In Thomson's effect we deal with only metallic rod and not with thermocouple as in Peltiers effect and Seebeck's effect. (That's why sometimes it is known as homogeneous thermo electric effect. When a current goes through an unequally heated substance, there is an evolution or absorption of heat in the body of the substance. This is called Thomson's effect.
(i) Positive Thomson's effect : In positive Thomson's effect it is found that hot end is at high potential and cold end is at low potential. Temperature is evolved when current is gone from hotter end to the colder end and heat is absorbed when current is passed from colder end to hotter end. The substances which provide positive Thomson's effect are Ag, Cd, Cu, Sn, Zn... etc.
(ii) Negative Thomson's effect: In the elements which show negative Thomson's effect, it is found that the hot end is at low potential and the cold end is at higher potential energy. Temperature is evolved when current is gone from colder end to the hotter end and heat is absorbed when current flows from hotter end to colder end. The substances which show negative, Thomson's effect are Bi, Pt, Fe, Co, Hg... etc.
Thomson's co-efficient
In Thomson's effect it is found that heat released or absorbed is proportional to QΔθ i.e. H ∝ QΔθ H=σQΔθ; where σ = Thomson's coefficient. It's unit is Joule/coulomb°C or volt/°C and Δθ = temperature difference.
If Q = 1 and Δθ = 1 then s = H so the amount of heat energy absorbed or evolved per second between two points of a conductor having a unit temperature difference, when a unit current is passed is known as Thomson's co-efficient for the material of a conductor.
It may be given that Thomson co-efficient of the material of conductor 
; where b = Thermo electric constant =dS/dt
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