The Law of Intermediate Metals for thermocouples states that…
The Law of Intermediate Metals for thermocouples states that inserting a third metal at junctions held at the same temperature does not affect the measured voltage. What practical implication does this have? It means thermocouples only work with two metals, and a third metal will always distort readings. It allows solder, connectors, and measuring instruments to be inserted into the circuit without introducing error, provided their junctions are isothermal. It requires the reference junction to always be at 0°C. It proves that the voltage produced at the junction of two metals does not depend on temperature.