Question

With increase of hot junction temperature in thermocouple thermo emf :

• A. always increases with temperature
• B. is always positive
• C. can be positive (or) negative
• D. first increases with temperature and then decreases with temperature

Answer 1

The correct option is D first increases with temperature and then decreases with temperature

In a thermocouple when the hot junction temperature is raised, the galvanometer shows a deflection which is directly proportional to the thermoelectric current and hence the thermo e.m.f. When the temperature is further increased, there is a state reached when the thermo e.m.f is maximum.
This value called neutral temperature (${\theta }_n$). The given thermocouple has a neutral temperature that has a fixed value and does not depend on the colder junction.
After further increasing the temperature, it is found that at a particular temperature we would see the thermo e.m.f decreases finally reaching zero. The temperature of the hot junction where there is reversal in the direction of e.m.f is called the temperature of inversion (${\theta }_i$).