76 
MR. C. T. R. WILSON: INVESTIGATIONS ON LIGHTNING DISCHARGES 
a given point by the induced charge on the hemisphere is readily seen to be equal 
to Ya 3 / r ' 3 where a is the radius of the hemisphere and r is the distance of its centre 
from the given point. In the actual case the correction amounts to 6 per cent. 
The charge on the exposed earthed test-plate (the surface of which is at ground 
level) is similarly diminished by the presence of the hut; the correction to be applied 
amounts in this case to about 1 per cent. A somewhat larger correction—-estimated 
at 1‘5 per cent.—has to he made for the effect of the induced charge of the earth- 
connected cover and its supporting arm. Apart from these small corrections the 
relation between the potential gradient F at ground level and the charge Q on the 
exposed earthed test-plate, of area A, is given by 47rQ/A = F, when the quantities 
are expressed in C.G.S. electrostatic measure. The effective area of the plate is 
2220 sq. cm. 
For the measurement of the quantities of electricity which passed between the 
exposed conductor and earth through the connecting wire, the capillary electrometer 
described in the previous paper was used. By means of a ^-inch microscope 
objective, placed with its axis vertical above the electrometer, an image of the 
meniscus was formed on a horizontal slit. The slit coincided in position with the 
image of the axis of the capillary tube and was almost in contact with the sensitive 
surface of a photographic plate kept in uniform motion at right angles to the slit. It 
was made by ruling a line with a razor blade on an exposed and developed “ process ” 
plate; it was protected by a strip of microscope cover-glass cemented with Canada 
balsam to the gelatine surface—the thin cover glass was next the moving photo¬ 
graphic plate and was only a small fraction of a millimetre distant from it. The 
breadth of the slit was about Yu mm. 
The carrier of the photographic plate was clamped to the middle portion of a wire 
stretched horizontally over two pulleys; a weight was attached to one end of the 
wire, while the other was attached to a piston, the motion of which in its cylinder 
caused oil to be driven through a fine hole in a brass disc. By turning the disc any 
one of a graduated series of holes could be brought into action according to the speed 
of travel desired. 
The light from the source of illumination—a paraffin lamp—could be cut off 
momentarily by means of a shutter which was worked by a cord from outside the 
hut. In this way it was possible to record on the photographic plate the times of 
the beginning and ending of thunder. In the records reproduced (Plates 2 to 5) 
these momentary interruptions of the illumination are represented by vertical black 
lines ; a single line indicates the beginning, a double line the end of a peal of thunder. 
The interval on the photographic record between the vertical portion of the trace, 
which represents the sudden change of field due to the passage of a lightning flash, 
and the dark line which marks the moment when the thunder resulting from the 
flash began to be heard, afford data for obtaining an estimate of the approximate 
distance of the discharge. 
