Barns — Displacement Interferometer. 81 



13. The Same. Further experiments. The interesting feature 

 of the preceding result is the large range of variability in the 

 sensitiveness of" the apparatus. It was therefore thought worth 

 while to throw further light upon the investigation by purposely 

 tipping the apparatus, in order that the disk might lie on one 

 side or the other of the guard ring. 



rrv, • • <• 7 /A -AT Ma D % 



The approximate equation for Ay A JS = . ,» -r^ 2 — 1 



applied to the experiments did not give results quite as 

 consistent as was anticipated, inasmuch as the same value of 

 k did not reappear when the apparatus" was tipped back 

 apparently to its original position. Moreover, the central 

 position did not clearly correspond to k=0. Hence it seems 

 that & = is determined by the geometry of a non-uniform 

 field. Neither did the values of V s correspond very closely 

 to the values of k. The apparatus is more sensitive when the 

 disk V 1 is on the side of the plate V s and about equally so 

 when central or when on the side V 2 = 0. 



In other experiments the disk was weighted with a rider and 

 a similar series of data was taken, although V, = 250 volts was 

 carried by the disk only in cases where its position was nearly 

 central. This was even the case on adding heavier riders. 

 The computed values of V t were in all cases 2 or 3 times the 

 true values, though the proportionality is maintained. The 

 conditions are thus too complicated for consideration here. 



14. Case of the enclosed disk. With the object of obtaining 

 a more uniform field, the electrometer was now modified by 

 surrounding the disk with a short cylindric or drum-shaped tube 

 within the guard ring. The plates and the drum guard ring 

 are spaced, as before, by three hard rubber rings, with a hard 

 rubber screw passed through the perforations. 



Experiments were begun with the light disk, M — '607 

 gram, which was charged to 110 volts, as it would not carry 

 an appreciably higher voltage. Consequently the rider was 

 thereafter added, making M = l - 086 grams, and the disk 

 charged as far as 250 volts. The results are given in fig. 7 b. 

 These voltages, though obtained under very different condi- 

 tions, are practically coincident and the discrepancies are as 

 liable to be in one instrument (voltmeter) as the other. The 

 remaining experiments were of the same nature. 



To summarize / the enormous variation of the sensitiveness 

 of the disk electrometers as depending upon the position of the 

 disk, k, has, therefore, failed of interpretation ; i. e., the cor- 

 rection for V 3 does not seem to follow the above equations. 

 The field is, therefore, probably far from uniform, possibly 

 consisting of a conical tube of force between plate and disk on 

 the side toward which the disk leans and of a more nearly 

 Am. Jour. Sci.— Fourth Series, Vol. XXXVII, Ko. 217.— January, 1914. 



