A thermopile is a sensitive instrument used for the detection of heat radiations and measurement of their intensity. It is based on Seebeck Effect. It is a series arrangement of similar thermocouples. If there are n thermocouples in series, the net emf in the circuit is n times the emf produced by a single thermocouple. To reduce the overall resistance, thick bars of two metals A and B are used. Since antimony-bismuth thermocouple gives quite a large thermo emf, a thermopile is formed by alternately connecting antimony and bismuth bars. The free ends of the thermopile are connected to a galvanometer whose resistance is made equal to the resistance of the thermopile. One set of junctions on one side is coated with lampblack, whereas the other set of junctions on the other side is well polished. The arrangement is enclosed in a funnel-shaped vessel. Heat radiations coming from the side of funnel shape are made to fall on a set of Junctions coated with lampblack. Their radiations are absorbed and hence the temperature of this set of junctions is raised. The other set of junctions, being polished, reflect these radiations and remain cold. Due to this, thermo e.m.f. is developed in the thermopile and thermoelectric current so produced is measured by a galvanometer. Thus, the deflection in the galvanometer directly measures the intensity of heat radiations. The instrument is so sensitive that it can detect heat radiations from a match stick lighted at a distance of 50 metres from the thermopile.
Thermo Electric Refrigerator:
According to Peltier Effect, when current is passed through two junctions that are at a different temperature, then heat is absorbed at one junction and is evolved at the other junction. If on the whole, the heat is absorbed, then the thermocouple acts as a thermoelectric refrigerator. Such a refrigerator have no motors. The efficiency of thermoelectric refrigerators is small as compared to the efficiency of conventional refrigerators. If the region to be cooled is small and noise is not acceptable, then a thermoelectric refrigerator will be the most ideal choice.
The thermocouple selected for the use of thermoelectric refrigerators should have following properties-
- Joule heating should be minimum possible.
- The thermal conductivity should be low so that a large temperature difference must be maintained between the two junctions.
- The thermoelectric power should be large.