PROPERTIES AND USES OF THERMISTORS 193 



PART II— USES OF THERMISTORS 



The thermistor, or thermally sensitive resistor, has probably excited more 

 interest as a major electric circuit element than any other except the vacuum 

 tube in the last decade. Its extreme versatility, small size and ruggedness 

 were responsible for its introduction in great numbers into communications 

 circuits within five years after its first appUcation in this field. The next 

 five year period spanned the war, and saw thermistors widely used in addi- 

 tional important applications. The more important of these uses ranged 

 from time delays and temperature controls to feed-back amplifier automatic 

 gain controls, speech volume limit ers and superhigh frequency power meters. 

 It is surprising that such versatility can result from a temperature dependent 

 resistance characteristic alone. However, this effect produces a very useful 

 nonlinear volt-ampere relationship. This, together with the ability to pro- 

 duce the sensitive element in a wide variety of shapes and sizes results in 

 applications in diverse fields. (The variables of design are many and inter- 

 related, including electrical, thermal and mechanical dimensions. 



The more important uses of thermistors as indication, control and cir- 

 cuit elements will be discussed, grouping the uses as they fall under the 

 primary characteristics: resistance-temperature, volt-ampere, and current- 

 time or d^mamic relations. 



Resistance-Temperature Relations 



It has been pointed out in Part I that the temperature coefficient of elec- 

 trical resistance of thermistors is negative and several times that of the or- 

 dinary metals at room temperature. In Thermistor Material No. 1, which 

 is commonly used, the coefficient at 25 degrees centigrade is —4.4 per cent 

 per centigrade degree, or over ten times that of copper, which is +0.39 per 

 cent per centigrade degree at the same temperature. A circuit element made 

 of this thermistor material has a resistance at zero degrees centigrade which 

 is nine times the resistance of the same element at 50 degrees centigrade. 

 For comparison, the resistance of a copper wire at 50 degrees centigrade 

 is 1.21 times its value at zero degrees centigrade. 



The resistance-temperature characteristics of thermistors suggest their 

 use as sensitive thermometers, as temperature actuated controls and as 

 compensators for the effects of varying ambient temperature on other ele- 

 ments in electric circuits. 



Thermometry 



The application of thermistors to temperature measurement follows the 

 usual principles of resistance thermometry. However, the large value of 

 temperature coefficient of thermistors permits a new order of sensitivity to 

 be obtained. This and the small size, simplicity and ruggedness of thermis- 



