8 



DISPLACEMENT INTERFEROMETRY APPLIED TO 



The second modification (fig. 7) of the electrophorus was then used with the 

 plates quite separated and the micrometer-screw above. The insulation was 

 much better, the loss amounting to not more than 2 fringes in 10 minutes at 

 full charge. The pitch of the micrometer-screw being now o.i cm., the upper 

 plate was conveniently discharged when d' = i cm. above the hard-rubber 



ID 



SO 



surface. Large fringes (about 1.5 scale-parts) were installed. The results 

 (fringe-readings in terms of displacement y) obtained in the same way as the 

 preceding are shown in figure 9. The outgoing and incoming series practically 

 coincide. 



Figure 10 shows a series of results in which the plates were discharged at 

 different distances d' = i.o, 0.8, 0.6, 0.4 (clear distance between rubber and 

 metal faces) apart. If we write y* = Cx, x= i.$n (x in scale-parts, about two- 

 thirds fringe), the relations are 



= i.o 

 io 3 C=i8.o 



0.8 

 7.1 

 90 



0.6 



2.9 

 124 



0.4 cm. 

 0.8 



200 



These parabolas are only approximate ; in each case except the last, C decreases 

 appreciably as x increases. The frinpe displacement is in excess of the equa- 

 tion y z = Cx, owing, as I take it, chiefly to the escape of negative charge to the 

 electrometer as d' decreases. 



The relation of C and d 1 (fig. n) is again apparently parabolic in shape, but 

 remotely so; for if we write G i = cd > ', the values of c (fig. n) rapidly decrease 

 as d increases, probably for the reason just indicated. 



