Sec. 2-1] 



TEMPERA T URE TRA NSD UCERS 



[(>!> 



The output voltage of the thermoelement is proportional to the 

 temperature difference A/ between the two junctions only within a 

 narrow range of temperature. The output voltage E , as a function 

 of the temperature, follows more accurately the expression 



E„ 



At + \Bt 2 



&* 



(1) 



where / is the temperature of one junction, the other junction being 

 kept at a temperature of 0°C. The constants A, B, and C depend 

 upon the materials of which the thermoelement is made; for values 

 of A, B, and C see the International Critical Tables. AlsoEq. (1) can 

 be considered only as an approximation which is correct within a 

 limited temperature range (for instance, for the platinum-platinum 

 90-rhodium 10 element only between 630.5 and 1063°C). All 

 thermal emfs are reduced to zero in the vicinity of the absolute zero 

 point. 



The sensitivity (also frequently referred to as thermoelectric 

 power Q) of a thermoelement is 



S 



dE 

 ~dt 



A + Bt + Ct 2 



and is usually expressed in microvolts per degree centigrade. The 

 magnitude of S depends upon the chemical composition and the 



Table 10. Thermoelectric Sensitivity S = dEjdt of Thermoelement 

 Made of Materials Listed against Platinum, /*V/°C 

 (Reference junction kept at a temperature of Q C) 



physical treatment of the materials used and varies with tempera- 

 ture. Values given in standard tables vary considerably Approxi- 

 mate values of ft for thermoelements made from different materials 

 against platinum in the vicinity of 0°C are given in Table 10. The 

 output voltage from a thermoelement made from two metals listed 



