Sec. 4-3] ELECTRICAL TRASS nrCERS 225 



where F is the field strength prevailing at the point under operating 

 conditions and M the minimum field strength of the point at which 

 a discharge occurs. Both M and the proportionality constant a 

 depend upon the radius of curvature of the point. 1 The indication of 

 the point collector is influenced by the wind velocity. 2 



The system is primarily useful for the measurement of relatively 

 large potentials and field strengths (atmospheric studies of distant 

 thunderstorms) . 



flame collector. The collector contains a candle or a kerosene 

 burner, the escaping flame gases contain ions which carry the charge 

 off the probe. The system is simple and useful for orientating experi- 

 ments, but it is not very stable and is rarely used nowadays. 



radioactive collector. The probe contains a radioactive ma- 

 terial which ionizes the air in the vicinity of the collector. Materials 

 emitting a rays are preferred because the effect of a rays is limited to 

 the immediate vicinity of the probe, so that the field at greater dis- 

 tance from the point remains undisturbed. The radioactive material 

 is generally deposited on small metal plates or foils and protected by 

 a thin lacquer or enamel layer. The collector is most suitable for the 

 measurement of small, relatively steady potentials ("fair-weather 

 potential gradient"). 



4-32. Electron Optical System 



A simple system for the measurement of space potentials in elec- 

 tron tubes (concentric diodes) is described by Kenyon. 3 The inner 

 side of the anode is coated with a fluorescent layer, which can be 

 observed. A probe consisting of a fine tungsten wire is introduced in 

 the space between cathode and anode parallel to the tube axis, i.e., 

 in an equipotential surface, to avoid field disturbances. The probe is 

 connected to a potential between those of the anode and the cathode. 

 If the probe potential is equal to the space potential, the probe will 

 not alter the flow of electrons; if its potential is different from the 

 space potential, it will cause a shadow or a bright line on the fluores- 

 cent screen. 



The accuracy of the method is about 1 per cent for measurements 

 in the space between cathode and about three-quarters of the way 

 toward the anode, but diminishes as the probe approaches the 

 anode. 



1 F. J. W. Whipple and F. J. Scrase, Geophys. Mem. (London), no. 68. 



2 J. A. Chalmers and W. W. Mapleson, Repts. Progr. in Physics, 17, 101 

 (1954). 



3 D. E. Kenyon, Rev. Sci. Instr., 11, 308 (1940). 



