TEMPERATURE DETERMINATIONS 



147 



shown in Figure 4, leads to the rotary selector switch and the common 

 lead to the potentioinetric circuit are of copper (represented by solid 

 lines). The constantan leads (dotted lines) are shared by the refer- 

 ence junction and all other junctions to be read against it. It is 

 advisable to use a second reference junction inserted into the same 

 vacuum bottle with the first and provided with a switch so that they 



Outside Inside 

 shell shell 



junctions junctions 



-°^ ~"9 Air in junction 



-<^~~JAir out junction 



^''^'^ Room air junction 



„ - T C\UUM1 Ull JUlll-llU 



l' o<^~~j Animal junction 



\ ' o^'~-* Animal junction 



0'^'"'!' Animal junction 



(^'"j Animal junction 



Standard junctions 



Fig. 4. Potentiometric circuit for thermocouples without standard cell. 



ma}^ be read against each other to determine the zero point of the 

 galvanometer. This will eliminate the effects of extraneous voltages 

 from therm oj unctions that inadvertently occur in the potentiometric 

 circuit. One of the reference junctions is then used with the other 

 thermocouples. 



3. Calibration of Thermocouples 



Tables published by the National Bureau of Standards are avail- 

 able relating the temperature difference between junctions of the 

 most commonly used thermocouple metals and the voltage produced. 

 If thermocouple wire has been purchased from a standard supply 

 house (Driver-Harris or Leeds and Northrup), the tables published 

 can be used as a satisfactory calibration. If wire from an unknown 

 source is used, the thermocouples should be calibrated by comparison 

 with a mercury thermometer, keeping one junction at constant tem- 

 perature, e.g., in an ice bath. 



