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where  is the resistivity of the sample. The first term is the irreversible Joule heat. The second term is due to Thomson emf. |
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In metals such as copper and zinc, the hotter end is at a higher potential (as shown in the figure above). In such a situation if the current due to an external supply is in the same direction as the direction of decreasing potential, there is additional evolution of heat due to Thomson effect and the net heat produced is more than the Joule heat. If the direction of the current is reversed, heat energy is converted to electrical energy due to Thomson effect and the rate production of heat is reduced. This is known as positive Thomson effect . |
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An anomalous situation occurs in metals such as cobalt and iron. In these metals the hotter end is at a lower potential so that charge carriers move against the thermal gradient. The effect is opposite of what happens in case of positive Thomson effect. Such anomalous effect is known as negative thomson effect . Lead shows zero Thomson effect. The simple physical picture given above cannot explain the strange behaviour. |
