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LORD KELVIN, DR. M. MACLEAN, AND MR. A. GALT, ON THE 



Thus the 3'4 millims. filter is the most effective and the 7'0 millims. filter is 

 the least effective of these five filters of equal length. The following table shows 

 results for different lengths and different bores : 



60. A glass tube filter, 4 centims. long and 3 '3 millims. in bore, covered outside 

 with strips of tinfoil along its length, was similarly compared with the standard 

 filter. When newly put up, and as long as the glass was dry, it took out very little 

 electricity from the air ; but as the experiment proceeded, and the glass became less 

 dry by taking up moisture on its inner surface, the quantity of electricity taken out by 

 the glass tube became greater and greater. Thus in a first experiment q z lq\ = 28'5 ; 

 but after working the pump for one hour q z fq l = 3'8. 



61. Up to this time (December, 1895), we had not been able to find a filter 

 which could take all the electricity from the air, and we now proceeded to search for 

 a filter which would be able to practically do so. The first filters tried with this 

 object were tubes filled with very small pieces of fine copper wire, and closed at each 

 end by a plug of cotton wool and a disc of brass gauze. The diameter of the wire 

 was 0'00296 centim. The containing tube was in one case block-tin pipe 10 centims. 

 long and 1 centira. diameter : and in another it was a glass tube of the same length 

 and diameter, coated both inside and outside with longitudinal strips of tin-foil. 

 The diselectrifying power of each was calculated from the observations by the 



' Tube No. 7 had twelve wire gauzes inside it, and, as the table shows, its filtering efficiency was 

 more than that of the equal and similar tube No. 8, which was clear inside. 



