The constants a a

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Thermocouples are of two wire legs made from different metals. These wires legs are welded together at one end, creating a junction.

This junction is where the temperature is measured.

When the change in temperature occurs at junction, a voltage is created.

Now neutral temperature for the thermocouple.
is the temperature of the hot junction at which the thermo-emf in a thermocouple becomes maximum

For a thermocouple with constants a and b having the same sign, the neutral temperature will be less than the cold junction temperature of thermocouple, as

Hence, there will be no neutral or inversion temperature, since the hot junction temperature cannot be less than the temperature of cold junction of thermocouple.