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XXXI. On the Effect of Clouds on Ionization. By GrWiLYltf 

 Owen, M.A., I).Sc, and Joseph fi. T. Roberts, 31. Sc, 

 University of Liverpool*. 



DURING- some experiments on the electrical conductivity 

 of air containing certain vapours, we noticed that the 

 production o£ a cloud in the gas by an adiabatic expansion 

 caused a marked diminution in the ionization current. It 

 occurred to us that this expansion method of producing mists 

 and clouds would afford a reliable means of investigating the 

 effects of clouds of different densities upon the conductivity 

 produced in a gas by a constant ionizing source. 



In order satisfactorily to investigate the effect on the 

 conductivity of a gas of the presence in it of foreign particles 

 in suspension, it is essential that means be found 



(1) for producing repeatedly a set of particles of constant 



number and of uniform size ; 



(2) for varying the number and size of the particles at 



will. 



These means are readily secured by using as suspended 

 particles either the charged water-drops formed when moist 

 ionized dust-free air is subjected to an adiabatic expansion of 

 magnitude greater than 1/25, or the uncharged water-drops 

 formed when ordinary moist dust-free air is subjected to an 

 expansion greater than 1*38 t» 



Now since it is impossible in practice to study the ionization 

 current through the air at the instant of the formation of the 

 cloud, it seems likely that both types of cloud (viz., those in 

 ionized and those in normal air) will produce more or less 

 similar effects. For if a cloud be uncharged initially, it will 

 very soon contain under the action of the ionizing rays a fair 

 proportion of charged drops of both signs, while the cloud 

 which is charged to start with will very soon contain a 

 certain proportion of uncharged drops. Hence a cloud 

 obtained in this way, in addition to possessing the important 

 advantages mentioned above, is quite representative of any 

 set of suspended particles such as dust, smoke, mist, fog, &c, 

 which have to be considered in their bearing upon the 

 phenomena of atmospheric ionization. 



Experimental arrangements. 



A general idea of the final form of our apparatus is given 

 by the figure. A represents the metal ionization-chamber 

 connected as shown to a Wilson expansion apparatus. By 

 connecting A to one terminal of a battery of small cells, the 



* Communicated bv the Authors. 



t C. T. K. Wilson/ Phil. Trans, clxxxix. p. 265 (1897). 



