56 CONDENSATION OF VAPOR AS INDUCED BY NUCLEI AND IONS. 



electrometer. The voltages here to be measured must of course be small, 

 and hence all connecting wires are to be inclosed in earthed metal pipes. 

 The core in question is then removed from the electrical condenser and 

 put into the axis of a dust-free fog chamber (fig. 18), where the nucleation 

 (ionization) is found on condensation from the constants of the coronas, 

 or vice versa. Here there are some outstanding difficulties, for the 

 coronas are not the same throughout the length of the fog chamber, as 

 discussed in Chapter III. Even immediately around the radium core a 

 single corona may be green on one side and red on the other. In a fog 

 chamber 45 cm. long, the coronas may vary from the glass end to the 

 metal end of the chamber in a way to correspond to from 100,000 to 

 200,000 nuclei, respectively, while the radium core is fixed in the middle. 

 Inferring secondary radiation, one might naturally expect to obtain still 

 larger coronas near the metal end if the radiimi core, thoroughly sealed, 

 is placed there instead of in the middle of the chamber. But this is not 

 the case, the coronas being markedly smaller than before, decreasing 

 uniformly in size toward the glass end. As the sealed aluminum tube 

 is within the chamber, this behavior is clearly of great importance. 



Jl L 



1 



FlG. 1 8. Fog chamber used as a cylindrical electrical condenser. AR, aluminum core. 



These difficulties are inherent in the phenomenon and merely exhibited 

 by the fog chamber. The latter has the great advantage that enormous 

 nucleations, like millions per cubic centimeter, are not excluded. Under 

 these circumstances the coronas alone are available for finding the 

 nucleation, inasmuch as all the fog particles evaporate before subsidence. 

 Finally, the occurrence of maxima of nucleation may in a large measure 

 be obviated by distributing the radium along the axial tube. 



45. The same. Preliminary data. To test the efficiency of the fog 

 chamber it is sufficient to make a preliminary measurement of Thom- 

 son's e. Let the radii of the electrical condenser be R t and R 2 , its length 



