﻿Effect of Clouds on Ionization. 367 



Summary . 



(1) The problem investigated is the effect of suspended 

 particles upon the ionization current in air, the ionizing 

 agent being the y rays of radium. 



(2) As suspended particles the writers have utilized the 

 cloud particles formed when moist ionized dust-free air is 

 subjected to an adiabatic expansion. 



('6) This method, so far as we know, is the only method 

 (applicable to the problem) of producing repeatedly a set of 

 minute particles of constant number and of uniform size, the 

 number and size beino- readilv controllable. 



(4) The ionization current in cloud-laden air is smaller 

 than the current in cloudless air, the extent of the redaction 

 depending upon 



(a) the intensity of the electric field, i. e, the ionic 



velocities ; 



(b) the density of the cloud. 



A cloud falling at the rate of 1 cm. in 20 seconds diminished 

 the ionization current under an electric force of 2 volts per 

 cm. by 75 per cent., and that under an electric force of 

 120 volts per cm. by 10 per cent. 



(5) The denser the cloud the greater is the percentage 

 diminution in the current, but when the percentage diminution 

 reaches a certain value, of the order of about 75 per cent., 



. any further increase in the density of the cloud does not 

 result in any further reduction in the ionization current. 



(6) For this reason a cloud produced on equal numbers of 

 positive and negative ions his very little greater retarding 

 effect upon the current than the cloud produced on the 

 negative ions alone. 



(7) The number of nuclei of Wilson's foggy condensation 

 caught by an expansion greater than 1'38 is greatly reduced 

 by the presence of ions in the gas. 



(8) An investigation of the manner in which the effect of 

 a cloud upon the current varies with the age of the cloud 

 indicates that in the early stages after its formation a cloud 

 exposed to ionizing rays undergoes certain rapid changes. 



(9) Incidentally the experiments illustrate in a new way 

 the results obtained by C. T. R. Wilson, viz. : the catching 

 of the negative ions at an expansion of 1*25, positive ions at 

 1-31, and production of cloudy condensation in unionized air 

 at an expansion of 1'38. 



In conclusion we wish to express our thanks to Professor 

 Wilberforce for many valuable suo-gesfcions during the 

 progress or this work. 



Holt Physics Laboratory, 

 University of Liverpool. 



