no 
MR. C. T. R. WILSON: INVESTIGATIONS ON LIGHTNING DISCHARGES 
of the cloud. If the normal rate of production of ions in the air below the cloud had 
alone to be taken into account, the current would be small; but we have to add 
the ions supplied by evaporation of charged drops falling from the cloud and those 
(of opposite sign) due to point discharges from earth-connected conductors, such 
as the leaves of trees or even the tips of blades of grass, under the action of the 
intense electric field of the central area below the cloud. If the rain reaches the 
ground the former of these sources of ionization is absent, but there is a further 
source of ionization in the splashing of the rain on the ground. In addition to the 
ionization current we have also the convection current carried to the ground by 
charged rain-drops. The total current between the lower pole of the cloud and the 
ground now consists of the convection current carried by the falling charged drops 
and the conduction current carried mainly by the upward stream of ions set free by 
point discharges and splashing at the surface of the ground. The ratio of the 
convection current to the conduction current will be less near the ground than 
higher up, since the falling drops will lose more and more of their charge as they 
penetrate farther into the stream of upward moving oppositely charged ions; these 
again as they are carried upwards by the electric field are continually diminished in 
number by union with the drops. The greater the supply of ions from the ground 
the smaller will be the charge retained by the drops ; if the current carried by the 
upward stream of ions is sufficient, the drops may lose the whole of their charge or 
even have it reversed before they reach the ground. The charge carried to the 
ground by rain-drops is thus by no means necessarily a true measure of the vertical 
current in a shower : nor does the sign of the charge carried by the drops when they 
reach the ground necessarily indicate the sign of the current between the ground and 
the base of the cloud. 
Thus a large part of the current from the upper pole of a cumulo-nimbus cloud 
is likely to reach the conducting layers of the upper atmosphere, while that from 
the oppositely charged lower pole goes mainly to earth. A current is thus 
maintained from the earth through the cloud to the upper atmosphere or in the 
reverse direction according to the sign of the polarity of the cloud. 
Discharges between the ground and the lower pole of the cloud and between the 
upper pole and higher portions of the atmosphere contribute to the total current 
between the ground and the upper atmosphere ; discharges between the two poles or 
between the upper pole and the ground diminish the electric field which maintains 
the vertical current without contributing anything to the current. 
I 
XX. Differences Between the Electrical Effects of Shower-clouds of Positive and 
Negative Polarity. 
We may define the polarity of a shower-cloud as being positive when the upper 
charge is positive, negative when the upper charge is negative, the current through 
it being upward in the former case, downward in the latter. 
