Question

If the temperature of the cold junction of a thermocouple is lowered, its neutral temperature:
(A) Decreases.
(B) Increases
(C) Remains the same.
(D) May increase or decrease depending upon the nature of metals in the thermocouple.

Answer 1

Hint We know that the cold junction is independent with neutral temperature, it means it will not be affected by the change in temperature of the cold junction.

Complete step by step answer
It is given in the question that if the temperature of the cold junction of the thermocouple is lowered then we have to find the change in its neutral temperature.
The thermocouple is a device having sensors that measure temperature. It consists of two different types of metals joined together at one end. Whenever the junction of the two metals is heated or cooled, a voltage is created that can be correlated back to the temperature.
The thermocouple is based on the See back effect principle. According to the See back effect when a closed-circuit is formed by joining two dissimilar metals at two junctions are maintained at different temperatures then an electromotive force (emf) is induced in this closed circuit.

In a thermocouple we have two wires of different metal wires, P is representing hot junction, Q is representing cold junction and this P and Q is maintaining a difference in temperature which is inducing an electromotive force.
The point Q will act as a temperature measuring junction whereas the point Q is usually placed in crushed ice or water so that it will remain at zero degree Celsius. But the emf at the cold junction remains the same when the temperature is lowered. It means it does not affect the neutral temperature of the junction. It will remain the same.

Therefore, option C will be the correct answer.

Additional information
The thermocouple has a very small-time lag with the change in temperature so it can be used in the application where a very rapid temperature change takes place but it is also noted that thermocouple has very low accuracy so they cannot be used for very high precision measurement.

Note
One can think that when the temperature of the cold junction is lowered then the emf of the junction will increase this will increase the neutral temperature of the circuit. But please note that cold junction is independent with neutral temperature thus, the neutral temperature remains the same.