, _ LIGHTNING PEEK 179 



where the spark takes place over a series of clouds by a miiltigap 

 action and can, in fact, be shown in the laboratory. When the 

 flash is from cloud to ground or from one cloud to another through 

 clear air, as is usual with lightning, it is not cJear how progressive 

 action can occur. While the field produced by the charge is fairly 

 uniform, it is probable that at the instant before spark-over a 

 needlelike streamer forms and breakdown then corresponds to the 

 needle-gap distance. Needle-gap spark-over requires less than 20 

 per cent of the 30 kv./cm. required for uniform field. The spark- 

 ing distance should usually correspond to a continuous voltage, 

 because there is generally no large transient until after the spark 

 starts. 



It thus appears, by approximately direct measurement, that the 

 order of voltage of a severe lightning stroke to ground may be 



■,sp»« ^^-^^ Table 



^''"Distance /rom Poin/ Voltage Induced 



y ■ Directly on a Line 



/ Beneath Cloud q Iwenfy ft High 



' OMile lOOKv/t. ZOOOHv. 



-^4 Mile 3l9Kv/ft. «8 Kk 

 y^Mile l2.4Hv/ft 248 Kv. 

 /Mile 3i6Kv/n. IZi^v. 



'" ---^IMile 0.9K^/ft. /5Ki! 



I 



-i-C Cloud y::'"r~^% ""\ 



.flotyc/ 1.000 feet Above 

 "\ Earth and at a 

 \ Potential of \ 

 \ 100.000.000 Volts, 



g-IOOlW. g-3l.9Kv. g-llAKv. g=5.dj{v 



per ft. per ft. per ft per ft. 



Fig. 14 



about 100,000,000. This is about one million times the voltage of 

 the ordinary lighting circuit or one thousand times the voltage of 

 a very high voltage transmission line. The lightning voltage dur- 

 ing a storm will, of course, vary over a very wide range, sometimes 

 much higher but generally lower than the value above. It has been 

 observed that during a severe thunderstorm there may be many 

 induced strokes at very low voltages, a less number of moderate 

 voltages, and so on to very few at the extreme voltages. 



It will be noted that the above conditions require a gradient of 

 100 kv./ft. (330 kv./m.) in the most dense part of the electric field 

 where the flash occurs and a gradient of less than a third of this 

 a short distance away. See Figure 14, where the gradients are 

 given for points near the surface of the earth. 



It is estimated from the voltage and the size and height of the 

 clouds that the current is of the order of 80,000 amperes and the 



