Thomsons Effect

Consider a copper bar AB heated in the middle at the point C. A current is passed from A to B. It s observed that heat is absorbed in the part AC and evolved in the part CB. This is known as Positive Thomson effect. Similar effect is observed in metals like Ag, Zn, Sb and Cd.

In the case of an iron bar AB, heat is evolved in the part AC and absorbed in the part CB. This is known as Negative Thomson effect. Similar effect is observed in metals like Pt, Ni, Co and Bi.
For leads, the Thomson effect is zero.
The Thomson effect is reversible.
In the case of copper, the hotter parts are at a higher potential than the colder ones. It is opposite in the case of iron. Heat is either absorbed or evolved when current passes between two points having a difference of potential. Therefore, the passage of electric current through a metal having temperature gradient results in an absorption or evolution of heat in the body of the metal.

When a current flows through an unequally heated metal, there is an absorption or evolution of heat throughout in the body of the metal. This is known as ‘Thomas effect.’

Thomson Coefficient. TheThemson coefficient σ of a metal is defined as the amount of heat energy absorbed or evolved when a charge of 1 coulomb flows in the metal between two points which differ in temperature by 1C
Thus, if a charge of q coulomb flows in a metal between two points having a temperature difference of 1C, then
heat energy absorbed or evolved = σ q joule.
But if E volt be the Thomson emf developed between these points
. :    σq = Eq
or     σ  = E.
Thus the Thomson coefficient of metal, expressed in joule per coloumb per C, is numerically equal to the emf in volt, developed between two points differing in temperature by 1C.
Hence it may also be expressed in volt per C.
σ is not a constant for a given metal. It is a function of temperature.