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The thermo e.m.f E in volts of a certain thermocouple is found to vary with temperature difference θ in °C between the two junctions according to the relation E = 300 − θ² / 15 The neutral temperature for the thermocouple will be

1. 30°C
2. 450°C
3. 400°C
4. 225°C

Correct answer: 225°C

Solution

The neutral temperature is the temperature at which the thermo e.m.f. is maximum. To find this, we differentiate E with respect to θ and set it to zero: dE/dθ = -2θ/15 = 0. Solving gives θ = 0 or θ = 225°C. Since θ = 0 corresponds to no temperature difference, the neutral temperature is 225°C.

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