Barus — Displacement Interferom eter 



77 



II— M — 1-0930 grams. 



Drum 



10 S &N 



v 3 



Voltmeter 



24-9 



6-22 



21-9 



21 



47'3 



11-82 



41-6 



41 



Fig. 4. 



40 





a 



y/6 











zo 



1 











y 









t - 



S3* 



VCi 



%-* 













1 



1 z 



4 







6 



9 



9 20 40 



Fig. 4. Electrometer data compared with actual voltages. 



The differences in this table are more liable to be of the 

 voltmeter than of the electrometer. The needle easily carried 

 a potential of V, = 250 volts. With V 3 = 100 volts, the 

 needle became asymmetric and observations could no longer be 

 taken. The sensitiveness is about -0003 centimeter of double 

 displacement per volt for the short suspension, so that - 1 volt 

 per ring is the equivalent datum. The present form is thus 

 easily made as sensitive as the preceding form, besides being 

 much simpler in general design and installation. 



The experiment was now pushed a step further in the direc- 

 tion of sensitiveness by using an even lighter paper cylinder 

 with paper wings, the needle having the constants given in the 

 second series of Table 2 and fig. 4b. The sensitiveness is now 

 about '0006 centimeter of double displacement per volt, twice 

 that of the preceding case as it should be. This apparatus did 

 not, however, behave as satisfactorily as the other, there being- 

 greater tendency of the movable cylinder to cling to the stable 

 parts, whenever V[ was high. In case of smaller potentials or 

 of the idiostatic method there would be no difficulty in this 

 respect. No doubt, if greater time and patience were spent on 

 the work (a cylinder of celluloid suggests itself) the final result 

 could be somewhat improved ; but the limit found (6X10 -4 cen- 

 timeter or 20 rings per volt) will not easily be exceeded in 

 practice. 



11. Disk electrometer. Apparatus and experiments. — The 

 disk electrometer deserves special attention because it is 

 simpler in design and practically more sensitive than the 

 cylinder types. At the same time, however, it is more treach- 

 erous and without special precautions there is danger of short- 

 circuiting the highly charged disk, while the sensitiveness is 

 enormously variable in response to any unsymmetrical position 



