Sec. 2-1] TEMPERATURE TRANSDUCERS 163 



Gruess 1 has used thermistors made of sintered aluminum oxide at 

 temperatures between 800 and 1100°C. The transducers have a life- 

 time of several months. 



Carbon resistors have been used for the measurement of very low 

 temperatures (below 20°K) either in the form of carbon strips con- 

 sisting of a thin conductive layer of carbon granules held with a 

 binder and mounted on the object to be measured 2 or in the form of 

 commercial radio resistors of about 2 to 150 ohms. 3 Both types 

 exhibit a large increase of resistance in the range from 20 down to 

 almost 0.1 5°K. The useful range of the carbon-strip resistor may be 

 varied by varying the size of the carbon granules. The radio resistor 

 is, in general, more stable than the carbon strip; in the range from 

 20 to 1°K the resistance value is reproducible within about 0.2 per 

 cent. Both types are relatively insensitive to the influence of mag- 

 netic fields. They should be sealed off from exposure to liquid helium. 



Most thermistors of the oxide type exhibit an aging effect, i.e., an 

 increase of resistance with time. About 0.5 to 1.5 per cent of the 

 resistance variation takes place during the first day or week of its 

 use, but changes of as much as 1 per cent may still occur after several 

 months. Preaging (exposing the thermistor to a temperature slightly 

 higher than that to which it is to be subjected) is recommended 

 where high stability is required; preaged thermistors may vary by 

 as little as 0.2 per cent/year. Thermistors with pure electronic con- 

 duction exhibit less variation with time than such with ionic or 

 mixed conduction. Enclosure of the thermistor in glass or other 

 suitable materials to exclude chemical alterations diminishes the 

 aging effect. 



The advantages of thermistors over metal resistance ther- 

 mometers are, besides their high temperature coefficients, their rela- 

 tively small size, which makes localized temperature measurements 

 possible. Thermistor beads ranging in diameter from 0.006 to 0.1 in. 

 are commercially available. Because of their low specific heat, ther- 

 mistors draw practically no heat from the object to be measured. 

 Temperature differentials as low as 0.001°F have been measured 

 with thermistors. Besides this, thermistors are physically rugged 

 and have large resistance values which permit adequate impedance 

 matching with the associated electronic equipment and, for many 



1 H. Gruess, Arch. tech. Messen, J 221-2, December, 1937. 



2 W. F. Giauque, J. W. Stout, and C. W. Clark, J. Am. Chem. Soc, 60, 1053 

 (1938); T. H. Geballe et al., Rev. Sci. Instr., 23, 489 (1952). 



3 J. R. Clement and E. H. Quinnel, Rev. Sci. Instr., 23, 213 (1952), and 

 J. R. Clement et al., Rev. Sci. Instr., 24, 545 (1953). 



