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INDEX
The image above shows a thermocouple, it means the association of two different metals.
Experiment
The experiment consists in imposing a difference of temperature for 64 serial thermocouple and observing the tension generated.
The heat source is a thermostat, the cold source is a becher with water and ice mixed in order to have a stable temperature near O deg celsius. We take the measure of the heat source with a presision thermometer and check that cold source remain stable. We note the difference of temperature measured and the tension generated.
click to enlarge.
The next table presents the result of the experiment :
It is noticed whereas the tension obtained is proportional to the difference in temperature (the shift of 12mV can be explained by the effects of junctions between the sources). It correlation is called "Seebeck effect".
where U is the potential difference (in V), S is the coefficient of Seebeck (in V.K -1) and dT the difference in temperature (in K).
This formula remains valid as long as dT remains weak. When that becomes larger, one can approach the potential difference as being a polynom funtion of the difference in temperature. The coefficient of Seebeck (S(1,2)) of the unit depends only on the nature of materials employed.
Moreover :
where S(1) and S(2) are the coefficients of respective Seebeck of materials 1 and 2. The order of magnitude of size of the coefficient of Seebeck is 10 µV for a number of metal couples, the mV for high performance semiconductor couples.
In order to obtain an exploitable tension, one thus associates a great number of thermocouples in series electrically, but thermically in parallel.
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