682. Mr. C. T. R. Wilson on the Condensation Method 
Further experiments showed * that the number of drops 
produced by expansions between the above-mentioned limits 
in air exposed to Rontgen rays, is reduced in a very strikiag 
manner when a sufficiently strong electric field is maintained 
across the air before expansion, thus proving that the nuclei 
move in an electric field and are therefore electrically charged, 
and presumably identical with the ions to which the con- 
ducting power is due. On the other hand, similar experiments 
made in the absence of ionising agents failed to show any 
diminution of the number of drops by the action of even very 
strong fields. The absolute identity of the degree of super- 
saturation required to cause condensation upon ions and upon 
the nuclei to which the rainlike condensation is due, made it 
difficult to believe that the latter are not ions also, and to 
explain their non-removal by an electric field it was suggested 
that they might be ions produced in some way as a result of 
the expansion. When, however, subsequent experiments + 
on the leakage of electricity from conductors suspended 
within closed vessels, showed that a continual slight ionisation 
of the air is always going on in such vessels, it appeared 
more likely that the rainlike condensation really is due to 
this ionisation, and that the failure to detect any diminution 
in the number of drops under the action of an electric field is 
due to some defect in the conditions of the experiments. In 
the experiments thus far made the vessels used had been small, 
and to permit of a strong electric field being applied the air 
was enclosed between conducting surfaces generally only a 
centimetre or less apart ; in many cases one of the conductors 
was a layer of water at the bottom of the vessel, the other 
being a horizontal metal plate coated with wet filter-paper. 
The drops were under these conditions very few whether an 
electric field was applied or not; it was thought that if a 
much larger volume of air were used there would be more 
chance of detecting the diminution in number when an electric 
field was applied. This expectation has been realized. With 
the large apparatus described below the effect of an electric 
field in removing the nuclei which gave rise to the rainlike 
condensation is very striking. 
The construction of the apparatus { (shown in the figure) 
is the same in principle as in the experiments on condensa- 
tion nuclei which I have described in previous papers. On 
* Phil. Trans. vol. excil. p. 403 (1899). 
+ Geitel, Physikalische Zeitschrift, vol. ii. p. 116; C. T. R. Wilson, 
Roy. Soe. Proc. vol. Ixviii. p. 151. 
{ The apparatus was made by Messrs. W. G. Pye & Co., Cambridge. 
To Mr. Pye Iam indebted for many suggestions as to the mechanical 
details. 
