216 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 4 



Electrons and ions which possess sufficient kinetic energy to 

 ionize can also be measured with ionization chambers (5-2 la), Geiger 

 counters (5-216), and scintillation counters, as well as with the cloud 

 chamber, with a calorimeter, and by their photographic action or by 

 fluorescence. 



4-11. Electrode Systems 



a. Charged-plate Electrode. The simplest receiver for electric 

 charges consists of an insulated metal plate arranged in the beam of 

 the charged particles, as shown for negatively charged particles in 

 Figs. (4-1)1 and (4-1)2. The incident charged particles discharge the 







R 



H 



E n 







h 



Fig. (4-1)1. Transducer for elec- 

 tric charges; charged plate. 



Fig. (4-1)2. Charged plate 

 with grounded screen. 



plate P and form a current / through the (large) resistance R. To 

 avoid the escape of secondary electrons caused by the impact of 

 primary electrons and ions upon the plate or of reflected or back- 

 scattered primary electrons, the plate should be charged to a positive 

 potential with respect to the walls of the surrounding vessel, as in 

 Fig. (4-1)1, or with respect to a grounded screen located in front of 

 the plate, as in Fig. (4-1)2. The potential of the plate should be at 

 least that corresponding to the electron-volt velocity of the incident 

 charges. A charged electrode for the measurement of local electron 

 intensities in an electron microscope is described by Bahr, Carlsson, 

 and Lomakka. 1 



A modification of the charged plate is the so-called paraffin capaci- 

 tor, a metal plate covered with a thin layer of paraffin over which a 

 metal layer is deposited; the incident electrons induce charges in the 

 plate. 2 



b. Faraday Cage or Cup. The Faraday cage, shown in Fig. (4-1)3, 

 consists of a hollow metallic body C. A beam of charged particles 



1 G. F. Bahr, L. Carlsson, and G. Lomakka, Rev. Sci. Instr., 27, 749 (1956) 



2 P. Lenard, Ann. Physik, 64, 288 (1898); A. Becker, Ann. Physik, 13, 394 

 1904. R. Thaller, Physikal. Z., 29, 841 (1928). 



