PROPERTIES AND USES OF THERMISTORS 171 



temperature compensators and temperature control devices; 2) special 

 switching devices witiiout moving contacts; 3) regulators or volume limiters; 

 4) pressure gauges, flowmeters, and simple meters for measuring thermal 

 conductivity in liquids and gases; 5) time delay and surge suppressors; 6) 

 special oscillators, modulators and amplifiers for relatively low frequencies. 

 Before these uses are discussed in detail it is desirable to present the physical 

 principles which determine the properties of thermistors. 



The question naturally arises "why have devices of this kind come into 

 use only recently?" The answer is that thermistors are made of semi- 

 conductors and that the resistance of these can vary by factors up to a 

 thousand or a million with surprisingly small amounts of certain impurities, 

 with heat treatment, methods of making contact and with the treatment 

 during life or use. Consequently the potential application of semiconduc- 

 tors was discouraged by experiences such as the following: two or more 

 units made by what appeared to be the same process would show large 

 variations in their properties. Even the same unit might change its re- 

 sistance by factors of two to ten by exposure to moderate temperatures or 

 to the passage of current. Before semiconductors could seriously be con- 

 sidered in industrial applications, it was necessary to devote a large amount 

 of research and development efifort to a study of the nature of the conduc- 

 tivity in semiconductors, and of the effect of impurities and heat treatment 

 on this conductivity, and to methods of making reliable and permanent 

 contacts to semiconductors. Even though Faraday discovered that the 

 resistance of silver sulphide changed rapidly with temperature, and even 

 i though thousands of other semiconductors have been found to have large 

 \ negative temperature coefficients of resistance, it has taken about a century 

 i of effort in physics and chemistry to give the engineering profession this 

 j new tool which may have an influence similar to that of the vacuum tube 

 I and may replace vacuum tubes in many instances. 



If thermistors are to be generally useful in industry: 

 ! 1) it should be possible to reproduce units having the same character- 

 istics; 

 I 2) it should be possible to maintain constant characteristics during use; 

 the contact should be permanent and the unit should be chemically 

 inert ; 



3) the units should be mechanically rugged; 



4) the technique should be such that the material can be formed into 

 various shapes and sizes; 



5) it should be possible to cover a wide range of resistance, temperature 

 coefficient and power dissipation. 



Thermistors might be made by any method by which a semiconductor 



