200 BELL SYSTEM TECHMCAL JOURI^AL 



total resistance of the circuit has been increased only 6.1 per cent at the 

 upper temperature limit by the addition of a compensator. This increase 

 is small because of the high temperature coefficient of the compensating 

 thermistor. The characteristics of such a thermistor are so stable that the 

 resistance would remain constant within less than one per cent for ten years 

 if maintained at any temperature up to about 100 degrees centigrade. 

 Figure 15 shows aging characteristics for typical thermistors suitable for 

 use in compensators. These curves include the change which occurs during 

 the seasoning period of several days at the factory, so that the aging in use 

 is a fraction of the total shown. 



In many circuits which need to function to close tolerances under wide 

 ambient temperature variation, the values of one or more circuit elements 

 may var>' undesirably with temperature. Frequently the resultant overall 

 variation with temperature can be reduced by the insertion of a simple ther- 

 mistor placed at an appropriate point in the circuit. This is particularly 

 true if the circuit contains vacuum tube amplifiers. In this manner fre- 

 quency and gain shifts in communications circuits have been cancelled and 

 temperature errors prevented in the operation of devices such as electric 

 meters. The change in inductance of a coil due to the variation of magnetic 

 characteristics of the core material with temperature has been prevented by 

 partially saturating the coil with direct current, the magnitude of which is 

 directly controlled by the resistance of a thermistor imbedded in the core. 

 In this way the amount of d-c magnetic flux is adjusted by the thermistor 

 so that the inductance of the coil is independent of temperature. 



In designing a compensator, care must be taken to ensure exposure of the 

 thermistor to the temperature affecting the element to be compensated. 

 Power dissipation in the thermistor must be considered and either limited to 

 a value which will not produce a significant rise in temperature above am- 

 bient, or offset in the design. 



Volt-Ampere Characteristics 



The nonlinear shape of the static characteristic relating voltage, current, 

 resistance and power for a typical thermistor was illustrated by Fig. 9. 

 The part of the curve to the right of the voltage maximum has a negative 

 slope, applicable in a large number of ways in electric circuits. The par- 

 ticular characteristic showTi begins with a resistance of approximately 50,000 

 ohms at low power. Additional power dissipation raises the temperature 

 of the thermistor element and decreases its resistance. At the voltage 

 maximum the resistance is reduced to about one-third its cold value, or 

 17,000 ohms, and the dissipation is 13 milliwatts. The resistance becomes 

 approximately 300 ohms when the dissipation is 100 milliwatts. Such 

 resistance-power characteristics have resulted in the use of thermistors as 

 sensitive power measuring devices, and as automatically variable resistances 



