Thermocouple History
A thermocouple--also known as a thermocouple thermometer or thermocouple junction--is a device that consists of two wires joined together that contain two different metals. One junction has the temperature to be measured while the other junction has a fixed lower temperature. Thus, the current that the two ends generate is related to a temperature difference. A 19th century invention, the thermocouple is an inexpensive method of measuring temperatures.
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Types
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Several types of thermocouples exist. J and K thermocouples are particularly popular in the United States as they are easy to produce. The former is made of iron and constantan (a copper-nickel alloy) with a zero to 1,400 degrees Celsius temperature range, and the latter is made of chromel and alumel, with a zero to 2,300 degree Celsius range. Other types include E (chromel and constantan) and T (copper and constantan) thermocouples.
Voltage-Temperature Relationship
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Bear in mind that electrical current generated from the contact between the two metals has to be converted to temperature. This can be done by connecting the wires to a digital voltmeter that can convert the voltage reading to a temperature.
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Thomas Johann Seebeck
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Thomas Johann Seebeck (1770 to 1831), a German-Estonian physicist, is credited with discovering current generated by touching two dissimilar metals together at different temperatures to deflect a compass needle. Seebeck produced the first thermocouples, and experimented with metals of different kinds and shapes.
However, he thought that the current was magnetic, rather than electric. Indeed, his reports to the Prussian Academy of Sciences between 1822 and 1823 describes his observations as "the magnetic polarization of metals and ores produced by a temperature difference." Nevertheless, this thermoelectric effect is now referred to as the Seebeck Effect.
Leopoldo Nobili
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Using Seebeck's work as his foundation, Italian physicist Leopoldo Nobili (1784 to 1835) collaborated with another Italian physicist, Macedonio Melloni (1798 to 1854), to develop a thermoelectrical battery in 1826. Called the "thermo-multiplicateur" (thermomultiplier), it drew from Seebeck's discovery of thermoelectricity by combining a thermopile and galvanometer to measure radiation. For his work, some people credit Nobili as the inventor of the thermocouple as it is known today, or at least the archetype.
Today
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Thermocouples are used widely, particularly in space exploration missions, heating appliances and electronics manufacturing. They are popular in these industries because they are simple, durable, have no need for batteries, dependable and cover a wide range of temperatures.
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