Sec. 1-5] 



MECHANICAL INPUT TRASS 1)1 V 'KHS 



113 



10 



10 



10 



to 



10 



10" 



10" b 

 Pressure, mm Hg 



' 



io- 



pressure rises above 10~ 3 mm Hg, the impact of positive ions upon 

 the cathode tends to heat and destroy the cathode. 



At a pressure lower than 1 0~ 8 mm Hg, the following effect imposes 

 a limit to the application: The impact of electrons upon the grid 

 causes a soft X-ray radiation ; these X rays liberate electrons from 

 the plate. 1 At a pressure of less 

 than 10~ 8 mm Hg the electron- 

 emission current from the plate is 

 of the same order of magnitude 

 and indistinguishable from the ion 

 current to the plate. By using a 

 very small plate (a wire centrally 

 suspended in the helical grid space ; 

 the filament is outside the grid 

 helix), Bayard and Alpert 2 have 

 succeeded in reducing the photo- 

 electron current and in increasing 

 the useful range of the ionization 



, A ,„ tt q a t r- Fig. (1-5)11. Characteristics of a therm- 



gaugetolO- 10 mmHg. 3 Amodin- ionic ionization gauge for different 

 cation and improvement of the gases. 

 Bayard-Alpert gauge is described 



by Nottingham. 4 Another thermionic ionization gauge which per- 

 mits measurements of pressure down to the order of 10~ 10 mm Hg is 

 described by Lander. 5 Two gauge types for measuring pressures up 

 to the millimeter range are described by Schulz and Phelps. 6 



Since the efficiency of the ionization process varies for different 

 gases, the calibration will depend upon the type of gas in the gauge; 

 impurities in the gas will introduce errors in the pressure measure- 

 ment. Calibration curves for three different gases are shown in Fig. 

 (1-5)11. 



The usual method of operating the gauge consists in keeping the 

 grid current constant (about 1 to 20 mA) and measuring the plate 

 current. Various methods for keeping the grid current constant 

 have been suggested. Hariharan and Bhalla 7 have described a system 

 in which the ratio iji g is measured with a logarithmic differential 

 amplifier. This system, which forms an output current proportional 



1 W. B. Nottingham, J. Appl. Physics, 8, 762 (1937). 



2 R. T. Bayard and D. Alpert, Rev. Sci. Instr., 21, 571 (1950). 



3 See also G. H. Metson, Brit. J. Appl. Phys., 2, 46 (1951). 



4 W. B. Nottingham, Trans. Vacuum Symposium, 1954, p. 76. 



5 J. J. Lander, Rev. Sci. Instr., 21, 672 (1950). 



6 G. J. Schulz and A. V. Phelps, Rev. Sci. Instr., 28, 1051 (1957). 



7 P. Hariharan and M. S. Bhalla, Rev. Sci. Instr., 27, 448 (1956). 



