A thermocouple is a temperature-measuring device consisting of two dissimilar conductors that contact each other at one or more spots. It produces a voltage when the temperature of one of the spots differs from the reference temperature at other parts of the circuit. Thermocouples are a widely used type of temperature sensor for measurement and control, and can also convert a temperature gradient into electricity. The operation of thermocouples is based on a phenomena called as a seeback effect. It states that a current flows in a closed circuit made of two dissimilar metals if the junctions of two metals are kept at different temperatures. Due to this current flow, an emf proportional to the temperature difference is produced. The sensitivity of thermal element is given by:

S=e/t=a+bt+ct2.Where “e” is the emf generated corresponding to a temperature change of “t”.a,b,c are constants determined by measuring the value of “e” at three standard reference temperatures.

Thermoelectric laws are:

1. The application of heat to a single homogenous metal is in itself not capable of producing an electric current.
2. A thermoelectric emf is produced when the junctions of two dissimilar homogenous metals are kept at different temperatures.
3. In a thermocouple that has junctions with different temperatures, the emf developed will remain unaffected even if a third homogenous metal is included in the circuit, provided the temperatures of the two junctions are the same.
4. The algebraic sum of emfs produced in a circuit containing two or more thermocouples all at the same temperature is zero.
5. The total emf of a circuit containing two thermocouples is unaltered by the addition of more thermocouples at the same temperature.