[ 73 ] 
III. Investigations on Lightning Discharges and on the Elective Field of 
Thunderstorms. 
By C. T. U. Wilson, F.R.S., Reader in Electrical Meteorology in the University 
of Cambridge and Observer in Meteorological Physics at the Solar Physics 
Observatory , Cambridge. 
(Communicated by the Director of the Solar Physics Observatory .) 
Received March 15,—Read May 6, 1920. 
[Plates 2-5.] 
Contents. 
Section Page 
I. Methods of measurement. 74 
II. Some typical records.. 77 
III. On the prevailing sign of the sudden changes produced in the potential gradient by 
lightning flashes. 85 
IV. Magnitude of the changes produced in the electric field by lightning discharges at 
different distances.86 
V. Effects to be expected from different kinds of discharges at different distances .... 87 
VI. Electric moments of the discharges. 90 
VII. Quantity of electricity discharged in an average lightning discharge ....... 91 
VIII. Electric field of a thunder-cloud. .94 
IX. Conditions determining discharge.98 
X. Dimensions of the regions discharged by lightning flashes.101 
XI. Maximum potential attained before the passage of a lightning discharge..102 
XII. Mean density of the charge in a thunder-cloud immediately before a discharge . . . 103 
XIII. Charge associated with 1 c.c. of water in a cloud .. .103 
XIV. Disruption of drops by the electric field.104 
XV. Pressure within a charged portion of a cloud. .106 
XVI. Thunder resulting from sudden contraction due to loss of charge.106 
XVII. Energy dissipated in lightning flashes.106 
XVIII. Interpretation of “ recovery ” curves.107 
XIX. Electrical currents maintained in the atmosphere by thunder-clouds and shower-clouds . 109 
XX. Differences between the electrical effects of shower-clouds of positive and negative 
polarity. 110 
XXI. The normal potential gradient and air-earth current of fine weather.112 
XXII. Influence of the nature of the earth’s surface below a thunder-cloud or shower-cloud. . 113 
XXIII. Secondary thunder-clouds.114 
VOL. GCXXI.-A 584. M [Published August 27, 1920. 
