2 Scientific Proceedings, Royal Dublin Socielij. 



vertical air-ciirreuts which are a well-ascei'tained feature of the thunderstorm. There 

 is no doubt that Simpson's theory holds the field, not only as being the sole theory 

 which will bear a moment's examination in light of the known facts, but as offering 

 a reasonable explanation of all the electrical phenomena, and being fully in accord 

 with modern meteorological ideas as to the nature of these storms. On only one 

 point may some hesitation be felt, and that is the qviestion of magnitudes. It may 

 be asked have we any evidence that sujBBcient breaking up of water-drops occurs 

 to account for the observed charges on thunderstorm rain and for the potentials 

 developed in the atmosphere ? 



In this connexion the question also arises as to whether sufficient breaking up 

 of drops occurs under non-thundersiorm conditions to account for the charges 

 observed on ordinary rain. Simpson' quotes the experimental value (1'36 e.s. units 

 per c.c.) mentioned above, and states that " if about yV of the rain was broken up 

 once into fine drops the observed charge would be produced" (on ordinary rain). 

 Under thunderstorm conditions, of course, repeated breaking up of the water can 

 be assumed, and by imagining a sufficient number of repetitions of the process the 

 charge density can be pushed as high as we please. But it must be realized 

 that, on the evidence so far available, in order that any quantity of water may have 

 a charge of the order of one e.s. unit per c.c., it must all be broken into very fine 

 drops (radius circa 6 x 10"' cm.). The amount of shattering whicli drops are likely 

 to experience, even in the highly disturbed conditions associated with a tlumdei'- 

 storni, is very small as compared with this complete pulverization. Some writers 

 go so far as to deny that shattering of the rain-drop can occur in nature.^ While 

 this, no doubt, is an extreme view, the experiments of Hochschwender, quoted by 

 Lenard,' tend to show that, except in the case of the largest drops, a very con- 

 siderable counter-acceleration is required to produce rupture. The question of 

 magnitudes, therefore, remains a difficulty. There is a gap between the amount of 

 charging produced in the laboratory corresponding to a certain degree of breaking 

 up and the amount of charge observed on I'ain — thunderstorm and ordinary — 

 considered in the view of the amount of breaking up it is likely to have experienced. 

 The experiments now described were undertaken with a view to a more complete 

 examination of the phenomenon in the hope of bridging this gap. The investiga- 

 tion was successful to this extent, that it indicated the possibility of obtaining 

 charge densities of an order ten times as great as those previously reported 

 corresponding to the same degree of breaking up of water. 



Experimental Methods. 



In work of this kind there is no difficulty about the electrical observations; 

 the charges produced are high and easily measured. But, as indicated in the 

 previous paper, the degree of breaking up of the water is always difficult of 

 determination. Drops covering a wide range of sizes are present in the water 

 from a sprayer, and while for some purposes of rough estimation it may be accurate 

 enough to calculate air average size by estimating the total number of drops and 

 the total volume of water, still it is clear that this method may in some cases 

 involve very great errors. Before entering on the details of the work, a preliminary 

 experiment which is of some interest may be described. It occurred to us that 



1 G. C Simpson, Phil. Mag., loc. cit. 



2 See J, Rey, " L'ionisation de I'air par les chutes d'eau," p. 73. Gautliier-Villars, 1912. 



3 P, Lenard, Ann. d. Physik, 15, 1921. 



