed to Fessenden.

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.

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 there is an oscillating current from the antenna 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 current are rendered as sound.

External links

Patents

*, "Current Actuated Wave Responsive Device" – August, 1902 ("barretter" detector)
*, "Receiver for Electromagnetic Waves" – May, 1903 (improved "barretter")

Other

* [http://home.luna.nl/~arjan-muil/radio/history/wireless-age/electrolytic.html Detectors of electrical oscillations]
* [http://www.northwinds.net/bchris/define.htm Tech Definitions - Radio Concepts]
* [http://earlyradiohistory.us/index.html United States Early Radio History ]
*Secor, H. Winfield (January, 1917). [http://earlyradiohistory.us/1917de.htm Radio Detector Development] . "The Electrical Experimenter", pages 652+, accessed 2007-12-20.