2\n 



Mi;. C. T i;. WILSON ON TI1K KKFiriKNCY AS CONDENSATION 



of the negative ions when v. t /v l amounts to T25, practically all the negative ions are 

 caught when r z /r, exceeds 1'28 ; while to make water condense on any of the positive 

 ions t'j/t'i must exceed alxnit 1 '-'51 ; all the ions positive and negative Keing caught 

 when 2 /y, exceeds 1'35. 



Experiments were no\v made in which the expansion did not take place till after 

 the rays had l>een cut oft'. 



The ions which are l>eing attracted from the ionised layer to the lower plate, have 

 at the most ahout :3| millims. to travel, while the upward moving ones have from 17 

 to 20 millims. to travel. One would expect, therefore, alnmt six times as long a time 

 to be taken for the removal of the latter as is required for the removal of the former. 

 There is thus a considerable range of time available for making the expansion, so that 

 the majority of the ions of one kind shall still be present, while all the ions of the 

 other kind have reached the lower plate. Using one LECLANCHE cell to maintain the 

 electric field between the plates, an interval of about 1 second between cutting off 

 the rays and making the expansion was found to Ije suitable. 



The results were in agreement with those previously obtained ; the drops, when 

 the field was such as to give positive ions and v. i jv l was less than 1'31, were now no 

 more numerous than if the expansion were made without exposure to the rays at all. 

 The method, therefore, enables us to obtain ions entirely of one kind. 



To test to what degree accidental variations in the time allowed to elapse between 

 cutting off the rays and making the expansion could affect the result, some 

 experiments were made in which this interval was varied. A metronome, giving 90 

 ticks per minute, was used ; the interval before the expansion was varied by 

 switching off the current from the coil, as the metronome made a tick, and pulling 

 the trigger of the expansion apparatus at the moment of the first, second, or any 

 subsecpuent tick thereafter. The following results were obtained, one LECLANCHE 

 cell being used to maintain the field. The expansion was such as to catch negative 

 ions only. 



(1) NEGATIVE Ions moving upward. 



