Definition: When the two junctions of a thermo couple are maintained at different temperatures, then a light starts going through the loop known as thermo electric current. The potential energy difference in between the junctions is called thermo electric emf which is of the order of a few micro-volts per degree temperature difference (mV/°C).

See beck Series: The value and direction of thermo emf in a thermocouple depends not only on the temperature difference between the hot and cold junctions but also on the nature of metals constituting the thermo couple.

(i) Seebeck arranged different metals in the decreasing order of their electron volume. Few metals creating the series are as below.

            Sb, Fe, Zn, Ag, Pb, Hg, Au, Cr, Sn, Cd, Mn, Cu, Pt, Co, Ni, Bi

(ii) Thermo electric emf is directly proportional to the distance between the two substances in series. Farther the substances in the series forming the thermo couple greater are the thermo emf. Thus maximum thermo emf is obtained for Sb-Bi thermo couple

(iii) The current flow at the hot junction of the thermocouple is from the metal happening later in the series towards that happening earlier, so, in the copper-iron thermocouple the current flows from copper (Cu) to iron (Fe) at the hot nods. That can be remind easily by the hot coffee.

Variation of thermo emf with temperature: In a thermocouple as the temperature of the hot junction increases keeping the cold junction at constant temperature (say 0°C). The thermo emf increases till it becomes maximized at a certain temperature.

(i) Thermo electric emf is given by the equation E= αt + 1/2βt² where α and β are thermo electric constant having units are volt/°C and volt/°C² respectively (t = temperature of hot junction). For E to be maximum (at t = t_n)

           dE/dt=0

(ii) The temperature of hot junction at which thermo emf becomes maximum is called neutral temperature (t_n). Neutral temperature is constant for a thermo couple (e.g. for Cu-Fe, t_n = 270°C)

(iii) Simple temperature is not dependent of the temperature of cold junction.

(iv) If temperature of hot junction increases beyond neutral temperature, thermo emf start losing and at a particular temperature it converts into zero, on heating slightly further, the direction of emf is saved. That temperature hot junction is known as temperature of inversion (t_i).

(v) Relation between t_n, t_i and t_c is t_n = t_i+t_j/2

Thermo electric power: The rate of modification of thermo emf with the change in the temperature of the hot junction is called thermoelectric power.

It is also provided by the slope of parabolic curve representing the variation of thermo emf with temperature of the hot junction, as discussed in previous section.

The thermo electric power  is also called Seebeck coefficient. Differentiating both equations of thermo emf with respect to t, we got thermoelectric power

Laws of thermoelectricity

(i) Law of successive temperature : If initially temperature limits of the cold and the hot junction are t₁ and t₂, say the thermo emf is E_i. When the temperature limits are t₂ and t₃, then say the thermo emf is E_i =E₁+E₂  then E_i where  is the thermo emf when the temperature limits are E₀¹

(ii) Law of intermediate metals : Let A, B and C be the three metals of Seebeck series, where B lies between A and C. According to this law,  E_i =E₁+E₂

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