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



Curves I. and II. shows clearly that the effect of the clond 

 diminishes with increasing fields, i. e., with increasing ionic 

 velocities ; thus the diminution of the current by the cloud is 

 79 per cent under 2 volts, and only 10 per cent under 240 volts. 

 Curve III., lying all the way above Curve II., givos the 

 currents through the clouds obtained on expansion with the 

 field on. 

































400 









































8 



















I 



300 



























«, 



r 







/ 



/1 

























§200 





// 



























C3 



o 



/ 



1 / 



























100 



/ 































\ 































f 





























a 



2£0 



W 



80 120 180 200 



VOLTAGE ON IONIZATION-CHAHBLR. 



It will be convenient at this stage to discuss the cause of 

 this marked diminution of the ionization current by the 

 cloud. 



The effect upon ionization of foreign particles in the gas 

 was first observed by R. B. Owens *, who found that 

 introducing tobacco smoke into the ionized air greatly 

 reduced the ionization current. More recently A. S. Eve f 

 has studied, by means of an Ebert instrument, the effect of 

 dust and smoke upon the ionization produced in air by the 

 7 rays of radium. J. J. Thomson J explains the effect of 

 dust or smoke as follows : — " The effect produced by dust is 

 easily explained, as the dust particles are in all probability 

 very large compared with the ions ; thus if a positive ion 

 strikes against a dust particle and sticks to it, it forms a 

 large system which is much more likely to be struck by a 

 negative ion and neutralized than if the positive ion had 



* Phil. Mag. OcL 1899. 

 t Phil. Mag. May 1910. 

 + ' Conduction of Electricity through Gases.' p. 20. 



