Variation of emf with temp.
The magnitude of & direction depends upon nature of materials of the wires forming thermocouple & difference in temperature of two junctions. For majority of the thermocouples, the temperature dependence of the thermo-emf can be expressed by a quadratic equation,
_ _ _ _ (1)
where α and β are constants for the given pair of metals, called thermoelectric coefficients while θ is the temperature difference between hot and cold junctions.
The above relation holds for a limited range of temperature. Also the variation of thermo emf drawn against temperature difference between hot and cold junctions is found to be parabolic as shown.
Following experimental setup may be used to the effect of temp. on thermo emf for a given thermocouple:
One junction is kept hot by immersing in hot oil bath & the other junction is kept cold by immersing in ice. The temperature of the hot junction can be measured by a thermometer T. The deflection in the sensitive galvanometer G gives a measure of thermo emf.