88 
MR. C. T. R. WILSON: INVESTIGATIONS ON LIGHTNING DISCHARGES 
The vertical electric force at a point on the ground is given by 
F 
2QH _ 2Q 
L 3 1 + 
H 
L 2 
2X.3 
H 2 1 + 
JJ_\i 
IF 
2QH 20 
becoming 3 and ~ when L is large and small respectively compared with H 
±j ri 
where L is the distance of the point from the axis pp'. The curve I in fig. 3 represents 
