302 



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



that is, to the two-thirds power of the volume of each drop, or for a given ex- 

 pansion to l/n 2/J where n is the number per cubic centimetre. The time, t, taken to 

 fall a given distance is thus proportional to n 2 ', or the number of drops is proportional 

 to t l/ *. Now, when v^/Vi exceeds 1'35 the times taken by the drops on the two sides 

 to fall a given distance certainly do not differ by as much as 1 part in 10. If 

 t t /t t = ri then 71,/Ha = ri s/2 = 1'15. Thus the number of ions is the same on the 

 two sides to within 15 per cent. 



The equality of the fogs on the two sides is, in fact, rather more exact than we 

 would expect ; for the positive ions, according to ZELENY, take 1*25 times as long as 

 the negative to travel a given distance. We would expect then (the strength of the 

 field on each side being the same) that, if equal numbers of positive and negative 

 ions were produced in a given time, the negative ions would be more quickly removed, 

 and a somewhat larger number of drops should have been produced in the half 

 containing mainly positive ions. 



The absence of any indication of this slight excess of positive ions is of the less 

 consequence for the present purpose, since it strengthens rather than weakens the 

 evidence against the view that a larger number of positive than of negative ions is 

 produced. As a test of the trustworthiness of the method for detecting a difference 

 in the number of drops produced on the two sides, experiments were made in which 

 the direction of the field on both sides of the central plate was such as to drive 

 negative ions outwards towards the side plates ; the strength of the field being, 

 however, different on the two sides. The ratio of the fields was as 3 to 2, the 

 stronger field being produced by two Leclanche cells. The following results were 

 obtained : 



The excess in the number of drops on the side of the weaker field is plainly shown. 



It appears, therefore, that the difference in the behaviour of the positive and 

 negative ions is not to be explained by a difference in the quantity of electricity 

 carried by positively and negatively charged ions respectively. 



Some experiments were now tried with the object of determining whether the rain- 

 like condensation, which takes place in dust -free air, even when not exposed to any 

 ionising agents, is due to ions. 



