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MR. C. T. R. WILSON : INVESTIGATIONS ON LIGHTNING DISCHARGES 
The potential gradient (negative) in the central area below the cloud of positive 
polarity will be greater than the positive potential gradient in the corresponding area 
below the cloud of negative polarity, the central positively charged area below the 
cloud of positive polarity being also larger than the negatively charged area below 
the cloud of negative polarity. Again, the positive potential gradient at the ground 
in the outer zone will be less (on account of the smaller charge on the upper pole) in 
the case of the cloud of positive polarity than the negative potential gradient in the 
corresponding region due to the cloud of negative polarity. Thus in each area 
negative potential gradients tend to be greater than positive. 
The electric field in the central area below the lower pole being stronger in the case 
of the cloud of positive polarity, the current carried by the stream of positive ions 
from the ground will be increased, and therefore also the tendency to neutralisation 
or reversal of the negative charge on the falling rain-drops.. 
If lightning discharges occur, they are more likely to pass between the ground and 
either the upper or the lower pole if this is negative than if it is positive, since the 
charge of the pole is greater when negative. Thus discharges carrying positive 
electricity from the earth to the atmosphere will be more frequent than negative 
discharges. Discharges will tend to occur especially between the ground and the 
upper, negative, poles of clouds of negative polarity and the lower, negative, poles of 
clouds of positive polarity. In the latter case the discharges are an additional source 
of loss or reversal of the negative charge on falling rain-drops. 
Essentially similar results are reached if, instead of assuming the same potential 
difference,to be maintained between the poles, whether the clouds are of positive or 
of negative polarity, we assume that the same vertical current is maintained in 
both cases. 
Thus, if we assume that shower-clouds may have polarity of either sign, the 
differences in the mobilities of the positive and negative carriers of electricity in the 
higher portions of the atmosphere will account for the preponderance in showers : 
(l) of negative potential gradients ; (2) of upward or positive lightning discharges ; 
and (3) of positively charged rain. It also affords (4) a possible explanation of the 
normal positive potential gradient of fine-weather regions. 
XXI. The Normal Potential Gradient and Air-eartli Current of Fine Weather. 
A thunder-cloud or shower-cloud is the seat of an electromotive force which must 
cause a current to flow through the cloud between the earth’s surface and the upper 
atmosphere. In the case of thunder-clouds the records of the changes produced in 
the electric field by the passage of lightning flashes give us means of forming some 
estimate of the magnitude of such currents, and it would appear from them that the 
current through a few square kilometres of the surface of the ground below a 
thunder-cloud may amount to some amperes. In shower-clouds in which the 
