on the Ions produced in Air by IRontgen Rays. 43.3 



put up close to the apparatus having a vertical slit in it 

 through which only the central portion of the illuminated part 

 of the cloud could be seen. This slit and the beam of light 

 were each about half a centimetre wide, so that the portion 

 of the cloud which was observed was that occupying a vertical 

 prism half a centimetre square at the axis of the tube AB 

 between the disks. 



The disks were never more than one centimetre apart, and 

 consequently very little circulation of the air could take place 

 between them. When a cloud is formed by expansion in a 

 large vessel, the walls of the vessel heat up the air near them 

 which produces a circulation of the air upwards near the 

 walls and downwards in the middle. If this sort of thing 

 happened in these experiments, v x and v 2 would both be 

 obtained too high ; but it was found that when the disks 

 were not more than a centimetre apart the circulation which 

 occurred near the glass walls of the tube did not extend to 

 the centre, and the surface of the cloud between the disks 

 remained plane as the cloud fell. 



The disks were always connected together until the expan- 

 sion had taken place, when, if it was desired to determine v 3 , 

 they were immediately connected to the battery by turning 

 the commutator lever. If they were connected to the battery 

 before the expansion took place no cloud was obtained because 

 the field removed the ions as fast as they were formed. 



In making a measurement of e the time of fall from the 

 upper disk to the lower one was measured with a stop-watch, 

 alternately with and without the electric field. i\ and v 2 

 were then calculated from the mean results for the times of 

 fall. 



C. T. R. Wilson (Phil. Trans. A, 1899, p. 440) found that 

 with an expansion of nearly 15 cms. of mercury only the 

 negative ions acted as nuclei, but with greater expansions 

 condensation occurred on both positive and negative ions. 

 These results were easily verified with my apparatus. With 

 an expansion of 15 cms. charging the upper disk negatively 

 caused the whole cloud to fall more quickly than it fell when 

 the disks were uncharged, while charging the upper disk 

 positively reduced the rate of fall of the cloud. It was clear, 

 therefore, that the droplets were negatively charged. 



Also with considerably larger expansions than 15 cms. 

 some of the droplets fell more slowly and some more quickly 

 when the disks were charged than when they were not 

 charged, showing that both positively and negatively charged 

 droplets were present. However, there always seemed to be 

 more negatively charged droplets present than positively 



