6 Dr. G. C. Simpson on the 



can only be reversed when the excess of positive electricity 

 which is normally in the air has been replaced by an excess 

 or! negative electricity. We have just seen that the influence 

 effect still further increases the positive electricity in the- 

 air ; therefore the effect which reverses the field must 

 remove positive electricity from the air faster than the rain 

 leaves it behind. Elster and Geitel have realized this 

 difficulty, and say " We very much doubt whether the 

 earth's normal field must always be considered as the exciting 

 field, (a) Selective absorption on the precipitation particles 

 during and after the condensation, and (b) the breaking of 

 drops in the air as investigated by Simpson, act as electro- 

 motive forces, (c) Further, the shifting of electrified cloud 

 masses by wind, and (d) lightning discharges in the later 

 stages, could cause electrical fields which are quite inde- 

 pendent of the earth's normal field " *. 



We can neglect (cT) as we are at present considering the 

 initial stages. At the stage we are considering the cloud 

 masses will be positively charged by the influence effect, 

 hence (c) may be neglected. If (a) and (b) are sufficiently 

 efficient to produce rain so highly charged with positive 

 electricity as to overbalance the rain charged negatively by 

 influence, then they should be considered as the chief cause 

 of the electricity of precipitation. 



It appears to me useless to explain the general electricity 

 of rain by one process, and then to have to introduce a more 

 powerful effect to explain the large changes in the electrical 

 field which accompany it. 



This consideration, which is damaging to the theory with 

 non-thunderstorm rain, becomes more important with 

 thunderstorms. ,• The most marked feature in a thunder- 

 storm is the rapid reversal of the high electrical field. It 

 does not appear helpful to explain the processes at work by 

 an effect which under no circumstances can reverse a field. 

 The mistake made by Elster and Geitel is that they have 

 inverted cause and effect. They make the electrical field 

 the cause of the charge on the rain, while in reality it 

 is the charge on the rain which gives rise to the electrical 

 field. 



One could go much more fully into the failure of Elster 

 and Geitel's theory, but enough has been said to prove it 

 unsatisfactory. The theory will die a natural death if it can 

 be shown that there is another which fits the facts better. 

 We will therefore now discuss the alternative theory. 



* Elster & Geitel, Phys. Zeit. xiv. p. 1290 (1913). 



