434 Mr. H. A. Wilson : Determination of the Charge 



charged ones, and unless the expansion used was nearly 

 enough to produce a fog even in the absence of any ions, 

 the positively charged droplets were not very easy to observe. 

 There seemed, in fact, to be a large excess of negative ions 

 present between the disks and not equal numbers of positive 

 and negative ions. The explanation of this is, no doubt, to be 

 found in the secondary radiation emitted by brass under the 

 action of Rontgen rays. This secondary radiation has been 

 proved to consist of negatively charged corpuscles, so that 

 an excess of negative ions in the air near the disks might 

 have been expected. The excess would, however, not have 

 been expected to be as great as appeared to be the case. It 

 is perhaps possible that when both positive and negative 

 ions are present together, condensation takes place mainly on 

 the negative ions, although when either kind are present 

 alone, there is very little difference between the supersatura- 

 tions required to produce condensation on the positive or 

 negative ions. 



An expansion of from 16 to 17 cms. of mercury was always 

 used in the experiments described below, and all the results 

 are for the charge on the negath T e ions. 



All the droplets did not always fall at the same rate when 

 the electric field was applied. This appeared to be nearly 

 always the case, but was especially so when strong rays were 

 used. There appeared to be several sets of droplets, each 

 set falling all at the same rate. The rate of fall of the most 

 numerous set indicated that the droplets in it had the smallest 

 charges. The observations given below refer to this set only, 

 the other sets will be considered later. 



Since the cloud begins to evaporate soon after it is formed, 

 it is very important to get the measurement of its rate of fall 

 over as quickly as possible. 1 therefore generally only 

 allowed it to fall about half a centimetre, and applied the 

 electric field in the direction which increased the rate of fall. 

 Another reason why a very small distance between the disks 

 had to be used, was that the available P.D. was only 2000 

 volts, so that unless the disks were near together, the electric 

 field between them was not strong enough to appreciably 

 alter their rate of fall. For these various reasons nearly all 

 the observations were made with the disks as near together 

 as possible, because it was clear that reliable results could 

 not otherwise be obtained. For the same reason the maximum 

 P.D. available was used in nearly every case. It would of 

 course have been more satisfactory if observations could have 

 been made with a greater variety of distances between the 



