LIGIITNIXG CURRRXTS IX BURIED CABLE 



299 



is the distance to the stroke. If the trigger arrangement in the apparatus 

 mentioned in Section 2.5 is sufficiently sensitive to be operated by strokes 

 so remote that the thunder cannot be distinguished above noise on the wire 

 record, then the energy in the sound wave would be controlling, in the sense 

 that it would determine the making of a usable record. On the other hand 

 if the triggering should occur only for strokes of such substantial intensity 



5.0 



3 4 5 6 7 



RADIUS OF OBSERVATION AREA IN MILES 



Fig. 13. — Apparent incidence of strokes to ground, per square mile per ten thunder 

 storm days, as a function of radius of observation area. 



that some of the more remote strokes of low intensity would not be recorded, 

 then the energy in the electromagnetic wave would be controlling. Simi- 

 larly, in the case of direct visual-aural observation, the light radiation from 

 the stroke may be assumed proportional to /-/''"• If, for any of these 

 methods the energy density is taken as ii = cP/r^ where c is a constant 

 and the minimum energy required for observation is tio, then only stroke 

 currents in excess of / = {iio/cy^r would be observed. The observed or 



