Sec. 2-1] 



TEM I'ERATURi: T /.'. I NS I XJCERS 



161 



and sintering the mixture at high temperature into a solid mass. 

 Electrodes are applied by firing on metal colloids, by applying metal 

 pastes or paints, or by shrinking the material around wires. Typical 

 forms of thermistors are shown in Fig. (2-1)5. Depending upon the 

 composition, the resistivity of thermistor materials can have values 

 ranging from 10 _1 to 10 9 ohm-cm, but it is usually in the range 



% 



o 



Fig. (2-1)5. Typical thermistors. 



between 10 2 and 10 6 ohm-cm. Small admixtures of impurities or 

 physical alterations caused by heat treatment can result in a per- 

 manent change of the resistivity by large factors. The resistivity 

 decreases, in general, with the amount of impurities present. Figure 

 (2-1)6 shows the variation of the resistivity with temperature of two 

 different thermistor materials, and also, for comparison, of platinum. 

 Between and 300°C the resist- 

 ance of the thermistors decreases 

 by a factor of about 1,000, while 

 the resistance of platinum in- 

 creases in the same range by a 

 factor of 2. 



The resistivity variation fol- 

 lows approximately the empirical 

 equation 



9 (Blt-Blt ) 



Pt = Po e 



-200 -100 



300 400 



100 200 

 Temperature, °C 



Fig. (2-1)6. Resistivity of platinum 

 and of two thermistor materials versus 

 temperature. 



where p t is the resistivity at the 

 temperature t, and p is the resis- 

 tivity at the temperature t (t and 

 t are expressed in degrees Kelvin) . 

 B is a constant which varies for 



different materials and is usually in the order of 4,000. A more 

 precise expression for the resistivity is given by Becker, Green, 

 and Pearson. 1 



1 J. A. Becker, C. B. Green, and G. L. Pearson, Trans. AIEE, 65, 3 (Novem- 

 ber, 1949); see also O. J. M. Smith, Rev. Sci. Instr., 21, 344 (1950). 



