176 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 2 



The voltage comparison is made by repetitive switching from one 

 resistor to the other. The output voltage is amplified, rectified, and 

 integrated. The switching method eliminates differences in amplifier 

 characteristics and drift. The temperature probe of Garrison and 

 Lawson is a resistance in the kilohm range. It consists of a platinum 

 Wollaston wire (0.0001 in.) enclosed in a protective platinum cylin- 

 der, as shown in Fig. (2-1)19, and it is suitable for temperatures up to 

 1400°C. For higher temperatures, tungsten can be used. 



The noise thermometer can measure temperatures over a wide 

 range (100 to 1700°K) with good accuracy; 0.1 per cent has been 

 claimed over the entire range with very careful selection of the elec- 

 tronic components, while 1 per cent requires no special precautions. 

 The resistance element itself requires no calibration. There are no 

 thermal hysteresis effects, and any metal can be used. Carbon resis- 

 tors are not suitable because of inhomogeneities in their structure. 

 The technical difficulties in overcoming stray capacitance, pickup, 

 and drift are considerable. 



