Thermo-Electric Diagrams

A thermocouple is formed from two metals A and B. The difference of temperature of the junctions is TK. The thermo emf E is given by the equation.

        E = aT + bT²
A graph between E and T is a parabola.
        dE/dT = a + 2bT
dE/dt is called thermoelectric power.

A graph between thermoelectric power (dE/dT) and difference of temperature T is a straight line. This graph is called the thermo-electric power line or the thermo-electric diagram. Thomson coefficient of lead is zero. So generally thermoelectric lines are drawn with lead as one metal of the thermocouple. The thermoelectric line of a Cu-Pb couple has a positive slope while that of Fe-Pb couple has a negative slope.

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