Laws for thermocouples

Law of homogeneous material
A thermoelectric current cannot be sustained in a circuit of a single homogeneous material by the application of heat alone, regardless of how it might vary in cross section. In other words, temperature changes in the wiring between the input and output do not affect the output voltage, provided all wires are made of the same materials as the thermocouple. No current flows in the circuit made of a single metal by the application of heat alone.

[edit] Law of intermediate materials

The algebraic sum of the thermoelectric emfs in a circuit composed of any number of dissimilar materials is zero if all of the junctions are at a uniform temperature. So If a third metal is inserted in either wire and if the two new junctions are at the same temperature, there will be no net voltage generated by the new metal.

[edit] Law of successive or intermediate temperatures

If two dissimilar homogeneous materials produce thermal emf1 when the junctions are at T1 and T2 and produce thermal emf2 when the junctions are at T2 and T3 , the emf generated when the junctions are at T1 and T3 will be emf1 + emf2,provided T1<T2<T3.