174 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 2 



which is fed from a small d-c voltage source E A . The bridge becomes 

 unbalanced for any change of the temperature-sensitive resistor R t 

 and furnishes an output voltage which compensates for the change 

 of thermal emf occurring at the reference junction. 



A further means to reduce the effect of temperature variation of 

 the reference junction is to use thermoelements with curved charac- 

 teristics, as shown in Fig. (2-1)17. The platinum-platinum rhodium 



Fig. (2-1)16. Alternate circuit for 

 reference -temperature error compen- 

 sation. 



200 400 600 800 1,000 

 Temperature, °C 



Fig. (2-1)17. Thermoelements with 

 curved characteristics which eliminate 

 the error resulting from temperature 

 variation of the reference junction. 



thermoelement curve rises slowly in the vicinity of 0°C and faster at 

 higher temperature, thus leading to a reduction in the error produced 

 by the reference-temperature variation. The curves for the nickel 

 copper-nickel iron thermoelements show almost no increase of the 

 thermal emf between and 200°C. If the reference junction of such 

 a thermocouple is held within this range, any temperature variation 

 will cause a reading error of less than 2°C for a reference-temperature 

 variation of lOC^C. 1 



For measuring procedures on thermocouples, see Wm. F. Roeser and T. 

 Lonberger, Methods of Testing Thermocouples and Thermocouple Materials, 

 Natl. Bur. Standards Circ. 590, February, 1958. 



2-15. Noise Thermometer 



The electrons in a conductor at temperatures above the absolute 

 zero point are in constant random motion. This motion of the 



1 \V. Rohn, Z. Metallic., 16, 297 (1924); Kulbush and Kalinin, Prdzisions- 

 industrie (Moscow), 1933, nos. 3 and 4; referred to by G. Keinath, Arch. tech. 

 Messen, J 241-3, July, 1934. 



