Sec. 2-1] TEMPERATURE TRANSDUCERS 167 



composition, it can have a negative or positive temperature 

 coefficient. Three of the specimens presented in this paper have 

 the following properties: 



a. Dielectric constant at room temperature, 46; temperature co- 

 efficient, — 0.0003/°C. 



b. Dielectric constant at room temperature, 490; temperature 

 coefficient, — 0.003/°C. 



c. Dielectric constant at room temperature, 18; temperature co- 

 efficient, +0.0001/°C. 



In the temperature range from —40 to -f-160°C, the dielectric con- 

 stant changes linearly with the temperature. 



Some materials (e.g., some barium titanates in the vicinity of the 

 curie point) exhibit very large changes of dielectric constant with 

 temperature, but show a large hysteresis of the dielectric constant 

 versus the temperature function which limits their application to 

 temperature-sensing elements. Other materials, such as polyvinyl 

 alcohol, show a variation of the dielectric constant from 8 to 500 in 

 a temperature range from 25 to 85°C as measured at 60 cps. However, 

 these materials also show an increase of dielectric losses with tem- 

 peratures of such magnitude as to make their application for tem- 

 perature measurements impractical. 



For numerical values of the temperature constant of some dielectrical mate- 

 rials, see Dwight E. Gray (ed.), "American Institute of Physics Handbook," 

 sec. 5d, McGraw-Hill Book Company, Inc., New York, 1957. 



2-14. Thermoelectric Transducers (Thermoelements or 

 Thermocouples) 



The contact potential between two dissimilar metals varies with 

 the temperature of their junction. If two dissimilar metals A and B 

 are joined together as shown in Fig. 

 (2-1)11, and if their junction points P x 

 and P 2 are kept at the temperature t x 

 and t 2 , respectively, an electromotive 

 force (emf ) arises which causes a current 

 in the circuit. The thermal emf is to a \ / \l 



first approximation a linear function of YpHw t -J %P~^ 



the temperature difference At = t 2 — t x | 



and is independent of the temperature ^ ,„,»., ™ 



, .. r i. n i Fig. (2-1)11. Thermoelements, 



and the temperature gradient of the basic system. 



wires themselves. The thermal emf can 



be used, therefore, to measure the temperature difference At between 



the two contact points P x and P 2 . The order of magnitude of the 



