164 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 2 



Copper elect rodes- 



practical purposes, eliminate the influence of the lead- wire resistance 

 upon the temperature measurements. 



For literature references, see Smith, op. cit., p. 350, and H. Kanter, Arch. 

 tech.Messen, Z 119-3, July, 1955; Z 119-4, August, 1955; Z 119-5, September, 

 1955; Z 119-6, December, 1956. 



c. Electrolytic-resistance. Temperature Transducers. The electric 

 resistance of electrolytes decreases with increased temperature. The 

 resistance variation is, in general, of the order of —2 per cent/°C but 



can reach values as high as 8.9 per cent/°C 

 (for a 42.7 per cent sodium hydroxide solu- 

 tion). Polarization, the development of 

 gas, and instability are likely to cause 

 difficulties. The use of a-c circuits is 

 recommended in order to reduce polari- 

 zation. 



A stable electrolytic sensing element 

 that can be used for d-c operation is de- 

 scribed by Craig. 1 It consists of a solution 

 of cuprous chloride, hydrochloric acid, 

 and ethyl alcohol in a capillary tube with 1 mm bore. The electrodes 

 are made of copper. The resistance-temperature coefficient is in 

 the order of — 2 per cent/°C. Temperature measurements obtained 

 with this transducer are accurate to within 1°C. Figure (2-1)8 



Fig. (2-1)8. Electrolytic tem- 

 perature transducer. 



Table 9. Resistivity and Resistance -temperature Coefficient 



of Electrolytes at 25°C 



shows an illustration of the electrolytic temperature transducer of 

 Craig. Table 9 gives values of resistance-temperature coefficients of 

 some electrolytes. 



d. Ionized Gases. The electrical conductivity of a gas depends 

 upon the degree of ionization, which, in turn, depends upon the 



1 D. N. Craig, J. Research Natl. Bur. Standards, 21, 225 (1938). 



