The Thermopile

Radiation from a hot body may be detected by converting it into electrical energy.

In the diagram above a copper and iron wire are wound together to form a junction, with the free ends connected to a galvanometer. On warming the junction, perhaps by holding it, a voltage is produced and the galvanometer deflects. This is called the thermoelectric effect.

Bismuth and antimony display the thermoelectric effect very well and may be used to make a thermopile.

In order to magnify the effect, many pairs of antimony and bismuth bars are joined in series. The radiation falls on the set of junctions, and each junction produces a small voltage, which add to give a much larger voltage, since the pairs of bars are connected in series. The end bars are connected to a galvanometer and a voltage and current is produced.

The size of the voltage produced increases with the temperature, so the thermopile may be used to indicate temperature.

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