308 Mr Vincent, On the Action of 



was attached to the outside of the cloud chamber in the position 

 indicated in the lower part of the figure. To render the cloud 

 visible an arc light A was placed in the same horizontal plane 

 as the spark and the cloud chamber, and a glass lens LL focussed 

 the light where desired. The tap T served to introduce air or 

 other gases into the chamber, and the rubber pipe P enabled 

 the depth of the water W to be regulated. This pipe was 

 connected to a water reservoir (not shown in the figure), which 

 could be raised or lowered so as to subject the air in the chamber 

 to successive expansions. In this way all dust particles could 

 be readily removed from the air in the chamber. The effective 

 height of the chamber could be varied by allowing different 

 depths of water to stand in it. By connecting the top and 

 the bottom plates D to the two terminals of a battery of small 

 accumulators an electric field of known strength could be ob- 

 tained. 



An arc light was first employed as a source of ultra-violet 

 rays, but this Was not a satisfactory method. Only a very long 

 arc taking a large current would produce the clouds. The intro- 

 duction of aluminium into the arc lessened the time required 

 to produce the cloud but no such effect followed the introduction 

 of iron. The clouds were generally obtained in a few seconds 

 using the aluminium spark, although this was not always so. In 

 fact the production of the cloud seems to depend on some circum- 

 stances which are not under control. Sometimes the cloud could 

 not be obtained at all. This was at first attributed to the forma- 

 tion of a film of dirt on the quartz window but the removal and 

 cleaning of the window was not universally effective. 



When the cloud is subjected to electric forces while the 

 ultra-violet rays are still shining on it the droplets can be seen 

 to move. A convenient intensity of field is 200 volts per cm. 

 Under this force the particles exhibit a brisk motion, some moving 

 in one direction, others in the other, while some remain stationary. 

 If this be allowed to continue for a minute or so and then the field 

 reversed, the whole cloud gathers towards the middle of the 

 chamber then spreads out again, the upper and lower portions 

 having crossed over the space occupied by the uncharged particles. 

 There is no doubt that there are three sets of droplets charged 

 positively, charged negatively, and uncharged. 



If a similar test is applied to a small isolated patch of cloud 

 from which the ultra-violet light has been cut off as soon as the 

 cloud is visible, it can be split up into three parts and the two 

 outer portions can be made to cross over the space occupied by 

 the uncharged cloud. The field used in this experiment must be 

 much weaker than before. 



Attempts to determine the charges on the drops failed owing 



