What is the law of intermediate metals?

According to the Thermocouple Law of Intermediate Metals, illustrated in the figure, inserting any type of wire into a thermocouple circuit has no effect on the output as long as both ends of that wire are the same temperature, or isothermal 


Consider the circuit in the next figure.



Both circuits are quite similar but a short length of constantan wire has been inserted just before junction J3 and the junctions are assumed to be held at identical temperatures. Assuming that junctions J3 and J4 are the same temperature, the Thermocouple Law of Intermediate Metals indicates that the circuit in the figure on left is electrically equivalent to the circuit of the figure on right. Consequently, any result taken from the circuit in the figure on left also applies to the circuit illustrated in the figure on right.

What is a typical construction?

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