It is defined as the rate of change of thermo-electric e.m.f with temperature.

Mathematically, thermo- electric power at temperature T is given by

P = dE
      dT

where dE is the thermo-electric-e.m.f. developed in a thermocouple having a temperature of hot junction increased by dT from T^oC

Since the equation of thermo-electric e.m.f is written as

E = p_AB T + 1q_AB T²
                  2
Differentiating both sides with respect to T we get

dE  = p_AB + q_AB T
dT
Thermo-electric power , P = dE
                                        dT


It is of the form y = mx+ c and therefore graph between thrmo-elelctri power dE and temperature is a
                                                                                                             dT
straight line. This straight line crosses the temperature axis at a neutral temperature point i.e. when T = Tn = -p_AB .
       q_AB

At neutral temperature. dE  = 0
                                  dT

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