196 BELL SYSTEM TECHNICAL JOURNAL 



10"^° amperes per millimeter per meter will readily indicate a temperature 

 change of 0.0005 centigrade degree. For comparison a precision platinum 

 resistance thermometer and the required special bridge such as the Mueller 

 will indicate a minimum change of 0.003 centigrade degree with a similar 

 galvanometer. 



Several thermistors which have been used for thermometry are shown in 

 Fig. 17. Included in the group are types which are suited to such diverse 

 applications as intravenous blood thermometry and supercharger rotor 

 temperature measurement. In Fig. 17, A is a tiny bead with a response 

 time of less than a second in air. B is a probe type unit for use in air streams 

 or liquids. C is a meteorological thermometer used in automatic radio 

 transmission of weather data from free balloons. D is a rod shaped imit. 

 E is a disc or pellet, adapted for use in a metal thermometer bulb. Discs 

 like the one shown have been sweated to metal plates to give a low thermal 

 impedance connection to the object whose temperature is to be determined. 

 F is a large disc with an enveloping paint finish for use in humid surroimd- 

 ings. The characteristics of these types are given in Table III. 



The temperature of objects which are inaccessible, in motion, or too hot 

 for contact thermometry can be determined by permitting radiation from 

 the object to be focussed on a suitable thermistor by means of an elliptical 

 mirror. Such a thermistor may take the form of a thin flake attached to a 

 solid support. Its advantages compared with the thermopile and resistance 

 bolometer are its more favorable resistance value, its ruggedness, and its 

 high temperature coefficient of resistance. It can be made small to reduce 

 its heat capacity so as rapidly to follow changing temperatures. Flake 

 thermistors have been made with time constants from one millisecond to 

 one second. Since the amount of radiant power falling on the thermistor 

 may be quite small, sensitive meters or vacuum tube amplifiers are required 

 to measure the small changes in the flake resistance. Where rapidly vary- 

 ing temperatures are not involved, thermistors with longer time constants 

 and simpler circuit equipments can be utilized. 



Temperature Control 



The use of thermistors for temperature control purposes is related closely 

 to their application as temperature measuring devices. In the ideal tem- 

 perature sensitive control element, sensitivity to temperature change should 

 be high and the resistance value at the control temperature should be the 

 proper value for the control circuit used. Also the temperature rise of the 

 control element due to circuit heating should be low, and the stability of 

 calibration should be good. The size and shape of the sensitive element are 

 dictated by several factors such as the space available, the required speed 

 of response to temperature changes and the amount of power which must 



