The thermocouple is a temperature-measuring device made from two dissimilar metals which, thanks to the Seebeck effect, generates a voltage proportional to temperature.

Any two different metals or metal alloys exhibit this thermoelectric effect, but only a few are used as thermocouples. These have well-characterized temp/voltage curves and the most popular have letter designations (J, K, etc) The typical material combinations: antimony and bismuth, copper and iron, or copper and constantan. Platinum, either with rhodium or a platinum-rhodium alloy, is usually used in high-temperature thermocouples.

Since all metal-metal junctions have this property, attention to detail when wiring up thermocouple circuits is essential. Solder joint? Thermocouple. Connector? Thermocouple. Long cable? Resistor which saps some of your already tiny (millivolts) signal.

Omega Engineering will send you a tome about thermocouples and temperature measuring design for the asking: