256 



BELL SYSTEM TECHNICAL JOURNAL 



Photographic observations indicate that a hghtning discharge is usually 

 initiated in the cloud by a so-called "stepped leader," except in the case of 

 discharges to sufficiently tall structures where this leader is initiated at 

 the ground end. After the leader reaches the ground, or the cloud in the 

 case of a tall structure, a heavy current "return stroke" proceeds from the 

 ground toward the cloud at about yV the velocity of light. The main surge 

 of current in the return stroke, which usually lasts for less than 100 micro- 

 seconds, may be followed by a low current lasting for -yg- second or so. 



220 



200 



180 



160 M 



140 a 



B 

 cd 



120 rH 



100 



50 



40 <u 



20 



.02 .C5 .1 »2 o5 1 2 5 10 20 50 100,^ 



Percentage of Lightnlns Strokes in V.Tilch Current Exceeds Ordinate 



Figure 1 — Distribution of crest currents in lightning strokes. 

 Curve 1: Currents in strokes to transmission line ground structures, based on 4410 

 measurements, 2721 in U. S. and 1689 in Europe. 



Curve 2: Currents in strokes to buried structures, derived from curve 1. 



There may then be a second leader, which does not exhibit the stepped 

 character of the first leader and always proceeds from the cloud, and a 

 second return stroke. This may be followed by a third leader and so on, 

 the average number of strokes in multiple discharges being about 4 and the 

 average time interval between strokes about yV second. Single-stroke dis- 

 charges are, however, most common, discharges having more than 6 strokes 

 being quite rare although discharges with as many as 40 strokes have been 

 observed. 



The crest value of currents in lightning discharges varies over wide limits. 



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