156 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 2 



Through the use of fixed series and parallel combinations of resist- 

 ances, the characteristics of resistance thermometers can be altered 

 to approximate a prescribed function, e.g., a linear one. 1 



a. Metallic-resistance Temperature Transducers. Most metals 

 show an increase of resistivity with temperature which is first 

 linear and then increases in accelerated fashion. In particular iron, 



400 



E 



/*■ 



300 



?200 



£100 

 



E 



-200 200 400 600 800 1,000 

 Temperature, °C 



Fig. (2-1)1. Resistance thermom- 

 eter, schematic diagram: 1,1, 

 current terminals; E,E, voltage 

 terminals. 



Fig. (2-1)2. Resistance variation with 

 temperature of a platinum and a nickel 

 resistance thermometer having a resist- 

 ance of 100 ohms at 0°C. 



nickel, and cobalt behave in this way. Only platinum shows a less 

 rapid increase in resistivity at higher temperatures, as shown in 

 Fig. (2-1)2. 



The resistance-temperature coefficient of all metals depends to a 

 large extent upon their purity and the thermal treatment. Pure 

 metals have relatively high coefficients; the coefficients of alloys are 

 usually smaller and can even be negative in certain ranges of tem- 

 perature (manganin). Change of the physical character of the metal 

 (annealing, recrystallization) can cause a change of the temperature 

 coefficient which in some cases, e.g., at the curie point, can be dis- 

 continuous. 



Nickel can be used below 300°C. Tungsten and tantalum have 

 high temperature coefficients and high melting points but oxidize in 

 air at temperatures above 400°C. The resistance variation of com- 

 mercial cojiper can be used to determine the temperature of machine 

 or transformer windings. In the range below 120°C a gold-silver 

 alloy can be used which has the same characteristics as platinum. 2 

 For low-temperature measurement certain phosphor-bronze alloys 

 are satisfactory. These materials exhibit a large change of resistivity 

 below 7°K and a small variation above this range. The sensitivity in 



1 F. Lieneweg, Arch. tech. Messen, J 222-2, December, 1949. 



2 G. Keinath, Arch. tech. Messen, J 221-1, January, 1933. 



