NMVLEI OK POSITIVELY A\I> NEGATIVELY CIIAKCKI* IONS 



295 



The negative inns tlnis take l>et\veeii :5 ;mcl J seconds t<> travel from the ionised 

 laver to the upj>er plate, and a large proportion of them are still present more than 

 2 seconds after turning off the rays. The field was now reversed. 



(2) NEGATIVE Ions moving downwards. 



All the negative ions have now reached the lower plate in two-thirds of a second 

 after turning off the rays. That the positive ions are still present was proved by 

 increasing the expansion to 1'34 (which is sufficient to cause water to condense 

 mi them also); a fog was now obtained when the interval was 1^ seconds. The 

 actual time taken for the positive ions to disappear was not measured, but as 

 ZELENY'S experiments show that they move more slowly than the negative, they 

 would doubtless have been found to take at least as long as the negative to travel to 

 the upper plate. 



The above results show that any accidental differences in the length of the time 

 between cutting off the rays and making the expansion are of little importance 

 when the interval amounts to about 1 second. 



The time taken for the ions to 1 removed, when they have to travel to the upper 

 plate, is somewhat longer than the time we obtain by calculation, using RUTHER- 

 FORD'S* value for the velocity of the ions. The time calculated in this way amounts 

 to about 2 seconds. The want of uniformity in the field is sufficient to account 

 for the actual time being longer than the calculated. 



The following tables contain the results of observations in which the expansion 

 was effected ulxmt 1 second after cutting off the supply of Rontgen rays. 



* Phil. Mag.,' vol. 44, p. 422, 1897 



