CHAPTER II. 



THE REPULSION OF TWO METALLIC DISKS, NEARLY IN CONTACT. 



26. Apparatus. The apparatus shown in fig. 28 was originally constructed 

 with the expectation of testing the horizontal pendulum for the measurement 

 of the Newtonian constant ; or, conversely, to graduate the horizontal pendu- 

 lum by means of that constant. Here AB suggests the parts of a Fraunhofer 

 slide micrometer, capable of moving the slide about 6 cm. and graduated in 

 o.ooo i cm. On this the two brass disks DD, (originally) 15 cm. in diameter 

 and about 0.6 cm. thick, are mounted in parallel, rigidly, normally and ver- 

 tically. To adjust the disks the steel plugs c and c are provided, fitting radial 

 holes in the plate. They are further held by the semicircular frame e and e, 

 screwed to the slide below and attached above to the disks by aid of the pairs 

 of screws, a and 6, on opposite sides of the diameter. The screw a is sunk into 

 the disk, while 6 presses against its outer surface. As the disks are to be fitted 

 nearly true to the slide and the frame, but slight adjustment at a and b is needed. 



FIG. 28. FIG. 29. 



The interior of the smaller disk d, (originally) about 10 cm. in diameter and 

 0.6 cm. thick, is suspended vertically by two fine wires / from the end of the 

 arm of the horizontal pendulum, just below the grating. The disks D, d, D 

 are coaxial, while d is relatively stationary; D or D may be brought as near 

 to d as desirable by aid of the slide micrometer, the other disk being removed 

 at the same time. 



49 



