thermoelectricity When two different metals are placed in contact, a contact potential develops. This can be explained by considering the fact that different metals have different conduction electron density. When two metals are brought in contact the electron density tries to equalize across the junction. Migration of electrons produces depletion of electron density in one and increase in the other. As a result potential gradient develops which stops further migration of electrons. The contact potential is approximately the difference between the work functions * for the two metals. Contact potentials are in the range of one to several electron volts. If a complete circuit is made using several metals, the contact potentials cancel and there is no net emf in the circuit. However contact potential vary with temperatures. Therefore when two junctions making a complete circuit are at different temperatures there is a net emf equal to the difference between the two emf’s. This is called Seeback effect. The emf called thermo emf is ~ 10^-6 V per degree temperature difference.

The converse of Seeback effect is called Peltier effect. A current through a bimetallic junction produces either cooling or heating depending upon the direction of the current.

A temperature gradient in a single metal strip generates an emf that is generally from the lower to the higher temperature. This is called Thomson effect. It occurs because the kinetic energy of the free electrons in a metal depends upon the temperature. The electrons with higher kinetic energy will diffuse faster than those with smaller kinetic energy. The density gradient of the free electrons produces a counter emf stopping the further flow of electrons.

Thermocouple: A typical graph showing the variation of thermo emf with temperature difference between the junctions is shown in fig. t3. The linearly varying portion of the curve can be used to measure temperature by maintaining one of the junctions at a constant known temperature and using the other as a probe. Joining an intermediate metal into the circuit does not alter the emf, provided that the points where the intermediate metal is joined are at the same temperature. Copper- constantum couple is useful in the range 200^oC to 400^oC. For higher temperatures (up to 1700^oC) platinum rhodium against platinum is used. An accurately calibrated millivoltmeter can read the temperature directly. For more accurate work a potentiometer and standard cell should be used. The junctions have very low heat capacity. They therefore attain the required temperature rapidly and without altering the experimental condition. This is a major advantage of using thermocouple as thermometer.