Properties and Uses of Thermistors — Thermally 

 Sensitive Resistors ' 



By J. A. BECKER, C. B. GREEN and G. L. PEARSON 



A new circuit element and control device, the thermistor or thermally sensitive 

 resistor, is made of solid semiconducting materials whose resistance decreases 

 about four per cent per centigrade degree. The thermistor presents interesting 

 opportunities to the designer and engineer in many fields of technology for ac- 

 complishing tasks more simply, economically and better than with available 

 devices. Part I discusses the conduction mechanism in semiconductors and the 

 criteria for usefulness of circuit elements made from them. The fundamental 

 physical properties of thermistors, their construction, their static and dynamic 

 characteristics and general principles of operation are treated. 



Part II of this paper deals with the applications of thermistors. These include : 

 sensitive thermometers and temperature control elements, simple temperature 

 compensators, ultrahigh frequency' power meters, automatic gain controls for 

 transmission systems such as the Types K2 and LI carrier telephone systems, 

 voltage regulators, speech volume limiters, compressors and expandors, gas pres- 

 sure gauges and flowmeters, meters for thermal conductivity determination of 

 liquids, and contactless time delay devices. Thermistors with short time con- 

 stants have been used as sensitive bolometers and show promise as simple com- 

 pact audio-frequency oscillators, modulators and amplifiers. 



PART I— PROPERTIES OF THERMISTORS 



Introduction 



THERMISTORS, or thermsMy sensitive resistors, are devices made of 

 solids whose electrical resistance varies rapidly with temperature. 

 Even though they are only about 15 years old they have already found im- 

 portant and large scale uses in the telephone plant and in military equip- 

 ments. Some of these uses are as time delay devices, protective devices, 

 voltage regulators, regulators in carrier systems, speech volume limiters, 

 test equipment for ultra-high-frequency power, and detecting elements for 

 very small radiant power. In all these applications thermistors were 

 chosen because they are simple, small, rugged, liave a long life, and require 

 little maintenance. Because of these and other desirable properties, ther- 

 mistors promise to become new circuit elements which will be used exten- 

 sively in the fields of communications, radio, electrical and thermal 

 instrumentation, research in physics, chemistry and biology, and war tech- 

 nology. Specific types of uses which will be discussed in the second part 

 of this paper include: 1) simple, sensitive and fast responding thermometers, 



* Published in Elec. Engg., November 1946. 



The authors acknowledge their indebtedness to Messrs. J. H. Scaff and H. C. Theuercr 

 for furnishing samples for most of the curves in Fig. 4, and to Mr. G. K. Teal for the data 

 for the lowest curve in that figure. 



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