98 
MR. C. T. R. WILSON: INVESTIGATIONS ON LIGHTNING DISCHARGES 
ground, (cl) the upper charge of the cloud and the conducting layer of the 
atmosphere. 
If uniform stratiform conditions over a wide area be assumed, the conditions are 
simpler than in the case of the cumulo-nimbus cloud. The field at the ground below 
such a cloud, if the effects of the conducting layer be ignored, would be the difference 
between the fields due to the upper and lower charges, and its sign would be that of 
the field due to the larger charge. The effect of the conducting layer, as in the case 
of the cumulo-nimbus cloud, is to reduce the potential gradient produced at the 
ground by the upper charge of the cloud : firstly by the action of the opposing field 
of the charge induced on the conducting layer above the cloud, and secondly by the 
actual diminution of the cloud charge by the ionization current from the conducting 
layer. 
IX. Conditions Determining Discharge. 
In order that a lightning discharge may begin, it is clear that the electric force 
must somewhere exceed the sparking limit, which amounts at the ordinary 
atmospheric pressure to about 100 electro-static units or 3,000,000 volts per metre; it 
is not necessary that the electric force along the whole length of the path of discharge 
should previously have approached the sparking limit. As Larmor has pointed out,* 
if we suppose that an initial discharge occurs along a narrow line of length 
equal to the distance (possibly very small) over which the sparking value of the 
electric force was originally exceeded, and that this approximately equalises the 
potential along its path, there will be concentration of charge and intense local fields 
at the ends of this line; the discharge will thus be lengthened. The conditions are 
in fact momentarily much the same as if a conducting wire were placed along the 
path of this initial discharge. The maximum value of the electric force at the ends 
of the conducting track of the initial discharge will thus greatly exceed the critical 
* Sir Joseph Larmor and J. S. B. Larmor, ‘ Roy. Soc. Proc.,’ A, vol. 90, p. 312, 1914. 
