AND ON THE ELECTRIC FIELD OF THUNDERSTORMS. 
Ill 
It was first proved by Simpson, # and has been confirmed by many observers, that 
rain on reaching the earth’s surface is much more often positively than negatively 
charged. This, as we have seen, does not necessarily imply that shower-clouds are 
always or even prevailingly of negative polarity. It is therefore of interest to 
consider some of the differences to be expected between the electrical effects of clouds 
of positive and of negative polarity. 
Recent experiments have shownt that the carrier of negative electricity in 
hydrogen, helium and nitrogen even at atmospheric pressure is the free electron, and 
that its mobility is some hundreds of times that of the carrier of positive electricity, 
the positive ion. In ordinary atmospheric air, as the pressure is reduced, the 
average mobility of the carriers of negative electricity increases relatively to that 
of the positive ions ; quite an appreciable proportion of the negative carriers, 
consisting, according to Wellisch,| of free electrons when the pressure is reduced to 
8 cm. of mercury, the proportion increasing rapidly as the pressure is further reduced. 
Thus, while the carriers of positive electricity dragged out of the conducting upper 
atmosphere by a cloud of negative polarity consist of ordinary ions, the negative 
carriers dragged down by a cloud of positive polarity are originally to a large extent 
free electrons, and a considerable proportion are likely to remain in this condition till 
quite moderate elevations are reached. The conductivity of the air between a shower- 
cloud and the upper atmosphere will thus be considerably greater if the cloud is 
of positive than if it is of negative polarity. 
Let us compare two shower-clouds which differ only in the sign of their polarity 
and consider the effect of the greater conductivity of the atmosphere above the cloud of 
positive polarity. Let us suppose that the two clouds act as generators capable of main¬ 
taining equal potential differences between their poles. Let V 2 , — Yjbe the potentials 
of the upper and lower poles of the cloud of positive polarity, and — V 2 \ +Vd the 
potentials of the upper and lower poles of the cloud of negative polarity, let V 2 —Vj = 
Vf—Vs 1 . Then the current from the ground to the upper atmosphere maintained 
by the cloud of positive polarity will be greater than that from the upper atmosphere 
to the ground maintained by the cloud of negative polarity, since the total resistance 
of the circuit is less in the former case. 
The ratio V 2 /Vi is less than Vf/Vd, the upper and lower potentials being proportional 
to the resistance of the portions of the circuit above the upper and below the lower 
pole respectively^ Thus V/ is greater than V 3 and is greater than Vf ; in other 
words the potential (and charge) of both the upper and the lower pole is greater 
when negative than when positive. 
* Simpson, loc . cit . 
t FPvANCK and Pohl, ‘ Verhandl. Deutsch. Physik. Gesellschaft,’ 9, p. 69, 1907. 
I Wellisch, ‘ Pkil. Mag.,’ vol. 34, p. 33, 1917. 
§ The potential of the conducting layers of the upper atmosphere is assumed to remain small in 
comparison with the E.M.F. of the thunder-cloud. 
