10 DISPLACEMENT INTERFEROMETRY APPLIED TO 



Results are apt to be even larger than these, much depending on the time 

 during which the glass plate is left within the electrophorus, or on the quick- 

 ness within which x can be read with the subsidence of motion of fringes. 

 The chief causes of discrepancy, however, are the large values of d' ' ', 

 since in this case the dielectric plate operates in strong fields and the 

 descending metallic plate of the electrophorus in weaker fields. Thus the 

 results for d' ' = 0.7 cm. and below are the most uniform. In other words, the 

 difference of d and y is so small that any slight charge on the plate has a rela- 

 tively large effect ond e y and vitiates the result. The glass plates virtually 

 conduct. If they happened to touch the upper plate a throw of the fringes 

 resulted. 



The same difficulties are present in a less degree even with hard rubber, of 

 which the following data are examples : 



d' d e y x K 



0.8 cm. 0.32 cm. 0.23 cm. 5.5 s. p. 3.4 



i.i 0.64 0.45 6.0 3.4 



0.7 0.156 o.ii 5.5 3.4 



results again too large, but quite apt to be larger still. 



Baeckelite usually leaves a charge on withdrawing the plates and the 

 method quite fails. 



Similarly, in case of hard rubber flamed to clean its surface, as the temper- 

 ature rises in successive trials, K rapidly increases. Thus in the above case 

 the effective K rose from 3 to 4, 6, 10, etc., and finally, when the rubber was 

 purposely well warmed, became practically infinite. This recalls the rapid de- 

 crease of viscosity under the same circumstances. Again, if one side of a cold 

 hard rubber is charged and placed contiguously with the charged surface of the 

 electrophorus, the electrometer, initially at zero, shows no appreciable effect; 

 but if the two charged surfaces are both up, the deflection is enormous. In 

 such a case the charge has virtually passed from bottom to top of the rubber 

 plate, as it probably actually does in small part in case of hot rubber and glass. 

 On the other hand, if the rubber is cooled to freezing or below, K decreases to 

 a limit. 



10. Allowance for the electrometer. If the latter has considerable rela- 

 tive capacity, the equations in the same notation as above change to 



, , 



where q is the charge which has gathered in the electrometer of area a, specific 

 inductive capacity of dielectric (glass) K, and distance between plate and 

 mercury d. Treated in the way given these equations reduce to 



(13) 



