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BELL SYSTEM TECHNICAL JOURNAL 



tors adapt them to a wide variety of temperature measuring applications. 

 VV^hen designed for this service, thermistor thermometers have long-time 

 stability which is good for temperatures up to 300 degrees centigrade and 

 excellent for more moderate temperatures. A well aged thermistor used 

 in precision temperature measurements was found to be within 0.01 centi- 

 grade degree of its calibration after two months use at various temperatures 

 up to 100 degrees centigrade. As development proceeds, the stability of 

 thermistor thermometers may be expected to approach that of precision 

 platinum thermometers. Conventional bridge or other resistance measuring 

 circuits are commonly employed with thermistors. As with any resistance 

 thermometer, consideration must be given to keeping the measuring current 

 sufficiently small so that it produces no appreciable heating in order that the 



Dissipation constant in still air, approx 4 milliwatts/C. deg. 



Thermal time constant in still air, approx 70 seconds 



Dimensions of thermistor, diameter approx 0.11 inch 



length approx 0. 54 inch 



thermistor resistance shall be dependent upon the ambient temperature 

 alone. 



Since thermistors are readily designed for higher resistance values than 

 metallic resistance thermometers or thermocouples, lead resistances are 

 not ordinarily bothersome. Hence the temperature sensitive element can 

 be located remotely from its associated measuring circuit. This permits 

 great flexibility in application, such as for instance wire line transmission 

 of temperature indications to control points. 



Table II gives the characteristics of a typical thermistor thermometer. 

 The dissipation constant is the ratio of the power input in watts dissipated 

 in the thermistor to the resultant temperature rise in centigrade degrees. 

 The time constant is the time required for the temperature of the thermistor 

 to change 63 per cent of the difference between its initial value and that of 

 the surroundings. As a sensitive thermometer, this thermistor with a 

 simple Wheatstone bridge and a galvanometer whose sensitivity is 2 X 



