THE LIGHTNING DISCHARGE 
circuit breaker duty and protective means for reducing 
the number of flashovers must be considered. 
Conclusions 
The physical phenomena of the lightning discharge 
are not entirely understood. Photographic evidence in- 
dicates a stepped-leader initiation of the stroke fol- 
lowed by a return stroke from its ground terminal. 
Theoretical stipulation of a pilot streamer has not been 
proven. 
To complete the understanding of the physical phe- 
nomena involved in lightning discharges, more informa- 
tion is required on these principal questions: 
1. The gradient at the point of origin of the stroke. 
2. Gradient distribution within clouds, as well as 
beneath clouds. 
3. Gradients at the ground end of a stroke. 
4, Existence and character of ground streamers prior 
to stroke contact with the ground. 
5. Ionization processes within the stepped leader, 
continuous leader, and the return stroke. 
6. Ionization and deionization of the stroke channel 
with special reference to continuing current discharges. 
7. Influence of ground conditions on the return-stroke 
process. 
For further progress on the protection problem, statis- 
tical evidence is desirable on: 
1. The wave shape of lightning stroke currents of 
both current peaks and continuing currents. 
2. The distribution of such currents in the ground 
network of protective installations, as well as in the 
earth. 
3. The lightning stroke density in various regions of 
the earth. 
4. The incidence of lightning to various structures as 
determined by height and physical location with regard 
to other structures and natural terrain. 
From the numerous investigations of lightning under- 
taken during the last twenty-five years, it has been 
possible to devise protective systems and practices 
which reduce damage due to lightning to a negligible 
factor on electrical installations as well as on buildings. 
REFERENCES 
The sources of information are extensive. References to 750 
papers are found in [1] and [2] below. To keep the number of 
references at a reasonable figure, only a few additional refer- 
ences are listed. 
143 
1. AJ.H.E. Lightning Reference Book, 1918-1935. New York, 
American Institute of Electrical Engineers, 1937. 
2. AL.H.H. Lightning Reference Bibliography, 1936-1949. New 
York, American Institute of Electrical Engineers, 1950. 
3. Frowrrs, J. W., “The Direct Measurement of Lightning 
Current.” J. Franklin Inst., 232: 425-450 (1941). 
4. Foust, C. M., and Kupunt, H. P., ‘“‘The Surge-Crest Am- 
meter.’’ Gen. elect. Rev., 35: 644-648 (1932). 
5. Hacencuta, J. H., “Lightning Recording Instruments.”’ 
Gen. elect. Rev., 43: 195-201, 248-255 (1940). 
6. Larsen, A., ‘Photographing Lightning with a Moving 
Camera.”’ Rep. Smithson. Instn., pp. 119-127 (1905). 
7. Ler, E.S., and Foust, C. M., ‘‘The Measurement of Surge 
Voltages on Transmission Lines Due to Lightning.”’ 
Trans. Amer. Inst. elect. Engrs., 46: 339-356 (1927). 
8. Lorn, L. B., and Merk, J. M., “The Mechanism of Spark 
Discharge in Air at Atmospheric Pressure.” J. appl. 
Phys., 11: 488-447, 459-474 (1940). 
9. Mauan, D. J., and Coutens, H., ‘Progressive Lightning, 
III.”’ Proc. roy. Soc., (A) 162: 175-203 (1937). 
10. McCann, G. D., ‘“The Measurement of Lightning Currents 
in Direct Strokes.” Trans. Amer. Inst. elect. Engrs., 63: 
1157-1164 (1944). 
11. McHacnron, K. B., ‘Lightning to the Empire State Build- 
‘ing.”’ J. Franklin Inst., 227: 149-217 (1939). 
12. —— and McMorris, W. A., “The Lightning Stroke: 
Mechanism of Discharge.’ Gen. elect. Rev., 39: 487-496 
(1936). 
13. Prerers, J. F., ‘The Klydonograph.” Elect. World, N. Y., 
83: 769-773 (1924). 
14. ScHonuanD, B. F. J., “Progressive Lightning, IV.’’ Proc. 
roy. Soc., (A) 164: 1382-150 (1988). 
15. —— and Coutsns, H., ‘‘Progressive Lightning.’’ Proc. roy. 
Soc., (A) 143: 654-674 (1934). 
16. ScHonzanp, B. F. J.,. Matan, D. J., and Coutens, H., 
“Progressive Lightning, II.’’ Proc. roy. Soc., (A) 152: 
595-625 (1935). 
17. Waener, C. F., and McCann, G. D., ‘“‘New Instruments 
for Recording Lightning Currents.”’ Trans. Amer. Inst. 
elect. Engrs., 59: 1061-1068 (1940). 
Further articles containing comprehensive references are: 
18. Bruce, C. E. R., and Gotpn, R. H., ‘‘The Lightning Dis- 
charge.”’ J. Instn. elect. Engrs., 88: 487-505 (1941). 
19. McEacuron, K. B., “Lightning and Lightning Protec- 
tion” in Encyclopaedia Britannica, Vol. 14. Chicago, 1948. 
(See pp. 114-116) 
20. —— and Hacrneura, J. H., “Lightning and the Protection 
of Lines and Structures from Lightning”? in Standard 
Handbook for Electrical Engineers, A. E. KNowuron, ed. 
New York, McGraw, 8th ed., 1949. (See pp. 2230-2254) 
21. Merex, J. M., and Perry, F. R., “The Lightning Dis- 
charge”’ in Reports on Progress in Physics, 10: 314-357. 
Phys. Soc., London, 1944-45. 
