[JWest]

Found on the web:

Accurate measurement of temperature is one of the most common and vital requirements in industrial instrumentation. It is also one of the most difficult objectives to achieve. Unless proper temperature measuring techniques are employed, serious inaccuracies of reading can occur, or otherwise useless data can result. The thermocouple is by far the most widely used temperature sensor for industrial instrumentation. Its favorable characteristics include good inherent accuracy, suitability over a broad temperature range, relatively fast thermal response, ruggedness, high reliability, low cost, and great versatility of application.

A discovery by T. J. Seebeck almost 150 years ago, opened the way for modern thermoelectric circuitry. In 1831, Seebeck discovered that an electric current flows in a closed circuit of two dissimilar metals when one of the two junction is heated with respect to the other.

In such a thermocouple circuit the current continues to flow as long as two junction are at different temperatures. The magnitude and direction of the current is a function of the temperature difference between the junctions and of the thermal properties of the metals used in the circuit. This phenomenon, is known as the Seebeck Affect.

The conductors can be of any two dissimilar metals, and when the hot junction is heated the current flow can be observed on a milliamp meter. If the position of the hot and cold junction is reversed, current will flow in the opposite direction.

In fact, a thermocouple circuit will actually generate a measurable, low voltage output that is almost directly proportional to the temperature difference between the hot junction and the cold junction. A unit change in this temperature difference produces some net change in the voltage.

Edit: didn't want to say I wrote this - I found it on the web.