212 Prof. Millikan : New Modification of the Cloud Method 



through which the cloud falls is the equilibrium temperature 

 after condensation — a quantity obtained from theoretical 

 considerations relating to the adiabatic expansion of saturated 

 vapours and the experimental curve expressing the relation 

 between the temperature and density of a saturated vapour. 



The results obtained by the method herewith presented 

 are freed from all of these sources of uncertainty. 



§2. The Temperature of the Cloud Chamber after Expansion. 



In order to be free from all uncertainty as to the tem- 

 perature of the cloud chamber I first attempted to measure 

 it with an ordinary mercury manometer, which was attached 

 to the chamber as shown in the figure. The diameter of the 



B ^ 



glass vessels A and C was about 5 cm., the figure being 

 drawn to scale but reduced to one-sixth actual size. The 

 expansions used in these test experiments corresponded to a 

 fall in pressure in the cloud chamber from about 75 cm. of 



