The hot-wire barretter was a demodulating detector, invented in 1902 by

Reginald Fessenden Reginald Aubrey Fessenden (October 6, 1866 – July 22, 1932) was a Canadian-born inventor, who did a majority of his work in the United States and also claimed U.S. citizenship through his American-born father. During his life he received hundre ...

, that found limited use in early

radio receiver In radio communications, a radio receiver, also known as a receiver, a wireless, or simply a radio, is an electronic device that receives radio waves and converts the information carried by them to a usable form. It is used with an antenna. Th ...

s. In effect, it was a highly sensitive thermoresistor, which could demodulate amplitude-modulated signals, something that the

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 20th century. Its use in radio was based on the 1890 findings of French physicist Édouard Bran ...

(the standard detector of the time) could not do. Tapan K. Sarkar, Robert Mailloux, Arthur A. Oliner, Magdalena Salazar-Palma, Dipak L. Sengupta, "History of Wireless", , January 2006, Wiley-IEEE Press, page 369. The first device used to demodulate amplitude modulated signals, it was later superseded by the

electrolytic detector The electrolytic detector, or liquid barretter, was a type of detector (demodulator) used in early radio receivers. First used by Canadian radio researcher Reginald Fessenden in 1903, it was used until about 1913, after which it was superseded ...

, also generally attributed to Fessenden. The barretter principle is still used as a detector for

microwave Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency ra ...

radiation, similar to a

bolometer A bolometer is a device for measuring radiant heat by means of a material having a temperature-dependent electrical resistance. It was invented in 1878 by the American astronomer Samuel Pierpont Langley. Principle of operation A bolometer ...

Description and construction

Fessenden's 1902 patent describes the construction of the device. A fine

platinum Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver". Pla ...

wire, about in diameter, is embedded in the middle of a

silver Silver is a chemical element with the symbol Ag (from the Latin ', derived from the Proto-Indo-European ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, white, lustrous transition metal, it exhibits the highest electrical ...

tube having a diameter of about . This compound wire is then drawn until the silver wire has a diameter of about ; as the platinum wire within it is reduced in the same ratio, it is drawn down to a final diameter of . The result is called

Wollaston wire Wollaston wire is a very fine (less than 0.01 mm thick) 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 wire is named after ...

. 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 increase of a metal resistivity with increasing temperature. The device is biased by a

direct current Direct current (DC) is one-directional flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or ev ...

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 In building design, thermal mass is a property of the mass of a building that enables it to store heat and provide inertia against temperature fluctuations. It is sometimes known as the thermal flywheel effect. The thermal mass of heavy structura ...

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 the much higher radio frequencies. The signal is demodulated because the current supplied by the biasing source varies with the changing wire resistance. Headphones are connected in series with the DC circuit, and the variations in the current are rendered as sound.

References

External links

Patents

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

Other

Detectors of electrical oscillations
*Secor, H. Winfield (January, 1917)

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The Electrical Experimenter ''The Electrical Experimenter'' was an American technical science magazine that was published monthly. It was established in May 1913, as the successor to ''Modern Electrics'', a combination of a magazine and mail-order catalog that had been publ ...