112 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



surrounding the gauge is sometimes recommended. Difficulties may 

 arise if the gauge is used in an atmosphere of gases that may react 

 with the hot filament. 



The construction of very small Pirani gauges (less than 0.5 cc) is described 

 by D. G. H. Marsden, Rev. Sci. Instr., 26, 1205 (1955). G. A. Slack, Rev. Sci. 

 Instr., 27, 241 (1956), describes a gauge of unusual construction for the measure- 

 ment of helium pressure at low temperature. For further modification and 

 references see H. Schwarz, Arch. tech. Messen, V 1341-3, January, 1952. 



1-57. Ionization Gauges 



a. Thermionic Gmige. The gauge system is constructed like a 

 triode, i.e., it contains, in a glass vessel, a heated electron-emitting 



cathode, a grid surrounding the cath- 

 ode, and a plate surrounding the grid. 

 The grid is maintained at a positive 

 potential ( 1 00 to 250 volts) with respect 

 to the cathode, the plate at a negative 

 potential (2 to 50 volts), as shown in 

 Fig. (1-5)10. 



Electrons emitted from the cathode 

 are accelerated by the grid; they will 

 collide with the gas molecules in the 

 vessel and will ionize them. The posi- 

 tive ions in the space between grid 

 and plate will be collected by the nega- 

 tive plate ; those formed in the space between cathode and grid will 

 migrate toward the cathode. The electrons and negative ions will be 

 collected by the positive grid. 



The rate of ion production is proportional to the amount of gas 

 present and to the number of electrons available to ionize the gas. 

 The ratio of positive ions, i.e., plate current i p , to the grid current i g 

 is, therefore, a measure of the pressure p within the tube; i.e., 



i + 

 p=k± 



h 



where k is a constant of the order of magnitude of 10 /.i Hg (mA///A) 

 varying with the geometry of the tube, nature of the gas, and 

 operating voltages. 1 The ionization gauge measures the total pres- 

 sure of all gases present. The indication is continuous and inertia-free. 



Thermionic ionization gauges can be used for pressure measure- 

 ments in the range between 10~ 3 and 10~ 8 mm Hg. Within this range 

 the output varies almost linearly from 10 -4 to 10 -9 amp. If the 



1 S. Dushman, Phys. Rev., 17, 7 (1921), and 23, 734 (1924). 



Fig. (1-5)10. 

 zation gauge. 



Thermionic ioni 



