198 BELL SYSTEM TECHNICAL JOURNAL 



with other thermistors designed for both higher and lower temperature 

 operation. In the lower temperature applications, typical thermistors 

 maintained their calibrations within a few tenths of a centigrade degree. 

 In general, electron tube control circuits dissipate less power in the ther- 

 mistor than relay circuits do. This results in less temperature rise in the 

 thermistor and leads to a more accurate control. While the average value 

 of this temperature rise can be allowed for in the design, the variations 

 in different installations require individual calibration to correct the errors 

 if they are large. The corrections may be different as a result of variations 

 of the thermal conductivity of the surrounding media from time to time or 

 from one installation to another. The greater the power dissipated in the 

 thermistor the greater the absolute error in the control temperature for a 

 given change in thermal conductivity. This follows from the relation 



^T = W/C (22) 



where AT is the temperature rise, W is the power dissipated and C is the dis- 

 sipation constant which depends on thermal coupling to the surroundings. 

 For the same reason, the temperature indicated by a resistance thermometer 

 immersed in an agitated medium will depend on the rate of flow if the tem- 

 perature sensitive element is operated several degrees hotter than its sur- 

 roundings. 



The design of a thermistor for a ventilating duct thermostat might pro- 

 ceed as follows as far as temperature rise is concerned : 



1 . Determine the power dissipation. This depends upon the circuit 

 selected and the required overall sensitivity. 



2. Estimate the permissible temperature rise of the thermistor, set by the 

 expected variation in air speed and the required temperature control accur- 

 acy. 



3. Solve Equation (22) for the dissipation constant and select a thermistor 

 of appropriate design and size for this constant in the nominal air speed. 

 Where more than one style of thermistor is available, the required time 

 constant will determine the choice. 



Compensators 



It is a natural and obvious application of thermistors to use them to com- 

 pensate for changes in resistance of electrical circuits caused by ambient 

 temperature variations. A simple example is the compensation of a copper 

 wire line, the resistance of which increases approximately 0.4 per cent per 

 centigrade degree. A thermistor having approximately one-tenth the 

 resistance of the copper, with a temperature coefficient of —4 per cent per 

 centigrade degree placed in series with the line and subjected to the same 

 ambient temperature, would serve to compensate it over a narrow tempera- 



