84 
MR. C. T. R. WILSON: INVESTIGATIONS ON LIGHTNING DISCHARGES 
The principal discharge of fig. 12 occurred on May 29, 1917, at about 
15h. 23m. 10s.; the peal of thunder which followed began 21'5s. later (indicating a 
distance of about 7 km.) arid was audible for about 20 seconds. The sudden change 
produced in the potential gradient was negative and exceeded 1250 volts per metre. 
The record shows the characteristic curve of recovery of the field, interrupted at 
lm. 50s. after the discharge by the lowering of the sphere. The positive field due 
to the charge which the flash neutralised was nearly counterbalanced at the place 
of observation by a negative field, so that the resultant potential gradient before 
the passage of the discharge was only about +360 volts per metre. 
When the sphere was first raised, at 15h. 19m. 30s., the potential gradient was 
positive—about 150 volts per metre—and it increased up to the moment of the 
principal discharge. There were, however, during this time, small sudden changes 
of potential, some positive, others negative, none exceeding 50 volts per metre ; 
they were obviously due to very distant discharges; no thunder was recorded. 
Throughout the afternoon there were towering cumulus clouds in all directions, rain 
falling from some of them. 
The discharge of fig. 13 occurred about 15h. 9m. 30s. on August 15, 1917. The 
sphere was lowered at 15h. 10m., the characteristic curve of recovery of the field 
being thereby interrupted. The peal of thunder began while the sphere was being 
lowered, i.e., about 40 seconds after the discharge ; the beginning is not marked 
on the record, but the double dark line indicates that the peal of thunder ended 
about 55 seconds after the discharge. The potential gradient immediately before 
the discharge had a negative value exceeding 1000 volts per metre ; immediately 
after the discharge the potential was positive and equal to about 300 volts per 
metre. 
The first discharge of fig. 14 occurred at 13h. 50m. on June 13, 1917, just at the 
moment when the sphere had been raised to its exposed position. The potential 
gradient before the discharge was negative (— — 690 volts per metre). The 
discharge was a double one, causing an increase in the negative potential gradient 
of more than 980 volts per metre, followed by a sudden change of the opposite 
sign, which brought the potential gradient to within 260 volts per metre of its 
original value, the total duration of the double discharge being about one-fifth of a 
second. 
Two other double discharges of about the same total duration were recorded about 
22 seconds and 87 seconds later, the first giving sudden potential changes of +70 and 
— 30, the second of +100 and —115 volts per metre. The other discharges shown 
in the figure are noteworthy as not being followed by the usual recovery curve. 
The next three figures are further examples of double discharge records of the same 
type— i.e., of records showing the occurrence within a very short interval of time 
of two sudden changes of potential of opposite sign. They differ among themselves 
mainly in the relative magnitudes of the two sudden changes : the first change of 
