Sec. 2-1] 



TEMPERATURE TRANSDUCERS 



155 



Table 7 shows values of the resistance-temperature coefficient a for 

 different materials at room temperature. Values of a reported in the 

 literature vary considerably. 



Table 7. Resistance -temperature Coefficients a 

 at Room Temperature, °C _1 



The resistance-temperature coefficient a can be found from two 

 measurements of temperature and corresponding resistance : i? x at 

 the temperature t x and R 2 at the temperature t 2 . Under the assump- 

 tion that the resistance follows the linear equation ( 1 ), the coefficient 

 is 



R e 



a 



R x 



J*l*2 ' -ft 2^1 



The resistance variation is usually measured with bridge arrange- 

 ments, such as the Wheatstone bridge or the Thompson (Kelvin) 

 bridge. A number of special bridge circuits which eliminate the 

 influence of the supply voltage, of the temperature error of the indi- 

 cating meter, or of the line connecting the resistance element with 

 the reading setup are described by Eggers. 1 For bridges and circuits 

 for the measurements of temperature differences with two resistance 

 thermometers, see the paper by Eggers and also the paper by 

 Geyger. 2 The use of the ratio meter for resistance-temperature 

 measurements is described by Lorenz. 3 The potentiometer method 

 for resistance measurement furnishes very accurate values over a 

 large range of resistance variation, but is in general less convenient. 

 The use of resistance-thermometer elements with separate voltage E 

 and current / terminals as shown in Fig. (2-1)1 is recommended for 

 potentiometric measurements to eliminate the influence of a variable 

 resistance at the contacts. 



1 H. R. Eggers, Arch. tech. Messen, J 222-1, March, 1941. 



2 W. Geyger, Arch. tech. Messen, V 2166-2, 1931. 



3 J. Lorenz, Arch. tech. Messen, V 212-1, November, 1939. 



