182 ANNUAL EEPORT SMITHSONIAN INSTITUTION^ 1925 



by losses. The voltage that the line assumes at the instant of a 

 sudden discharge is that of the equipotential surface at the point 

 in which the line is located. This is approximately the gradient 

 in kilovolts per foot times the height in feet. This is a certain 

 percentage of the voltage of the lightning bolt. In studies in Colo- 

 rado, as already stated, the author has actually measured induced 

 lightning voltages on transmission lines as high as 500,000 volts. 

 Insulator flash-overs have occurred that indicate induced voltages 

 as high as 1,500,000 volts, although the greater percentage of volt- 

 ages induced on transmission lines are very much lower than this. 

 These figures, as already shown, afford a means of estimating the 

 voltage of a lightning flash. 



THE MAXIMUM VOLTAGE ON TRANSMISSION LINES 



From the above it is apparent that the maximum lightning voltage 

 that can appear on a transmission line under given conditions de- 

 pends directly upon the height of the line. The maximum voltage 

 that can appear on any line is equal to the maximum possible 

 gradient of 100 kilovolts per foot or 100,000 volts per foot times the 

 height of the line in feet. This extreme condition seldom occurs 

 because it requires the discharge to take place at a very rapid rate in 

 the immediate vicinity of the line, practically a direct stroke. The 

 chances of this condition are remote because when the storm center 

 is only a quarter of a mile away the gradient is reduced to one-third. 

 It may also be reduced by a slow discharging cloud permitting a 

 considerable dispersal of the charge along the line. Apparent 

 gradients of 20,000 to 30,000 volts per foot are fairly common on 

 actual lines. The tabulation in Figure 14 shows the induced light- 

 ning voltages on a 20-foot transmission line at different distances 

 from the rapidly discharged cloud. 



Practically, this shows that a high transmission line is undesir- 

 able from the lightning standpoint. It also affords a means of 

 determining the maximum lightning voltage that can appear on a 

 transmission line. 



The maximum voltage induced on the line is 1 or 2 per cent of 

 the cloud voltage. The energy is of the order of 1,350 watt-seconds. 

 The character of the discharge is generally impulsive and reaches 

 its maximum in a few microseconds. In fact, it corresponds closely 

 to the discharge from the lightning generator in voltage, energy, 

 and duration. 



TRAVEL OF LIGHTNING ON TRANSMISSION LINES 



A lightning disturbance travels over the line at the velocity of 

 light, and is dissipated to a considerable extent by losses; it may 



