In 1856 Thomson found that heat energy is absorbed or evolved not only at junctions when a current flows through a thermocouple but evolution or absorption of heat takes place even in an unequally heated conductor. Thomson effect can be started 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.
Fig.1. Positive Thomson Effect
Positive Thomson Effect
For metals like copper, silver, zinc, Antimony and cadmium etc., potential at high temperature is considered at higher potential than a portion at lower temperature. Hence heat heat energy is absorbed when current flows from a point at lower temperature to a point at higher temperature.
Let a copper wire LL’ be heated at its middle point H to a temperature, say C, and two ends kept at C, then two points A and B equidistant from H on either side will be at the same temperature. Due to positive Thomson effect the point H is at higher potential than the point 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.
Consider an iron wire for which Thomson effect is negative. Let this wire LL’ be heated at its mid point H to a temperature off C, and the two ends kept at C, than 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 Fig.2 then it is seen that heat is evolved in the potential LH because the current flows from higher potential to lower one and heat is absorbed in the potential to higher one. Hence the point B is at lower temperature than the point A due to negative Thomson effect.
Fig.2. Negative Thomson Effect