78 
MR. C. T. R. WILSON: INVESTIGATIONS ON LIGHTNING DISCHARGES 
ionization current which has entered the sphere during the 30 minutes’ exposure to 
the atmospheric electrical field. The record shows that this amounted to 21'8 E.S.U., 
while the mean charge induced on the earth-connected sphere during the exposure 
amounted to 23 E.S.U.—the equivalent of 96 volts per metre. The mean “ dissipation 
2f8 100 
factor ” for the period of exposure was thus — — x 
30 
23 
i.e., about 3 per cent, per 
minute. 
The readings obtained when the sphere is down form a series of points on a curve 
of which the vertical height above the initial horizontal part of the trace is a measure 
of the integrated ionization current which has entered the sphere from the atmosphere. 
This curve forms the zero line for potential gradient, i.e., the differences of the 
ordinates of this curve and of the actual trace obtained when the sphere is exposed 
give a measure of the potential gradient at any moment. 
In fig. 2 is reproduced the record of May 12, 1917, from 16h. 50m. to l7h. 35m. 
The sky was overcast and the weather conditions suggested thunder—a storm did in 
fact occur some hours later. The sphere was momentarily raised at 16h. 51m. ; raised 
again at 16h. 55m., and kept up till 17h. 23m., being however momentarily lowered 
into its case at 5-minute intervals during this time ; it was kept in its case after 
I7h. 23m. The potential gradient was 150 volts per metre at 16h. 51m. ; it gradually 
diminished till it reached negative values, and continued to be negative from 
I7h. 12m. 50s. till I7h. 18m. 10s., reaching a minimum of —80 volts per metre at 
I7h. 16m., becoming positive again and being equal to 260 volts per metre when the 
sphere was lowered at 17h. 23m. The negative potential gradient coincided in time 
with the passage overhead of a cloud discharging rain which did not reach the ground. 
The test-plate was uncovered from l7h. 25m. to 17h. 30m. : the displacement of the 
meniscus on uncovering and covering the plate is almost too small to be seen in 
the reproduction of the record but indicates the continuance of a positive potential 
gradient of about 300 volts per metre. The ionization current from the earth- 
connected sphere to the atmosphere during the period of negative potential gradient 
has been sufficient to neutralise approximately the flow from the atmosphere to the 
negatively charged earth-connected sphere during its exposure to the positive potential 
gradient. 
All the remaining records reproduced in the plates show the effects of lightning 
discharges (generally at a considerable distance) on the potential gradient. 
Fig. 3 (June 13, 1917, 14h. 11m. to 14h. 16m. 30s.). 
The sphere was exposed during the whole time represented by the record except at 
about 14h. 12m. 30s., when it was momentarily lowered ; the effect of lowering and 
raising the sphere is indicated by the prominence midway between 14h. 12m. and 
14h. 13m. The potential gradient at that moment was negative and equal to 
— 420 volts per metre. The summit of the prominence gives the zero line of potential 
gradient. The record begins with a negative potential gradient of about —430 volts 
