202 CONDENSATION NUCLEI. 



tically ions of only one kind present, those, namely, which are mov- 

 ing toward tlie upper phite. 



In tliis way it has I)een found that in order that water may con- 

 dense upon them to form visible drops the negative ions require an 

 expansion i'Ji-\ =■ 1.25, the positive an expansion \.?)\^ the corre- 

 sponding supersaturations being fourfold and sixfold, respectively. 



When ions of both kinds are present in approximately equal num- 

 bers, it is often possible to observe a difference in the density of the 

 resulting cloud according as the expansion is below or above the limit 

 corresponding to the degree of supersaturation necessary for the con- 

 densation of the positive ions. The increase in density was first 

 described by J. J. Thomson, and it was suggested by him that it 

 might be due to a difference between the positive and negative ions 

 in their efficiency as condensation nuclei ; he pointed out that such a 

 difference, if established, woidd have important bearings on the sub- 

 ject of atmospheric electricity. For an electrical field might be 

 expected to result in ionized air if such a degree of supersaturation 

 was reached that condensation took place on ions of one kind only, 

 these loaded ions being then carried down by gravity. That the 

 drops produced under these conditions are actually negatively 

 charged, as was to be expected from the greater efficiency of the 

 negative ion as a nucleus, was proved by H. A. Wilson, by observing 

 the movement of the drops in a strong electric field applied after their 

 formation by expansion. 



CHAROE CARRIED BY THE IONS. 



The most important use which has been made of the fact that ions 

 act as nuclei for the condensation of water vapor has been in the 

 determination of the quantity of electricity carried by each ion. Two 

 entirely different methods have been employed, botli recjuiring the 

 use of the expansion ap])aratiis. In the first, that of J. J. Thomson, 

 a measurement of the leakage of electricity through the air of the 

 cloud chamber allows >i, e, the product of the immber of ions and the 

 ionic charge, to be measui'ed ; «, the number of the ions is given by the 

 iniml)erof thedi'ops. The number has been obtained, not by direct 

 counting, but from a knowledge of the (|uantity of water condensed, 

 and the size of the drops as obtained from their rate of fall. The 

 second method (used by II. A. Wilson) in its simplest form reduces 

 itself to a determination of the strength of the electric field necessary 

 to maintain in sus})ension tlie drops condensed upoii the ions. We 

 then have F (\ the product of the strength of the field and (he charge 

 on the drop, equal to its Aveight. The size of the dr()i)s, and hence 

 their weight, is again deduced from the rate of fall in the absence of 

 the field. 



