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.
