804 



MR. C. T. R. WILSON ON THE EFFICIENCY AS CONDENSATION 



secondary cells, between the central plate and one of the side plates, the other being 

 connected to the central plate, which was earthed. In one half, therefore, there was 

 a strong electric field, in the other half, none. A lens was used to examine simul- 

 taneously the drops formed on the two sides of the central plate. Any difference in 

 the number of the drops could thus readily be detected. 



In the strong field the maximum length of life of an ion will be the time taken to 

 travel from one plate to the other under the action of the field ; in the present case, 

 a distance of 8 millims., with a potential gradient of 320 volts in 8 millinis., or 

 400 volts per centimetre. The velocity of the ions produced by X rays would in such a 

 field be about 400 X 1 '6 centim. per second, if we take RUTHERFORD'S value of the 

 velocity for a potential gradient of 1 volt per centimetre. The 8 millims. would there- 

 fore be traversed in '8/(400 X 1*6) = 1/800 of a second. (The time would really be 

 somewhat longer on account of the plates being too small to give a sufficiently 

 uniform field.) Now the average length of life of the ions due to Rontgen rays when 

 they are destroyed by recombination alone is of the order of 1 second.* The fewer 

 the ions also the less rapid is the rate at which they recombine ; we would, therefore, 

 expect the average life of the very few nuclei with which we are now concerned, if 

 they really are ions, to be at least as long as 1 second in the absence of any electric 

 field. If then we have here simply a case of spontaneous ionisation due to molecular 

 encounters, we ought to obtain something like 800 times as many drops without the 

 field as with it. In similar experiments t made with the ions produced by Rontgen 

 or uranium rays, much weaker fields were in fact found sufficient to prevent almost 

 completely the production of fogs by expansion. 



In the experiments now made without external ionising agents, not the slightest 

 difference could be detected between the appearance of the showers on the two sides 

 of the apparatus. All degrees of expansion from Vj/Vj = 1'25 to v^/v, = 1'38 were 

 tried. 



If then we have here to do with a case of ionisation, it differs completely from the 

 iouisation produced by Rontgen rays. 



Very similar nuclei, requiring practically the same supersaturation to make water 

 condense on them as the ions, are produced by the action on moist air of sunlight and 

 of weak ultra-violet light. Former experiments J showed that the nuclei produced by 

 this volume effect of ultra-violet light (unlike those produced by its action on a 

 negatively charged zinc plate) are unaffected by electric fields strong enough to 

 remove the ions produced by Rontgen rays as fast as they are produced. More 

 severe tests were now made with the double apparatus, to see whether they are 

 altogether uninfluenced by the electric field. 



The ultra-violet light was produced by the spark discharge between aluminium 



* RUTHERFORD, loc. cU. 



t ' Phil. Trans.,' A, vol. 192, pp. 403-453, 1899. Also J. J. THOMSON, ' Phil. Mag.,' loc. tit. 



\ 'Phil. Trans.,' loc. at. 



