Sec. 1-5] MFJ'llASICAL IS PUT T R A XSDUCERS 109 



The voltage-current characteristic is different for positive or nega- 

 tive polarity of the point and changes strongly with the pressure, 

 and, to a lesser extent, with the gas velocity. The influence of humid- 

 ity is slight; that of temperature negligible. The instrument, there- 

 fore, measures primarily the pressure variation in the flowing gas. 

 The characteristic is also influenced by the geometry of the needle 

 point. 



The response of the system is extremely fast; pressure variations 

 in the megacycle region have been observed. In the range from 10 2 

 to 10 6 cps, the relative response does not change; the frequency- 

 response curve is flat throughout this range. The noise level can be 

 considerable. 



Other discharge forms, such as the Townsend discharge or the glow 

 discharge, have also been investigated as a means to measure velocity 

 and pressure. 



For a summarizing review, see W. M. Cady, Physical Measurements in 

 Gas Dynamics and Combustion, in R. W. Ladenburg (ed.), "High Speed Aero- 

 dynamics and Jet Propulsion," vol. 9, sec. C, 3, p. 146, Princeton University 

 Press, Princeton, X.J., 1954. For references see Werner, loc. cit., as well as 

 Werner and C.eronime, loc. cit. 



1-56. Thermal Gauges 



The following thermal transducer is designed primarily for opera- 

 tion below atmospheric pressure (i.e., as a vacuum gauge). It is 

 illustrated in Fig. (1-5)7 and it usually contains a fine wire F of 

 about 0.001 in. diameter mounted 

 in a glass or metal tube. The 

 tube is joined to the system where 

 the pressure is to be measured. 

 The wire is heated to a tempera- 

 ture between 75 and 400°C by a ( a ) (6) 

 current of the order of 10 to 100 



. , iiii i Fig. (1-5)7. Thermal vacuum gauges: 



mA and cooled by heat conduc- (o) fche temperature of the neated fila . 



tioil through the gas surrounding ment F is measured l>y the thermo- 

 the wire element Th; (b) the temperature of the 



At atmospheric pressures the 

 heat conductivity of gases is inde- 

 pendent of their pressures. However, as the pressure is reduced, a 

 point is reached (about 10 mm Hg) at which the heat conductivity 

 begins to decrease with decreasing pressure. The gas molecules 

 striking the hot wire carry off energy to the walls of the vessel; the 

 actual temperature of the wire is then controlled by the rate at which 



filament is measured by its resistance 

 variation. 



