Sec. 2-1] TEMPERATURE TRANSDUCERS 159 



The total resistance of the element can vary from 0.1 to hundreds 

 of ohms. Elements of thin wire and high resistance are preferred for 

 low-temperature measurements, since otherwise the resistance varia- 

 tion is too small for convenient measurement. The thermometer 

 element should be aged at the highest or lowest temperature to which 

 the thermometer is to be exposed, until the resistance at 0°C remains 

 unaltered. 



In constructing a resistance thermometer, care should be taken to 

 exclude excessive water vapor, since it is likely to be a cause of leak- 

 age resistance. Some workers recommend baking the support to 



Fig. (2-1)4. Construction of resistance thermometers. 



eliminate moisture. Annealing of the platinum wire and of the leads 

 after the wire has been applied to the carrier is recommended in order 

 to prevent strain. Protective tubes for resistance thermometers can 

 be made from glass, quartz, porcelain, or metal (nickel). For high- 

 temperature work, the tube is usually filled with air at £ to ^ atm; 

 for low temperature, the inside pressure should exceed the outside 

 pressure by about 1 atm. The protecting tube at low temperature 

 should be gas-tight to avoid condensation of water. For use below 

 — 190°C the tube should be filled with helium to avoid condensation 

 of air. For the connecting wire inside the protecting tube, gold or 

 silver is satisfactory. The external leads are usually made from 

 copper and connected to copper terminals, which should be mounted 

 close together in order to reduce the influence of thermoelectric 

 effects. Figure (2-1)4 shows some practical constructions of resist- 

 ance thermometers. 



Resistance thermometers are the most accurate of all temperature- 

 sensing devices. In the vicinity of 0°C or at room temperature, 

 measurements within an accuracy of 1 -4 °C can be made. Ordinarily, 

 however, the error is several thousandths of a degree. At 450° the 

 accuracy does not exceed several hundredths of a degree, and around 

 1000° the accuracy is not higher than about one-tenth of a degree. 

 To obtain an accuracy of 0.00 1°C, except at low temperatures, a 



