Seebeck of Berlin noticed in 1824 that an electric current flows in a circuit consisting of two dissimilar metals if one junction,say A is kept cold and the other, say B, is heated as shown in Fig. seebeck effect This experiment is remarkable because no cell is required for sending current through the circuit. Fig. Seebeck Effect
At the hot junction B, the current flows from copper to iron while at cold junction A the current flows from iron to copper. The current flows from iron to copper. The current is called thermoelectric current and the combination of two dissimilar metals is known as thermocouple. The phenomenon of production of emf by a thermocouple when the two junctions are kept at different temperatures is known as Seebeck effect, and emf so produced is known as thermoelectric emf.
EMF of thermocouple depends upon(i) the nature of materials of metals (ii) the difference of temperatures of two junctions.
EFFECT OF DIFFERENCE OF TEMPERATURES ON THERMO-EMF
The junction A is kept at any temperature, say at C, and the temperature of other junction B is slowly increase. A table of different of temperature of hot junction B and cold junction A and the thermo-emf developed is drawn and a graph between them is plotted. The graph is then a parabolic curve, as shown in Fig. From the graph we observe that
Fig. Effect of Difference of Temperatures on Thermo-EMF
(i) For small difference of temperatures of the two junctions, the curve, say OA, is practically a straight line meaning thereby that theremoelectric emf is proportional to the temperature difference of hot and cold junctions.
Mathematically for initial portion of curve OA,
where a is a constant.
the difference of temperatures of two ends.
(ii) For larger difference of temperatures, the emf first increase following a parabolic curve and attains a maximum value for a certain temperature of hot junction. This temperature of hot junction (say C, for Cu-Fe thermocouple), at which the thermo-emf is maximum, is known as neutral temperature.
(iii) At neutral temperature i.e. at point B on curve
Characteristics of Natural Temperature:
(i) It is constant for a given thermocouple.
(ii) It depends upon the nature of metals of the couple.
(iii) It is independent of temperature of cold junction e.g. the neutral temperature of Cu-Fe couple is always C, whatever may be the temperature of cold junction.
Temperature of Inversion
With the further increase of temperature beyond neutral temperature, the thermo-emf decreases and finally becomes zero at the temperature known as temperature of inversion, because thermoelectric current is reversed at this temperature.
Characteristic of Temperature of Inversion
(i) It depends upon the temperature of cold junction.
(ii) It is as much above the nature temperature as the cold junction is below the neutral temperature.
Let the temperature of cold junction
and temperature of inversion
If temperature of cold junction in Cu-Fe couple is , then temperature of inversion will be .