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The principle of thermocouples is that when two junctions of a pair of dissimilar metals are connected together and the two are kept at different temperatures, there exists a voltage between the ends, which is a function of the temperature difference and is called the thermo emf.
This can be used to measure temperature accurately and co control it. Platinum, platinum-Rhodium thermocouples can be used for very high temperatures.
I don't know how much information you need, but here is the standard Wikipedia article:
http://en.wikipedia.org/wiki/thermocoupl...
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