Principle of Thermocouples
When two dissimilar metals such a iron and copper are gained to form a closed circuit, current flow when one junction is at higher temperature and the other one is at lower temperature as shown in the figure.
| Figure 36.1 |
: Principle of measurement of temperature by a thermocouple |
The emf driving the current is called a thermoelectric emf and the phenomenon is known as thermoelectric effect or Seeback effect.
Usually a thermoelectric emf is very small. A pair of dissimilar metals welded together at their junction forms what is called a thermocouple. When several thermocouples are arranged in series, the emf is added together to give an appreciable output, this arrangement is called thermopile as shown in the figure.
| Figure 36.2: |
Arrangement of thermocouple to form a thermopile |
When two dissimilar metals are joined together, the free electrons move randomly across the junction. Because of the different atomic structure of each metal, electrons pass more readily across the boundary in one direction than in other. This results in displacement of charges, making one metal positive and other negative.
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