116 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



c. Philips- Penning Ionization Gauge. This gauge is shown in Fig. 

 (1-5)14. A high voltage is applied to the electrodes C\,C 2 , and A and 

 causes a discharge in the gas. The discharge current depends upon 

 the electrode geometry, the type of gas in the gauge, and the gas 

 pressure, and can be used as a means for the pressure determination. 



Magnetic field 



+ => 



Fig. (1-5)14. Philips-Penning gauge. 



At a pressure below 10~ 2 mm Hg, the mean free path becomes so 

 large that the probability of a collision and ionization is too small to 

 maintain a discharge. By applying a magnetic field and by using the 

 geometric arrangement shown in Fig. ( 1 -5) 1 4, Penning 1 has succeeded 



600i— — i 1 F 1 1 1 in increasing the path length of 



the electrons and in maintaining 

 a discharge beyond 10 -5 mm Hg. 

 Calibration curves of a Philips- 

 Penning gauge for different gases 

 are shown in Fig. (1-5)15. The 

 useful range of the gauge extends 

 from 10~ 3 to 10~ 5 mm Hg. Sev- 

 eral authors have succeeded in 

 extending this range. Penning 

 and Nienhuis 2 have technically 

 improved the gauge and been 

 able to use it down to 4 X 10 -7 

 mm Hg. Hayashi 3 has extended 

 the range to lower pressures ( 1 0~ 6 ) 

 and, in another modification by reducing the size of the anode ring, to 

 higher pressure (10 _1 mm Hg). However, at these high pressures 

 a considerable amount of sputtering occurs which leads to electric 



1 F. M. Penning, Physical, 71 (1937), and Philips Tech. Rev., 2, 201 (1937). 



2 F. M. Penning and K. Nienhuis, Philips Tech. Rev., 11, 116 (1949). 



3 C. Hayashi et al., Rev. Sci. Instr., 20, 524 (1949). 



mm Hg 



Fig. (1-5)15. Characteristics of a Philips- 

 Penning ionization gauge for various 

 gases. 



