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BELL SYSTEM TECHNICAL JOURNAL 



This family of curves will be modified by changes in ambient temperature 

 and where rather precise time delays are required, the ambient temperature 

 must be controlled or compensated. 



Since thermistors cover a wide range in size, shape, and heat conductivity 

 of surrounding media, large variations in //, C, and t can be produced. 

 The time constant can be varied from about one millisecond to about ten 

 minutes or a millionfold. 



One very important property of a thermistor is its aging characteristic 

 or how constant the resistance at a given temperature stays with use. To 

 obtain a stable thermistor it is necessary to: 1) select only semiconductors 

 which are pure electronic conductors; 2) select those which do not change 

 chemically when exposed to the atmosphere at elevated temperatures; 



TIME IN SECONDS 



Fig. 14. — Current versus time curves for six values of the battery voltage in the circuit 

 shown in the insert. 



3) select one which is not sensitive to impurities likely to be encountered in 

 manufacture or in use; 4) treat it so that the degree of dispersion of the 

 critical impurities is in equilibrium or else that the approach to equilibrium 

 is very slow at operating temperatures; 5) make a contact which is intimate, 

 sticks tenaciously, has an expansion coefficient compatible with the semi- 

 conductor, and is durable in the atmospheres to which it will be exposed; 

 6) in some cases, enclose the thermistor in a thin coat of glass or material 

 impervious to gases and liquids, the coat having a suitable expansion coeffi- 

 cient; 7) preage the unit for several days or weeks at a temperature some- 

 what higher than that to which it will be subjected. By taking these pre- 

 cautions remarkably good stabilities can be attained. 



Figure 15 shows aging data taken on three-quarter inch diameter discs 

 of Materials No. 1 and No. 2 with silver contacts and soldered leads. These 

 discs were measured soon after production, were aged in an oven at 105 

 degrees centigrade and were periodically tested at 24 degrees centigrade. 



