﻿362 Dr. G. Owen and Mr. J. H. T. Roberts on the 



less the further reduction due to a marked increase in the 

 number of cloud particles. 



Table IV. gives the results of observations made on the 

 current under a potential of —16 volts through the showers 

 and clouds obtained in normal air at different expansions. 



Table IV. 



Volts on Tube -16. 



Kadiuin absent when expansion was made, and Ihen 

 immediately placed in position B^ (strong ionization). 





Current 



O-irrent 







Expansion y- • 



before 

 expansion. 



after 

 expansion. 



C 2 . 



Observation. 



° 2 xioo. 



1 



1-300 



248 



248 



Few drops. 



100 



1-360 



239 



238-5 



Thin shower. 



100 



1370 



238-5 



233 



Heavy shower. 



97-9 



1-380 



236-5 



220 



Very dense shower. 



93-3 



1-388 



235 



196 



Cluud . 



83-4 



1-395 



234 



187 



Dense cloud. 



79 9 



1-406 



233 



106 



Fog. 



455 



1-420 



231 



81 





35-0 



1-436 



228 



73 



., 



32-0 



1-485 



226 



65 



" 



28-8 



In these experiments the radium was hidden behind a thick 

 lead screen when the expansion was made, and was brought 

 into position E a immediately afterwards. The interval 

 between the expansion and the unearthing was still 3 seconds, 

 but after some practice it was found that the radium could 

 be brought into the required position w r ell before the 3 

 seconds were up. The numbers in the last column are shown 

 plotted against the expansions in Curve III., fig. 3. From 

 this it is seen that a " shower " has very little effect upon the 

 current, consequently the curve only begins to drop at an 

 expansion of about 1*37. At an expansion of 1*385, however, 

 there is a decided increase in the slope of the curve ; this is 

 of course where the cloudy condensation commences. The 

 sudden increase in the density of the cloud now produces a 

 very large reduction in the current, but as is seen from the 

 curve this reduction does not proceed indefinitely, attaining 

 rather a saturation value at an expansion of about 1'45. 



Again, the marked diminution in the current at an ex- 

 pansion of 1'38, and also an inspection of the cloud itself, 

 suggested that the cloud obtained at an expansion greater 



