E. C. WINKLER 



these graphs do follow each other more closely than previously. A very 



similar situation is seen in the graphs of the mass of dust collected {Figure 7). 



A probable explanation of this is that the cheesecloth itself is insulating 



06 



0.4 



en 



I/) 



0) 



02 





 Figure 7. 



Earthed 



Insulated 



June 1957 Dec 1958 June 



Comparison of earthed and insulated screens — sample masses 



and only after a layer of conducting dust settled on it was there a noticeable 

 difference in the screens. The screens used by the Americans were of 

 stainless steel. 



{2) Sticky-paper method of collection and deposition of dust on roof with subsequent 

 collection 



A series of five measurements was carried out using the sticky-paper 

 method. The dust collected on the sticky paper should be comparable to 

 the 'fall-out' dust collected, although it is claimed by some workers that 

 the sticky paper also collects rainwater activity. 



It was found that the activity of the 'fall-out' dust from the roof was 

 approximately three times that of the week's sticky-paper samples but when 

 the fact that the collecting ai-eas were 16 ft.^ and 0-5 ft.-, respectively, was 

 allowed for, the roof collection had 10 per cent of the efficiency of the 

 sticky paper. 



If we assume that the sticky paper also collects rainwater activity, we find 

 that 'fall-out' dust plus rainwater efficiency varies from 14 per cent to 

 96 per cent of the gummed paper. Because of this large range of efficiency, 

 obviously due to differences in weekly rainfall, it seems likely that the sticky 

 paper has a near-zero efficiency for collection of rainwater activity, 



135 



