80 
MR. C. T. R. WILSON: INVESTIGATIONS ON LIGHTNING DISCHARGES 
indicated at 14h. 45m. 15s. when the plate was first uncovered is negative ( — 4570 volts 
per metre). The principal sudden changes of potential gradient (all in the neigh¬ 
bourhood of 3000 volts per metre) are negative, indicating the destruction of 
positive fields by the passage of lightning discharges. The times of the beginning 
and ending of the peals of thunder were in most cases marked as shown by the 
single and double black lines. The distances indicated by the intervals between 
the principal discharges and the beginning of thunder are all about 5 km. 
The characteristic curve of recovery after the passage of each discharge is well 
shown. 
Heavy black clouds were overhead at the beginning of the record and slight rain 
began about 14h. 48m. 30s. and became heavy at about the time when the record ceased. 
The effect of the rain is shown by the downward slope of the latter portion of the 
trace, which indicates a flow of positive electricity from the earth through the 
capillary electrometer to the test-plate. How much of this positive charge went to 
increase the induced positive charge on the test-plate (on account of increasing 
negative potential gradient) and how much to neutralise a negative charge carried 
down to the test-plate by rain drops, or by ions travelling under the influence of the 
negative potential gradient, remains undetermined owing to the fact that the cover 
was not replaced until after the record was completed. 
Fig. 5 (June 17, 1917, 20h. 23m. 23s. to 20h. 27m. 29s.). 
This is an enlargement of a portion of a record obtained while a severe storm was 
passing at a distance of 15 to 20 km. Between 20h. 20m. and 20h. 29m. the photo¬ 
graphic trace recorded 95 positive discharges (i.e., discharges causing a sudden 
positive change of potential gradient) and 40 negative discharges. The discharges 
were visible as vertical flashes passing between a cloud near the N.W. horizon and 
the earth, many of the flashes being multiple. The storm was seen to travel from 
W. to N. ; newspaper reports show that it passed over St. Ives, which lies from 10 to 
11 miles (about 17 km.) to the N.W., damage by lightning occurring there. The mean 
of the 95 sudden changes of potential gradient of positive sign amounted to 119 volts 
per metre, that of the 40 of negative sign to —80 volts per metre. 
The sphere was used in obtaining this record. 
Fig. 6 (June 16, 1917, 19h. 12m. to 19h. 23m.). 
The test-plate was used as the exposed conductor. 
The potential gradient was negative ( — 5400 volts per metre) at 19h. 12m. 45s. 
when the cover was removed, positive ( = 1000 volts per metre) at 19h. 22m. 15s. 
when the cover was replaced. Rain was falling throughout the duration of the 
record, and the charge carried down (by rain and ionization current) during the 
9|- minutes’ exposure was negative and amounted to 16 x 10 -12 coulombs per sq. cm., 
the mean current being thus about 27 x 10 -15 ampere per sq. cm. Two of the 
discharges recorded—at 19h. 17m. 4s. and at 19h. 20m. 55s.—were multiple, as is 
shown in the enlargements, figs. 18 and 19 of Plate 5. All the sudden changes of 
