A thermopile is a sensitive instrument used for detection of heat radiation and measurement of their intensity. It is based upon see beck effect.
A thermopile consists of a number of thermocouples of Sb –Bi all connected in series. One set of junctions on one side is coated with lamp black. The other set of junctions on the other side is well polished. The arrangement is enclosed in a funnel or horn shaped vassal. The two ends T1 and T2 of the thermopile are connected to a sensitive galvanometer.
Heat radiations coming from the funnel shaped side of the vessel are made to fall on the set of junctions coated with lampblack. These are absorbed and consequently the temperature of all the junctions in this set is raised. The other set of junctions being polished reflect these radiations and remain cold. Thermo is developed in each thermocouple. As thermocouples are connected in series, the thermoelectric current in each flow is in the same junction. The total current is therefore large and produces deflection in the galvanometer.
The deflection in the galvanometer indicates the existences of heat radiations. The deflection is calibrated to measure directly the intensity of heat radiations. This instrument is so sensitive that it can detect heat radiations from match stick lighted at a distance of 50 meters from the thermopile.
The working of thermo-electric refrigerator is based on pettier effect. According to pettier effect, if current is passed through a thermocouple heat is absorbed at one junction ands evolevedat the other junction of the thermocouple. If on the whole the heat is a boarded, the thermocouple acts as thermoelectric refrigerator. Such a refrigerator has no muter or compressor. Its efficiently is small in comparison to congenital refrigerator. Thermo electric refrigerator is very useful when the region to be cooled is very small and the noise is not acceptable.
The thermocouple selected for the use of thermoelectric refrigerator should have the following characteristics.
(i) Low resistivity so that the loss of energy in the form of joule heat is minimum.
(ii) Low thermal conductivity it will help in maintaining large temperature difference between the two junctions.
(iii) High thermo electric power.
The best material satisfying the above characteristics is the semi-conductor Bi2 Te3 (compound of bismuth and tellurium).
Thermocouple can be used to generate electric power using seek beck effect in remote areas. It can be achieved by heating one junction in a flame of kerosene oil lamp and keeping the other junction at room or atmospheric temperature. The thermo so developed is used to operate radio receivers or even radio transmitters.
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