PROPERTIES AND USES OF THERMISTORS 



101 



The percentage change in resistance over its initial value is plotted versus 

 the logarithm of the time in the aging oven. It is to be noted that most of 

 the aging takes place in the first day or week. If these discs were preaged 

 for a week or a month and the subsequent change in resistance referred to 

 the resistance after preaging, they would age only about 0.2 per cent in one 

 year. In a thermistor thermometer, this change in resistance would cor- 

 respond to a temperature change of 0.05 centigrade degree. Thermistors 

 mounted in an evacuated tube or coated with a thin layer of glass age even 

 less than those shown in the figure. For some applications such high 

 stability is not essential and it is not necessary to give the thermistors special 

 treatment. 



KD' 10^ lO'' 10^ 



TIME IN HOURS AT 105° C. 



Fig. 15. — Aging characteristics of thermistors made of Materials No. 1 and No. 2 

 aged in an oven at rG5°C. Per cent increase in resistance over its initial value versus 

 time on a logarithmic scale. 



Thermistors have been used at higher temperatures with satisfactory aging 

 characteristics. Extruded rods of Material No. 1 have been tested for stab- 

 ility by treating them for two months at a temperature of 300 degrees 

 centigrade. Typical units aged from 0.5 to 1.5 per cent of their initial 

 resistance. Similar thermistors have been exposed alternately to tempera- 

 tures of 300 degrees centigrade and —75 degrees centigrade for a total of 

 700 temperature cycles, each lasting one-half hour. The resistance of typ- 

 ical units changed by less than one per cent. 



In some applications of thermistors very small changes in temperature 

 produce small changes in potential across the thermistor which then are 

 amplified in high gain amplifiers. If at the same time the resistance is 

 fluctuating randomly by as little as one part in a million, the potential 

 across the thermistor will also fluctuate by a magnitude which will be 



