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



Then 



ft = Q ............ ghajfj (1), 



ft = n'(Q-ft) = n'Q(l-) ..... '..-(2). 



In the second experiment 



Let Q' = total quantity of electricity in air entering, 

 ft' total quantity of electricity taken out by A'B', 

 q.,' = total quantity of electricity taken out by AB. 

 Then 



7i' = 'Q' ............... (3), 



g s ' = (Q'-g 1 ') = nQ'(l-n') ....... (4). 



From these four equations we find 



_ r l\1\ ~ M . > _ Mi' - gag/ 



" 



13. By taking a movable plate of a small air-condenser charged to a known 

 potential, and applying it to the insulated terminal of the quadrant electrometers 

 used as described in 18, we could calculate ft, ft, ft', and q. 2 ', and hence find Q and 

 Q', the absolute density of the electrified air or gases in C.G.S. electrostatic units. 

 It was thus that we found 11 X 10~* and 22 X 10~ 4 mentioned in 64, 65. 



14. Up till the middle of December of 1895 the most efficient filter we tried had a 

 dist'.lectrifying power of about 0'8. During the Christmas holidays of 1895, we 

 succeeded in obtaining a filter of fine brass filings, as described in 62, which 

 abstracted so much of the electricity from the electrified air passing through it, that 

 what was left was not sufficient to show on a similar filter, A'B', attached to an 

 electrometer, E' (see fig. 7). It was not necessary now to have two experiments, 

 first with electrified air in one direction through two filters to be tested, and then 

 with electrified air in the reverse direction through them. It was sufficient to take 

 a filter, the diselectrifying power of which, determined as above ( 12), is found to be 

 very nearly unity, and attach it to electrometer E'. Then join the tested filter to 

 another electrometer, E, and allow the electrified air to pass this filter first, and 

 thence through the almost perfectly diselectrifying filter. 



With the same notation as in 8 12, we 



O ' 



' 



and 



1 



+ g 2 T , 1 g 2 

 1 + ' ft 



If n' = unity, as it is for a filter of fine brass filings ( 62), 



