76 Barus — Displacement Interferometer. 



meters in height. It was not thought worth while to do this, 

 and further experiments were abandoned. 



9. Cylindrical electrometer ; movable cylinder within. 

 Summary. — The above experiments were made' in the midst 

 of the turmoil of a large city and near the engineering labora- 

 tories of a University. It is rather remarkable, therefore, that 

 the ellipses were so easily found throughout and so easily 

 made use of ; but it was quite out of question to use the eva- 

 nescence of rings by which the sensitiveness could have been 

 increased over twenty fold. Though the work was carefully 

 done, it is intended merely to exhibit the general character of 

 the method, inasmuch as the storage battery which was drawn 

 upon was unavoidably in use elsewhere in the laboratory and 

 the potential may have fluctuated. 



The design of the apparatus with a movable inside cylinder 

 is probably the least interesting of those used. Thus it is 

 difficult to keep a paper cylinder quite smooth or to give it a 

 rigorous cylindrical shape and the correction for the ends can 

 scarcely be estimated. For high charges any asj^mmetry of 

 the movable cylinder is liable to place it in contact with the 

 fixed cylinders. Thus there is a limit of sensitiveness from a 

 purely instrumental point of view, not contemplated in theory. 

 In the above experiments with short suspension (21 centimeters) 

 "7X10" 4 centimeter per volt was the largest double displace- 

 ment practically obtained, which would mean about '04 volt 

 per vanishing ring. With the long suspension '006 volt per 

 ring may be estimated. It is far short of the theoretical datum 



of § 4 - 



10. Cylindrical electrometer ; movable cylinder without. — 



After a few preliminary experiments with the metal cylinder 

 of aluminum, the gilt paper cylinder was tried and was at once 

 successful. The mirror was attached to the paper wing needed 

 for damping by the aid of a bent piece of thin steel wire, 

 cemented on so as to give a horizontal axis on passing through 

 the perforation of the bit of cork holding the mirror. For a 

 vertical axis the whole apparatus is rotated (cf. figs. 1 and 2). 

 The results are given in Table 2 and fig. 4a. 



Table II. 

 Cylindrical electrometer ; movable cylinder (gilt paper) without. 

 1.-31= 2-075 g. ; R = T75 cm. ; r= 1'59 cm. ; 1 = 21 cm. ; 



v,-- 



= 250 volts. 











Drum 



10 3 AAT 



v 3 



Voltmeter 





11-2 



2-79 



18-7 



18 





22-9 



5-72 



38-2 



38 





35'3 



8-83 



59-1 



58 





27-6 



6-90 



46-1 



48 



