162 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 2 



Table 8 (taken from the same publication) shows values of the 

 resistance R, the temperature coefficient a, and the constant B for 

 a typical rod-shaped thermistor element. 



Table 8. Characteristic Values of a Thermistor at Different 



Temperatures 



15 



10 



The voltage-current characteristics of thermistors are linear up to 

 the range where the current causes a significant increase of the ther- 

 mistor temperature. For higher currents, the characteristics become 

 nonlinear, as shown in Fig. (2-1)7. The point at which the voltage- 

 current characteristics start deviating from linearity depends upon 



the dimensions of the thermistor 

 and the environmental conditions 

 (thermal contact). The power dissi- 

 pated before the resistance de- 

 creases by as much as 1 per cent is 

 about 1 to 10 mW/cm 2 of surface 

 in still air. 



The time constant of a thermistor 

 (i.e., the time in which a thermistor 

 reaches 07 per cent of its steady- 

 state resistance value after being 

 exposed to a new temperature) 

 varies for commercial thermistors 

 between 1 and 200 sec, although occasionally figures in the milli- 

 second range are found. For a study on the dynamic characteristics 

 of thermistors, see Smith. 1 



The range of temperatures in which thermistors have been used 

 extends from —100 to +300°C, although for high-stability require- 

 ments an upper limit of 100 to 150°C is frequently recommended. 

 1 Smith, ibid., p. 351. 





Milliamperes 



Fig. (2-1)7. Voltage-current charac- 

 teristic of a thermistor. 



