140 
ured was 130,000 amp. By making various assumptions, 
the lightning stroke currents responsible for tower cur- 
rents measured were deduced as shown in Item 3. 
The correctness of some of these assumptions is ques- 
tionable, and the values shown in the table are probably 
Tasie IJ. CHARACTERISTICS OF LIGHTNING STROKES 
Per cent of strokes with 
values in excess of 
| those shown in Table I | Maxi- 
Item 
bs ia mum 
90% | 50% 10% 
1. Current peaks measured | Min. | 2.2 | 6.0 20.0 | — 
in stroke path, kiloam- | Max. | 5.0 | 8.6 27.5 |160 
peres 
2. Current amplitudes in | Min. | 1.0 | 8.8 28.4 | — 
steel towers, kiloamperes| Max. | 5.3 |12.2 35.8 |130 
3. Lightning stroke cur- | Min. | 2.4 13.3 50.0 | — 
rents computed from | Max. |10.3 |40.0 {101.0 |220 
Item 2, kiloamperes 
Min 
4. Rate of rise of stroke ited Lasts 
Max. | 2.4 
currents, kiloamperes per 
psec 
5. Wave front of stroke | Min. | 0.2 | 1.0 5) | — 
currents, usec flax. | 0.9 | 2.Al iH.) |) Io) 
6. Wave tail of current | Min. /11.6 |30.3 #30 | = 
peaks—time to half | Max. |26.0 |45.5 80.0 |120 
value, usec 
7. Charges in current | Min. | — = — = 
peaks, cowlombs Max. 0.04) 0.23 1.03) 5.6 
8. Total stroke charges, | Min. | 2.3 110.4 &a).() | = 
coulombs Max. | 4.2 )22.2 |100.0 |165 
9. Total duration of | Min. | — | 0.0006) 0.2 | — 
strokes, sec Max. | 0.1 | 0.37 0.68) 1.6 
10. Number of current peaks} Min. | 1.0 | 1.8 4.0) — 
in lightning strokes Max. | 1.3 | 3.0 11.0 | 42 
11. Time interval between | Min. — | 0.02 0.1 — 
current peaks, sec Max. | 0.03) 0.088 | 0.18) 0.5 
too high. The statistical evidence mdicates, however, 
that only very few lightning strokes will have current 
peaks in excess of 60,000 amp. 
Wave Shape of Current Peaks 
Of great importance for evaluating the effect of 
lead length in lightning protective systems is the knowl- 
edge of the length of the front and the rate of rise of 
current of the current peaks. The number of records 
available is relatively small. However, the data taken 
by different investigators with different means of re- 
cording are in sufficient agreement to allow confidence 
in the results. The limits of wave fronts and rates of 
rise measured indicate that inductive drops are a serious 
consideration. In 50 per cent of the cases the rate of 
rise was 12,000 amp per usec or greater, which would 
result in approximately a 6000-v drop per foot of 
conductor. 
The duration of the current peaks is of importance 
in estimating or determining the strength of insulation 
subjected to lightning strokes. For practical reasons 
this is usually expressed in terms of microsecond dura- 
tion of the current wave while it rises from zero to 
crest and decays to half value. The statistical evidence 
shows that 50 per cent of the waves have a time to 
half value of approximately 38 usec or more. 
ATMOSPHERIC ELECTRICITY 
The charges in the current peak are related by an 
unknown factor to the charges laid down by the leader. 
The charges measured at the ground end of the stroke 
are expected to be lower than those in the leader be- 
cause of losses during the formation of the leader. The 
charges given in the table are based on the integrated 
product of current im amperes and time in seconds, 
using only the portion of the current peak between 
its start and its decrease to half value. In all cases but 
one, the charges thus determined were less than one 
coulomb and resulted from lowering a negative charge 
from the cloud. The maximum value of 5.6 coulombs 
resulted from lowering a positive charge from the cloud. 
The Composite Stroke 
The lightnmg stroke may consist of a number of 
current peaks and continuing current flow. The total 
duration of the stroke measured by different investi- 
gators varies between 0.0006 sec and 0.35 sec for the 
50 per cent level. The longest duration measured is 
about 1.5 sec. Fifty per cent of the strokes have two 
or more current peaks, while a maximum of 42 current 
peaks has been measured. The time interval between 
successive current peaks at the 50 per cent level varies 
between 0.02 sec and 0.09 sec, with a maximum of 0.5 
sec between successive discharges. 
Of considerable interest is the total charge in the 
lightning stroke. Thisis principally the charge conducted 
by the contimuous currents. The average charge meas- 
ured is approximately 18 coulombs, while a maximum of 
165 coulombs has been recorded. Many of these meas- 
urements were obtained from strokes to tall buildings, 
and in all cases they represent the charges at the ground 
end of the stroke. They represent the total charges 
conducted in the channel through the point of measure- 
ment. There should be some difference in the total 
charges conducted when the stroke is mitiated at the 
cloud and when it is initiated at a tall structure. In 
the latter case, the total charge in the channel can be 
measured, while for strokes initiated at the cloud, 
charges laid down from the cloud are not necessarily 
included in the measurement. 
Measurement at the ground end of the stroke will 
not necessarily record all charges involved in a light- 
ning stroke, particularly in the not-too-rare cases where 
the stroke changes its path at the ground end or at 
both ends. It is obvious that m such cases the total 
charges involved in the stroke mechanism can be con- 
siderably greater. 
Charge determination by means of electric field meas- 
urements of thunderstorms has indicated a maximum 
charge of 200 coulombs, a value about 25 per cent 
higher than at the Empire State Building. A further 
indication of total charges involved in lightning strokes 
has been obtained from damage produced by lightning 
strokes on metal parts, particularly of airplanes. Com- 
parison of such damage with similar effects produced 
in the laboratory has indicated charges at least as high 
as 300 coulombs, and possibly in excess of 500 coulombs. 
In some of these high total charges, strokes to ground 
were also involved, but since the total stroke mecha- 
