154 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 2 



between room temperature and the curie point, but seems potentially 

 a useful method for special purposes. 



Also the capacitive temperature transducer (2-13), which is based 

 upon the variation with temperature of the dielectric constant of 

 different materials, has found limited practical applications. How- 

 ever, considering the high sensitivity with which capacitance varia- 

 tions can be measured, it is potentially a useful system. 



Thermoelectric temperature transducers are described in 2-14. The 

 range of application of these transducers extends from about — 200 

 to about +2400°C, i.e., far into the range of optical pyrometry. The 

 platinum-platinum rhodium thermoelement is used for the definition 

 of the International Temperature Scale between 630.5 (zinc point) 

 and 1063°C (gold point). 



The noise thermometer (2-15) represents a promising method of 

 absolute electric temperature measurement over a wide temperature 

 range (100 to over 1500°K). However, the experimental difficulties 

 connected with this method at the present are considerable. 



There are, of course, indirect electrical methods of temperature 

 measurements, such as the combination of a bimetallic strip with a 

 displacement transducer. Such methods are not discussed in this 

 book. 



For general information on thermal transducers, see American Institute of 

 Physics, "Temperature, Its Measurement and Control in Science and In- 

 dustry," Reinhold Publishing Corporation, New York, 1955. 



2-11. Resistive Temperature Transducers (Resistance 

 Thermometer) 



A resistance thermometer consists of a resistive element exposed 

 to the temperature to be measured; the resistance varies with tem- 

 perature, and the resistance variation is detected in subsequent 

 stages. In general the resistivity of metals increases with increased 

 temperature (positive resistance-temperature coefficient); the resis- 

 tivity of electrolytes, semiconductors, and insulators decreases with 

 increased temperature (negative coefficient). Within narrow ranges 

 of temperature where the resistance-temperature coefficient may be 

 considered constant, the resistance of a conductor at the temperature 



t is 



R t = R [l + a(t - y] = R (l + a At) (1) 



where R is the resistance of the conductor at the temperature t , a 

 the temperature coefficient at t Q , and At — t — t . For larger tem- 

 perature ranges the resistance follows more accurately the form 



R t = R Q {1 +a At +£ A* 2 ) (2) 



