626 Dr. G. C. Simpson on the Wilson-Gerdien 



a second discharge could not take place until the stress o£ 

 30,000 volts per cm. had again been set up by a continuation 

 of the process under discussion, and that would take, as we 

 have seen, nearly another hour. Thus, i£ the process were 

 the simple one we have considered, lightning flashes could 

 only take place from any given area of the condensation 

 layer at intervals of about an hour each. 



But we must consider if it would not be possible, after the 

 first discharge had taken place, lor any other action to come 

 into play which would increase the rate at which the potential 

 gradient could grow, and so make it possible for more rapid 

 discharges to take place. 



Gerdien has suggested a "far stronger and more rapidly 

 acting source " of ions than natural ionization. He says*: — 



" Such a source must in fact come into action as soon as a 

 single negative ion, within the strong field caused by the 

 separation of the positive and negative ions, is able freely to 

 traverse the 'ionization difference of potential/ and so in- 

 troduce a disruptive discharge within the field f. In this 

 case ionization through ion impact takes place, and in a very 

 short time a degree of ionization is caused which exceeds the 

 ordinary ionization several million times. A part of these 

 newly formed ions will be drawn by the field into the discharge 

 current, and so neutralize a part of the charge separated by 

 the condensation ; the greater part will, however, disappear 

 through recombination. Nevertheless, for a considerable 

 time after the current has passed, a degree of ionization must 

 remain within the region traversed by the discharge, which 

 by far exceeds that present under normal conditions. Thus 

 each disruptive discharge will create a large number of ions 

 which the condensation process can immediately separate so 

 long as the ascending current is present to provide the 

 necessary energy." 



But this reasoning is not so conclusive as it would appear 

 on first sight. The discharge considered must take place 

 either upwards or downwards from the condensation layer. 

 In the former case the production of an infinite number of 

 ions would be of no use, for in the region above the conden- 

 sation layer the air is not supersaturated to a fourfold decree, 

 and so there would be no tendency for water to be deposited 

 on negative ions, no matter how numerous they were. On 

 the other hand, if the discharge took place downwards from 



* Gerdien, I c. p. 662. 



t " An irgendeiner Stelle ein negatives Ion die Ionisierungsspannung 

 frei durchlaufen kann und damit die selbstandige Stroinung innerhalb 

 des Feldes einleitet." 



