AND ON THE ELECTRIC FIELD OF THUNDERSTORMS. 
109 
the ground. A charged body suspended in the atmosphere under the conditions 
found by Wiegand at 8865 metres 'would lose about X U of its charge per second. 
Thus a charge of 20 coulombs in the head of a thunder-cloud at this height should 
lose more than 1 coulomb per second: to keep the charge constant the vertical 
current through the cloud would have to exceed 1 ampere. The presence of such a 
large charge would, it is true, not leave the conductivity of the surrounding 
atmosphere unaltered : it would tend to increase it by dragging down ions from 
upper layers of still greater conductivity. 
XIX. Electrical Currents Maintained in the Atmosphere by Thunder-clouds and 
Shower-clouds. 
Consider a cumulo-nimbus cloud of the type imagined in Section VIII. containing 
upper and lower charges—the latter being partly or, it may be, mainly carried by the 
rain below the cloud. Such a cloud may be regarded as an electric generator— 
whether essentially of the frictional type or of the influence machine type need not 
at present be discussed—capable of maintaining a potential difference between its 
poles of the order of Iff 1 volts. 
As pointed out in Section VIII. the potentials in the conducting layer of the upper 
atmosphere is likely to be insignificant in comparison with that in the head of a 
thunder-cloud, and the potential difference between them may thus be of the order 
of Iff' volts. 
There will be a flow of electricity along the lines of force belonging to the various 
groups enumerated in Section VIII. and indicated in fig. 5. The upper pole will 
continually be losing charge by currents flowing (l) to the lower pole; (2) to the 
earth’s surface (this portion of the current reaching the outer zone (Section VIII.) 
where the potential gradient is unlikely to reach high values); and (3) to the upper 
atmosphere. 
Unless the field in the shower-cloud approaches very near to the sparking limit, the 
conductivity within the cloud is likely to be small, since any ions liberated soon lose 
their mobility by becoming attached to cloud particles. The electrical resistance 
of the atmosphere between the upper pole of the cloud and the conducting layer of 
the upper atmosphere will be much less than that between the upper pole and the 
earth’s surface ; for the free ions will be dragged out of the conducting layer, and 
their mobility throughout the greater part of their course will greatly exceed that 
of the ions in the lower layers of the atmosphere. A large part of the current from 
the upper pole must thus go to the upper atmosphere. 
Consider now the lower oppositely charged pole of the cloud. Part of the charge 
is continually being neutralised by the direct return current between the poles, but 
this, as has already been pointed out, is likely to be small. The greater part of the 
charge lost by the lower pole will reach the ground. If no rain reaches the ground 
the loss of charge will be due to ions moving under the action of the electric field 
