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The rate of increase of thermo e.m.f. with temperature at the neutral temperature of a thermocouple
A
Is negative
B
Is positive
C
Is zero
D
Depends upon the choice of the two materials of the thermocouple
Detailed Solution
The thermo emf of a thermocouple varies with the hot-junction temperature as $E = \alpha t + \frac{1}{2}\beta t^2$, which is a parabola.
The neutral temperature is the temperature at which the thermo emf is maximum.
At a maximum, the slope of the curve is zero: $\frac{dE}{dt} = 0$
Below the neutral temperature the rate is positive and above it the rate is negative, but exactly at the neutral temperature it is zero for every thermocouple.
Hence the rate of increase of thermo emf at the neutral temperature is zero.
The neutral temperature is the temperature at which the thermo emf is maximum.
At a maximum, the slope of the curve is zero: $\frac{dE}{dt} = 0$
Below the neutral temperature the rate is positive and above it the rate is negative, but exactly at the neutral temperature it is zero for every thermocouple.
Hence the rate of increase of thermo emf at the neutral temperature is zero.