6 



DISPLACEMENT INTERFEROMETRY APPLIED TO 



To test the electrometer, figure 5, a screw electrophorus was constructed as. 

 shown in figure 6, where p and p' are the two plates and r the insulating layer 

 of hard rubber. The screw-post s, rising from the center of the plate p', 

 passes through the screw-sleeve of brass b, embedded in the hard-rubber handle 

 h, which in turn is firmly screwed into and supports the plate p. By rotating 

 the handle h, the plate p is raised or lowered by an amount shown on the 

 spring rod a, which is also the upper electrode. Fractions of a turn are read 

 off on the graduated top face of p. The lower electrode is at E. As the 

 insulation sbh appeared inadequate, the construction was modified by removing 

 s, raising b, and allowing a supporting screw, similar to s, to enter the handle k 

 from above, retaining the feature of rotation. The new form is shown in figure 

 7, k being the graduated head referred to the stem C. The plate d is clutched 

 by the same standard which supports p'. This left the space between r and 

 p clear for the insertion of dielectrics of different specific inductive capacity. 



In figures 6 and 7 , a and e are to be joined with a and e of figure 5 . It 

 would have been desirable, of course, to provide the condenser CM in figure 

 5 with a guard-ring. If B is an insulator, this would apparently occasion no 

 serious difficulty; for a fixed metallic ring or short cylinder coaxial with M 

 and partially submerged in it would suffice. The upper face of the ring 

 should be flush with M. The apparatus used, however, was not well adapted 

 for this -purpose. 



7. Fringes from a free mercury surface. By making the troughs of the 

 U-tube very shallow (a few millimeters), fringes were obtained rather easily. 

 They were entirely too mobile to be used with convenience. The curious fea- 

 ture was observed, however, that even with pools 6 cm. in diameter the rise and 

 fall of the mercury faces on the two sides was not rigorously in parallel. In 

 fact, the slit-images separated and the fringes vanished about as soon as when 

 floating glass plates were used. This discrepancy is to be referred to some type 

 of surface viscosity, which, if the mercury had been quite chemically pure, 

 would possibly have disappeared, rather than to an inequality of the electric 

 field at the surface. The experiments with the guard-ring alluded to were also 

 abandoned because of the mobility of the mercury surface even when using 

 thin glass plates i to 2 cm. in diameter. 



8. Equations. If we treat the case of the electrophorus as a closed cylin- 

 drical field of cross-section A, and if V is the potential of the charged hard- 

 rubber surface, we may write 



