Question

A thermoelectric thermometer is made of copper iron thermocouple. One junction is placed in melting ice and the other is placed in a bath, whose temperature is continuously increased from $0^{\circ}C$ to $600^{\circ}C$. The current in the galvanometer
A. remains constant
B. constantly goes on increasing
C. First increase and the decreases to zero
D. First increase becomes maximum and then decreases to zero and finally reverses its direction.

Answer 1

Thermocouple is a device which can measure the temperature, which is directly produced from an electric voltage. The thermocouple has two dissimilar electrical junctions , when some electrical potential or voltage is applied to the ends, there is transfer of heat between the junctions. This transfer of heat causes the change in temperature difference between them.

Complete step-by-step answer:
Given that in a Cu Fe thermocouple, the temperature increases from $0^{\circ}C$ to $600^{\circ}C$ with respect to the ice bath. Then this variation of temperature affects the emf of the circuit. Hence it emf increases, with the increase in temperature till it reaches the neutral temperature. Due to the increase in the temperature, there will be flow of electrons and hence the current in the galvanometer increases.
After the neutral temperature, if the temperature increases, the emf drops and the flow of electrons also decreases. This is observed till the temperature reaches the inversion temperature.
After the inversion temperature, the galvanometer reverses its direction.
Hence the answer is option D. First increase becomes maximum and then decreases to zero and finally reverses its direction.

So, the correct answer is “Option D”.

Additional Information:
If $T_{c}$ and $T_{h}$ are the temperatures of the hot and cold junction of the thermocouple, then the neutral temperature $T_{n}$ is given as $T_{n}=\dfrac{T_{c}+T_{h}}{2}$.
The various types of a thermocouple are:
1. the K type thermocouple where the junctions are Ni-Cr or Ni-Al
2. the J thermocouple where the junctions are Fe-constantan, to name a few.

Note: The reverse phenomenon of thermocouple is called the thermoelectric effect, where the difference in temperature results in the electric voltage. The thermocouple is used mostly in the temperature sensors and other scientific industries.