78 



Bar us — Displacement Interferometer, 



of the disk. The instrument used is shown in figs. 5 and 6, in 

 sectional side and front elevation. V x is the charged disk on 

 the bifilar suspension of very thin copper wire yy\ '007 

 centimeter in diameter. The disk is made of thin mica, 

 silver-plated, carried by the horizontal axial steel rod dd, and 

 surrounded by the guard ring of very thin copper T 7 /. Parallel 

 to the disk and equidistant (d) from it are the plates Y z and V 2 

 of the condenser at a distance D apart, so that D is twice d. 

 The plate V 2 is earthed and firmly held on the arm of the inter- 

 ferometer. The plates V % and V 2 are spaced at three points 



Fig. 5. 



Fig. 6. 



Fig. 5. 

 Fig. 6. 



Disk electrometer, vertical section. 

 Disk electrometer, vertical section. 



Front view. 

 Side view. 



by hard rubber gaskets, cc, through the holes of which hard 

 rubber screws are passed, the plates being ultimately secured 

 by nuts a, a 1 , and a 11 , on the outside of the plates. The nuts a 

 terminate in clamp screws. The guard ring T 7 / and the 

 suspension wires yy l are in metallic connection at the tops of 

 the suspension, the latter being the same already shown in 

 figs. 1 and 2. The small mirror, m, is attached to a small plate 

 of cork, n, which is slotted parallel to the rod dd, the latter 

 being clutched by the jaws of cork which make up the sides 

 of the slot. In this way m may be rotated around a horizontal 

 axis, while the apparatus as a whole may be revolved about a 

 vertical axis. 



If the grating is capable of being raised or lowered, it is not 

 difficult to adjust the apparatus and find the ellipses. The 

 damping of the disk is naturally good, though it may be 

 improved by surrounding it with the case shown in fig. 1. 



