Hot wire barretter
Encyclopedia
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The first device used to demodulate audio signals, it was later superseded by the electrolytic detector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. The barretter principle is still used as a detector for microwave
radiation, similar to a bolometer
.
wire, about 0.003 inch (0.0762 mm) in diameter, is embedded in the middle of a silver
wire having a diameter of about 0.1 inches (2.5 mm). This compound wire is then drawn
until the silver wire had a diameter of about 0.002 inch (0.0508 mm); as the platinum wire within it is reduced in the same ratio, it is drawn down to a final diameter of 6E-05 inch. 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 put under vacuum and then sealed.
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, the wire is further heated as the current increases and cools as the current decreases again. As the wire heats and cools, it varies its resistance in response to the signals passing through it. Because of the low thermal mass of the wire, it is capable of responding quickly enough to vary its resistance in response to audio signals. However, it cannot vary its resistance fast enough to respond to much higher radio frequencies. The radio frequencies are essentially removed, and the sound is demodulated because the current through the circuit varies with the changing wire resistance. Headphones are connected in series with the D.C. circuit and the variations in the current are rendered as sound.
Reginald Fessenden
Reginald Aubrey Fessenden , a naturalized American citizen born in Canada, was an inventor who performed pioneering experiments in radio, including early—and possibly the first—radio transmissions of voice and music...
, 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
Coherer
The coherer was a primitive form of radio signal detector used in the first radio receivers during the wireless telegraphy era at the beginning of the twentieth century. Invented around 1890 by French scientist Édouard Branly, it consisted of a tube or capsule containing two electrodes spaced a...
(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
Electrolytic detector
The electrolytic detector, or the bare-point electrolytic detector as it was also called, was a type of wet demodulator used in early radio receivers. This form of detector was in extensive use between the years 1902 and 1913, after which the superior vacuum tube diode became available...
, also generally attributed to Fessenden. The barretter principle is still used as a detector for microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
radiation, similar to a bolometer
Bolometer
A bolometer is a device for measuring the power of incident electromagnetic radiation via the heating of a material with a temperature-dependent electrical resistance. It was invented in 1878 by the American astronomer Samuel Pierpont Langley...
.
Description and construction
An extremely fine platinumPlatinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...
wire, about 0.003 inch (0.0762 mm) in diameter, is embedded in the middle of a silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
wire having a diameter of about 0.1 inches (2.5 mm). This compound wire is then drawn
Wire drawing
Wire drawing is a metalworking process used to reduce the cross-section of a wire by pulling the wire through a single, or series of, drawing die. There are many applications for wire drawing, including electrical wiring, cables, tension-loaded structural components, springs, paper clips, spokes...
until the silver wire had a diameter of about 0.002 inch (0.0508 mm); as the platinum wire within it is reduced in the same ratio, it is drawn down to a final diameter of 6E-05 inch. The result is called Wollaston wire
Wollaston wire
Wollaston wire is a very fine platinum wire clad in silver and used in electrical instruments. For most uses, the silver cladding is etched away by acid to expose the platinum core.- History :...
.
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 put under vacuum and then sealed.
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 currentDirect current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
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, the wire is further heated as the current increases and cools as the current decreases again. As the wire heats and cools, it varies its resistance in response to the signals passing through it. Because of the low thermal mass of the wire, it is capable of responding quickly enough to vary its resistance in response to audio signals. However, it cannot vary its resistance fast enough to respond to much higher radio frequencies. The radio frequencies are essentially removed, and the sound is demodulated because the current through the circuit varies with the changing wire resistance. Headphones are connected in series with the D.C. circuit and the variations in the current are rendered as sound.
See also
- Electrolytic detectorElectrolytic detectorThe electrolytic detector, or the bare-point electrolytic detector as it was also called, was a type of wet demodulator used in early radio receivers. This form of detector was in extensive use between the years 1902 and 1913, after which the superior vacuum tube diode became available...
a development of the barretter detector. - Iron-hydrogen resistorIron-hydrogen resistorAn iron-hydrogen resistor consists of a hydrogen-filled glass bulb like a light bulb, in which an iron wire is located. This resistor has a positive temperature coefficient of resistance. This characteristic made it useful for stabilizing circuits against fluctuations in power supply voltages...
Patents
, "Current Actuated Wave Responsive Device" – August, 1902 ("barretter" detector), "Receiver for Electromagnetic Waves" – May, 1903 (improved "barretter")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.