4 8 



NUCLEATION OF THE UNCONTAMINATED ATMOSPHERE. 



42. Fog chambers Filters with saturator. The long rectangular fog 

 chamber (Chapter II, section 16) of capacity ^=13,000 cc., having 

 the volume ratio v \V ^ = 0.13 to the volume V of the vacuum chamber, 

 was largely used. It is often difficult, however, to keep plate-glass 

 windows perfectly tight. Hence the cylindrical fog chamber (length 

 45 cm., diameter 12 cm.) was substituted for it, in which case 

 vIV -0.06. Both the former and the latter were often incased in 

 sheet lead, 0.14 cm. thick, leaving merely an open strip in the broad- 

 sides for observation of the coronas. 



T 



A 



FIG. 40. Filter with wet-sponge tube (saturator). 



In view of the difficulties encountered in the preceding chapter, the 

 attempt was finally made to counteract the effect of periodicity by 

 saturating the filtered air with water vapor before introducing it into 

 the fog chamber. To do this the U tube, figure 40, filled with pieces 

 of wet sponge was added to the filter F, the filtered air from B entering 

 the fog chamber very slowly by way of the stopcock C. This innova- 

 tion seemed at first to be remarkably successful, as the results of table 

 29 on page 50 will show. L,ater, however, there was a very definite 

 recurrence of periodicity, the true cause of which I ultimately traced 

 to the inevitable formation of water nuclei. In addition to the filter 

 mentioned, an ordinary dry-cotton filter and a Pasteur filter were often 

 used; but the latter was soon discarded, as it gave no additional free- 

 dom from dust and prolonged the time of filtration inordinately. 



fl-F 



A 



P 



w 



FIG. 41. Rectangular wood fog chamber A, with screening lead plates P and X-ray 



bulb B, exhaust pipe E, and filter pipe F. 

 FIG. 42. Cylindrical glass fog chamber A, with lead case L and bulb B. E is the 



exhaust pipe, F the filter pipe, p the plug for cleaning, and zc> the window. 



