160 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 2 



precision of resistance measurements of 2 to 4 parts in a million is 

 required. 



The resistance thermometer is primarily applicable for the 

 measurement of small temperature differences where the output 

 from a thermocouple would be too low for convenient measurement, 

 but it is also very accurate for the measurement of large temperature 

 differences and high temperatures. Errors resulting from undesired 

 thermoelectric potentials at the connections may be detected by 

 reversing the polarity of the supply voltage; a deflection of the null 

 detector, when the polarity is reversed, indicates a thermoelectric 

 effect. The error can usually be corrected by taking the average of 

 both readings. 



Disadvantages of resistance thermometers are the need for aux- 

 iliary apparatus and power supplies, and the relatively large size 

 which makes it impossible to measure temperature in a small volume, 

 as with a thermocouple or a small thermistor. 



If the measuring current is too high, it will heat the transducer 

 element and cause an error. This condition is indicated when the 

 temperature reading changes with a change of current through the 

 transducer. If a measurement with the current I x furnishes a reading 

 of temperature t x and a measurement with I 2 a reading of t z , the 

 corrected temperature 1 is given by 



I 2 



1 — 2 



A current of less than 10 mA through the transducer is usually safe 

 if the resistance wire is thicker than 0.05 mm and is immersed in a 

 medium of high thermal conductivity (water). 



b. Semiconducting-resistance Temperature Transducers {Thermis- 

 tors). Thermistors are solid semiconductors, usually with a large 

 negative resistance -temperature coefficient in the order of — 5 per 

 cent change of resistance per degree centigrade of temperature 

 change (i.e., about 10 times as large as that of metals). Also thermis- 

 tors with a positive temperature coefficient as high as +14 per cent/ 

 °C in a range from 50 to 225°C have been made. 2 Thermistors 

 consist of mixtures, mainly of the oxides but also of other compounds 

 (sulfides, silicates) of Mn, Ni, Co, Cu, U, Fe, Zn, Ti, Al, orMg. Carbon 

 is also used. Thermistors are in general produced by compressing 

 mixtures of such compounds in powder form to beads, rods, or disks 



1 H. Moser, in F. Kohlrausch, "Praktische Physik," 20th ed., sec. 4.162, 3. 

 B. G. Teubner, Veiiagsgesellschaft m.b.H., Stuttgart, 1955. 



2 H. A. Sauer and S. S. Flaschen, "ProceedingsIRE-RETMA-AIEE-WCEMA 

 Electronics Components Symposium," pp. 41-46, Engineering Publishers, 

 Babylon, N.Y., 1956. 



