284 



BELL SYSTEM TECHNICAL JOURNAL 



sistivity is increased by a factor k, or if co is decreased by the same factor, 

 it is evident from (4) and (5) that the same current will be obtained at the 

 distance xi = k^i'^x. Thus, if the earth resistivity is increased by yfe = 4, 

 Xi = 2x and the current attenuation will be half as great as before. This 

 rule applies to surge-currents of a given wave-shape as well, since they may 

 be regarded as composed of sinusoidal components, each of which would 



> 0.6 



0.5 



0.3 



40 50 60 



TIME IN MICROSECONDS 



Fig. 4 — Attenuation and distortion of surge current transmitted along a buried con- 

 ductor. 



travel farther by the factor ^^/^. Furthermore, it follows by the same rea- 

 soning that for surge-currents of congruent wave-shapes, but different dura- 

 tions, the rate of attenuation is inversely proportional to the square root 

 of the duration. That is to say, let in one case the current reach its crest 

 value at the stroke point in 5 microseconds and its half-value, in 65 micro- 

 seconds, and let the crest current at a given distance x have a certain value. 

 Then, if in another case the current reaches the same stroke-point crest 



