230 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 4 



4-51. Indirect Systems 



a. Space-potential Systems. Two probe electrodes (collectors) P 1 

 and P 2 of the type described in 4-31 are brought into the electric 

 field, as shown in Fig. (4-5) 1 . The distance between the probes in the 

 direction of the field is As; the voltage measured between the probes 

 is &.E. The field strength can be approximated from 



„ A# 

 As 

 Frequently only one probe is used, and the potential between 

 the probe and one field-terminating electrode (usually ground) is 



measured. In variational (a-c) electric fields, 

 if ' . \ the space-potential measurement is, in prin- 



_,. "/ ~"r~"{-~ ^-T"" — ? ciple, simpler than in d-c fields, since the field 



variation causes, by electrostatic induction, 

 the generation of an a-c potential between 

 "\" the electrodes, as in Fig. (4-4)2. The con- 



necting leads should be in the direction of 

 ig. ( - ) . ( ec no e ^ equipotential lines to avoid field defor- 



I with two probes, r x and ^ L 



p 2 . mation. 



b. Surface-charge Systems. If an insulated 

 electrode is exposed to an electric field, such as the Wilson plate 

 illustrated in Fig. (4-4)1, the field will cause a surface charge of the 

 electrode by electrostatic induction. The field strength F in the 

 immediate vicinity of the electrode is related to the surface-charge 

 density a by 



F = a = _Q = CE, 

 e e a e a 



where e is the dielectric constant, Q the charge of the electrode, a the 

 electrode surface, C the capacitance of the electrode against ground, 

 and E the voltage measured against ground. 



Any one of the surface-charge systems described in 4-4 can be used. 

 The Wilson plate is frequently used in atmospheric-electricity 

 studies, primarily for measurement during violent thunderstorms. 



4-52. Method Based upon Moving Charges (Electron-beam 

 Probe) 

 The following method is primarily applicable to the measurement 

 of electric fields in evacuated systems or such filled with gases under 

 reduced pressure (glow discharges). As shown in Fig. (4-5)2, an elec- 

 tron beam B emitted and focused by an electron gun G passes 

 through the electric field in a gas discharge that takes place between 



