SEEBECK'S DISCOVERY. 259 



CHAPTER IV. 

 THERMOELECTRIC CURRENTS. 



271. SEEBECK'S DISCOVERY. We have seen that a closed circuit 

 consisting of several metals at the same temperature, cannot give rise 

 to a current ; but this law no longer holds if the different parts of 

 the circuit, and particularly the solderings of the metals, are not 

 at the same temperature. The circuit is then traversed by what is 

 called a thermoelectrical current. 



This important discovery was made by Seebeck in 1821. 



In a circuit formed of a bar of bismuth, the ends of which are 

 joined by a strip of copper, the current goes from the bismuth to the 

 copper through the heated soldering ; the copper is then said to be 

 negative to the bismuth. With a couple antimony-copper, the current 

 is reversed it goes from copper to antimony through the heated 

 junction; the antimony is accordingly negative in reference to 

 copper. 



It is natural to suppose that the metals could be classed in a 

 regular series based on this new property, and that antimony, which 

 is negative to copper, is much more negative to bismuth. This, in 

 fact, is what experiment shows, and the electromotive force for the 

 same temperature at the junctions, is greater with the couple 

 bismuth-antimony, than with either of the two couples bismuth- 

 copper or copper-antimony. 



The electromotive force of a thermoelectrical couple may 

 be obtained by breaking the circuit at a point outside the 

 junctions, and determining the difference of potential at the two 

 ends. 



In a circuit consisting of a single homogeneous metal, it is 

 impossible to set up an electrical current by variations of tempera- 

 ture, whatever may be the shape and section of the conductors near 

 the heated points. Currents might, however, be produced if the 



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