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BELL SYSTEM TECHNICAL JOURNAL 



schematic of Fig. 25. The air pressure caHbration for such a manometer is 

 also shown. The characteristic will be shifted when a gas is used having a 

 thermal conductivity different from that of air. Such a manometer has 

 been found to be best suited for the measurement of pressures from 10~^ 

 to 10 millimeters of mercury. The lower pressure limit is set by practical 

 considerations such as meter sensitivity and the ability to maintain the zero 

 setting for reasonable periods of time in the presence of the variations of 

 supply voltage and ambient temperature. The upper pressure measure- 

 ment limit is caused by the onset of saturation in the bridge unbalance 



4~> ^ 



10-2 



4 6 8I0-' 



2 4-68! 2 46 8|0' 



CURRENT IN MILLIAMPERES 



4 6 810^ 



Fig. 24. — Characteristics of a typical thermistor manometer tube, showing the effect 

 of gas pressure on the volt-ampere and resistance-power relations. 



voltage versus pressure characteristic at high pressures. This is basically 

 because the mean free path of the gas molecules becomes short compared 

 with the distance between the thermistor bead and the inner surface of the 

 manometer bulb, so that the cooling effect becomes nearly independent of 

 the pressure. 



The thermistor manometer is specially advantageous for use in gases 

 which may be decomposed thermally. For this type of use, the thermistor 

 element temperature can be limited to a rise of 30 centigrade degrees or 

 less above ambient temperature. For ordinary applications, however, a 

 temperature rise up to approximately 200 centigrade degrees in vacuum 



