In 1856 Thomson found that heat energy is ab orbed or evolved not only at junctions when a current flows through a thermo-couple but evolution of absorption of heat takes place even in an unequally headed conductor. Thomason effect can be stated as-

The absorption or evolution of heat energy, if a current is allowed to flow in a conductor having its different parts at different temperatures is known as Thomson Effect.

Positive Thomson Effect : For metals like copper, silver, zinc, Antimont and Cadmium etc., portion at high temperature is considered at higher potential than a potion at lower temperature. Hence heat energy is absorbed when current flows from a point at lower temperature to a point at higher temperature.

Orection of Current

Let a copper wire LL be heated at its middle point H to a temperature say 100^oC and the two ends kept at 0^oC, then two points A and B equidistant from H on either side will be at the same temperature. Due to positive I homson effect point H is at higher potential than the points L and L. Now if a current is sent in copper wire LL we observe that heat is absorbed in the portion LH and evolved in the portion HL. Therefore, the point B will be at higher temperature than the point A due to the positive Thomson effect.

Negative Thomson Effect: For metals like Bismuth. Cobalt, Platinum and Nickel, portion at higher temperature possesses lower potential than a portion at lower temperature. Heat energy is therefore absorbed when the current flows from a point at higher temperature to a point at a lower temperature.

Orection of Current

Consider an iron wire for which Thomson effect is negative. Let this wire LL be heated at its mid point H to a temperature of 100^oC and the ends kept at 0^oC. then the two points A and B equidistant from point H on either side will be at the same temperature. Due to negative Thomson effect point H is at lower potential than point L and L. If a current flows along LL, as shown in then it is seen that heat is evolved in the portion LH because the current flows from higher potential to lower one and heated is absorbed in the portion HL because the current is to be driven from lower potential to higher one hence the point B is at lower temperature than the point A due to negative Thomson effect.

In case of lead, Thomson effect is not present,

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