52 



TH^ DIFFUSION OF GASES THROUGH 



difl&culties were encountered from the frequent presence of air bubbles 

 between the plate and the disk of the diver. It was difficult to remove 

 them completely, and all attempt at measurement failed. In general solids 

 take a permanent charge which can not be removed by any means offered 

 by the apparatus. They must therefore be avoided for condenser purposes. 

 A return to the oil-condenser with a number of minor improvements 

 showed the results recorded in table 14. 



Table 14. — Constants as in table 13, III. il/= 37.89 grams. 



These data appear to be trustworthy throughout and measure the un- 

 avoidable fluctuation of the potential of the electrical machine. They are 

 of the same order, moreover, as the spark potentials, remembering that 

 these had to be determined before and after the electrometer potentials. 

 Adequate precautions for the spark work could not be taken. The range 

 of D is large, increasing from 0.8 cm. for the largest admissible forces to 

 2.5 cm. Below 0.8 cm. pressure would have been needed to sink the diver. 

 In case of table 14 the diver is released suddenly and falls as a whole, giving 

 all necessary accuracy to its //'. Naturally this sudden drop is essential, 

 for the equations used are only true when the disk is flush with the guard 

 ring. A diver descending obliquely or sluggishly would not be trustworthy. 

 The apparatus need not be air-tight and a slight leak assists in the determi- 

 nation of H'. Other similar experiments were made with the same order 

 of values, which need not therefore be recorded here. 



Finally, certain experiments were improvised in the endeavor to measure 

 relatively low potentials of the order of 300 volts. Here the apparatus 

 should be quite free from leak, as the change of // under these conditions 

 is of the order of i mm. of mercury. Any thermal discrepancy in the 

 partially exhausted air, the presence of small air bubbles clinging to the 

 diver, would otherwise mask the effect to be measured. Finally, as the 

 distance apart of the plates is necessarily small, i mm. and less, special 

 precautions must be taken with this magnitude. The results obtained need 

 not be given here, as the work was undertaken merely to show that the 

 apparatus works smoothly even under these limiting conditions. 



From equation (7) above, 



AH = H-H' = 



8Mg (30oyD' 



