There are three laws of thermocouples.Refer diagram to grab more quickly.
Law of homogeneous material
A current cannot flow in a single wire loop made of same material by
application of heat alone.So if there is a temperature difference between two points along the length of a homogenous wire there will not be a voltage difference.
Law of intermediate materials
At uniform temperature the algebraic sum of the thermoelectric emfs in a circuit composed of any number of dissimilar materials is zero .So if a third metal is inserted in either of two wires of thermocouple and if the two new junctions are at the same temperature, there will be no net voltage generated by the new metal.
Law of successive or intermediate temperatures
Provided T1<T2<T3 then if a thermal emf1 is produces by thermocouple made by two dissimilar homogeneous materials whose junctions are at T1 and T2 and emf2 is produced when thejunctions are at T2 and T3 , the emf generated when the junctions are at T1 and T3 will be emf1 + emf2.