90 
MR. C. T. R. WILSON: INVESTIGATIONS ON LIGHTNING DISCHARGES 
learn something about the average lightning discharge by accumulating measurements 
of the effects produced by discharges at various distances. 
VI. Electric Moments of the Discharges. 
For each discharge recorded in fig. 2, FL 3 , the product of the vertical electric force 
and the cube of the distance of the discharge, has been calculated. This product, 
when the quantities are expressed in electrostatic C.G.S. units, may be taken as giving 
a lower limit for the electric moment 2 Hqh = 2QH, or 2Q(H 2 —H x ), becoming equal 
to it when L is large compared with H. 
The mean value of FL 3 for the 78 positive flashes for which L could be determined 
in the 1917 records is 7‘3x 10 5 in volts per metre x kilometres 3 ; for the 46 negative 
flashes the mean value obtained is identical with that found for the positive. We 
may take this value (the equivalent of 2'4 x 10 16 E.S.U. x centimetres, or about 
80 coulomb-km.) as a minimum estimate of the average electric moment of the 
lightning discharges. 
The observations of 1917 give values of FL 3 ranging between l/20 and 5 times the 
mean; but in more than half the discharges for which the necessary data are 
available FL 3 lies between one half and twice this mean value. Some of the eye 
observations made previously to 1917 lead to higher values, reaching in one case ten 
times the above mean. 
In Table I. are given the mean values of FL 3 for positive and for negative 
discharges at distances (l) below’ 5 km., (2) between 5 and 10 km., and (3) exceeding 
10 km. The number of observations used in getting the means is in each case 
inserted in brackets. FL 3 is given in volts per metre x kilometres 3 x 10 5 . 
Table I. 
1917 only . . 
1911') 
1915 y . , . 
1917 J 
Below 5 km. 
5-10 km. 
Above 10 km. 
Positive. 
' 
Negative. 
Positive. 
Negative. 
Positive. 
Negative. 
4-5(17) 
3-7 (37) 
. 
3-7 (8) 
3-7 (8) 
6- 0 (32) 
7- 4 (48) 
8-2 (29) 
7-8 (33) 
11-8 (29) 
14-6 (38) 
9-5 (8) 
15-5 (15) 
The mean values of FL 3 are not appreciably different for positive and negative 
discharges. 
For discharges at distances between 10 and 15 km., the mean value of FL 3 for the 
24 positive discharges of 1917 is 10*8 x 10 5 ; if the 5 discharges of previous years 
