148 



LAWRENCE R. PROUTY AND JAMES D. HARDY 



Temperature of the cold junction as well as that of the warm junc- 

 tion affects the thermoelectric force developed. If the thermocouple 

 is calibrated with the cold junction at to°C. and is used in an experi- 

 ment at ^i°C., it may be necessary to add the value (^i — to)k to the 

 observed temperature. The constant, k, varies with the bimetallic 

 junction used and the hot and cold junction temperatures. For- 

 tunately, k (sloi)e of the nearly straight portion of the curve in Fig. 

 2) is 1.0 for differences of to 50°C. between copper-constantan junc- 

 tions, to 100°C. between iron-constantan and chromel-alumel 

 junctions. Thus, a nearly linear relationship exists between tempera- 

 ture and electromotive force for these junctions in the ranges given 

 and the observed electromotive force can be considered as proportional 

 to the A< of hot and cold junctions without use of correction factors. 

 For precise measurements, even over a narrow range, the parabolic 

 nature of the curve must be taken into consideration. 



Thermocouples may be connected in either series or parallel as 

 shown in Figure 5. Greater sensitivity can be obtained "from thermo- 

 couples connected in series (thermopile) as the total voltage increases 

 in proportion to the number of thermocouples. For certain purposes, 

 parallel connection is satisfactory since the voltage produced by the 

 thermopile is then approximately the average of that of the individual 

 components. 



r — -1 r--i 

 /111 



r—i 



^^^ 



Fig. 5. Two thermocouple circuits. Le/<, series circuit ; 77 g/i/, parallel 

 circuit. Cold junction on right of each circuit, warm junctions on left. 



