﻿Effect of Clouds on Ionization. 359 



remained free ; in this way the presence of dust will facilitate 

 the recombination of the ions." 



On this view, therefore, the number of free ions present 

 at any instant in dusty or smoky air is less than in clear air. 

 Eve, on the other hand, suggests a different explanation, 

 namely, th it the presence of dust, smoke, and other centres, 

 charged or uncharged, in the air causes a transformation 

 from small mobile ions to large ions of very low mobility. 

 In this way " the total number of ions present may be 

 increased while the conductivity may be decreased." Eve's 

 explanation is supported by the fact that the ionization 

 current in his experiment was found to be increased by the 

 presence of smoke under strong electric fields (3000 volts 

 per cm.), whereas the smoke decreased the current under 

 moderate fields (600 volts per cm.). 



In our experiments, however, the maximum electric force 

 was only about 120 volts per cm. and was, therefore, far too 

 small to move appreciably in 3 seconds the cloud-particles 

 even if they were multiply-charged. Thus the currents 

 through the clouds must have been carried by small unloaded 

 ions, and consequently the diminution of the current by the 

 cloud must be due partly to the water-drops acting as 

 mechanical obstacles to the motion of the ions and partly to 

 the more rapid recombination of the ions or to their re- 

 placement by large ions of small mobility. The potential at 

 our disposal was too small to enable us to decide between 

 these two alternatives. 



II. The effects of clouds of various densities upon the 

 current under a constant electric field. 



Clouds of various densities can be readily obtained by three 

 different methods : — 



(a) by subjecting air under the action of a constant 



ionizing source to expansions of different magnitudes; 



(b) by subjecting normal air to expansions of different 



magnitudes, in which case we are dealing with 

 Wilson's " rain-like condensation^ or "cloudy con- 

 densation " according to the magnitude of the 

 expansion ; 



(c) by subjecting air to a constant expansion under ionizing 



rays of different intensities. 



In our present experiments we have adopted methods (a) 

 and (b) only, reserving (c) for future investigation. 



The results obtained according to method (a) are shown 

 in Tables II. and 111. 



