96 G. Barus — Repulsion of Two Metallic Disks. 



<f> = -01081 radians, F R = 42'9 AiT. The new brass plates 

 D, d, D', were identical in size, the mass being in = 468 grams 

 each, the diameter 2r = 20-3 cm , and the thickness *17 cm . 

 When all but in contact gravitational attraction should have 

 exceeded *508 dynes. The force P B corresponding to the 

 loaded pendulum is 



F\ = F R (1 4- mR/Mh) = 65-2 A JST 



The double gravitational attraction should, therefore, have 

 been at least equivalent to a difference of the displacement on 

 the two sides of •0156 fm , or about 31 drum parts of the 

 micrometer. In place of this a repulsion, equivalent on the 

 average to '0338 ctn or about 68 drum parts, was observed. 



The experiments were made at some length ; but as the 

 results corroborate the more elaborate work below they need 

 not be given here. 



4. The same. Metallic contact. — The next advance con- 

 sisted in placing the disks in electrical metallic contact, which 

 was easily done by joining the pivots of the horizontal pendu- 

 lum with the slide of the micrometer bearing the fixed disks, 

 by a copper wire. Moreover, since the position of equilibrium 

 is gradually reached in the lapse of minutes, the time of the 

 observation is taken in minutes. 



These experiments will also be omitted here as they showed 

 no essential difference. The mean repulsion observed for the 

 disks 20 cm in diameter was 2F R = 65-2 AN = 65-2 X '013 = -85 

 dyne at about d = l mm of air space. 



5. Retardation due to viscosity of air. — It will next be 

 necessary to examine the surmise, that the very gradual 

 approach of the suspended disk to its position of equilibrium 

 may be due to the viscosity of the interposed couche of air, in 

 view of the small forces and small displacements involved. 

 The case may perhaps be treated in terms of Poiseuille's law, 

 assuming that the flow is from the center of the two nearly 

 contiguous parallel disks radially toward the circumference. 

 Let y be the initial distance apart of the disks, and the time 

 t — seconds, measured from the fixed toward the movable 

 disk. Let y' be the final position of equilibrium of the mova- 

 ble disk, so that its excursion is y — y '. Let a small impul- 

 sive force P act normally on the outside of the movable disk, 

 by which it is put into the position y. The pressure generated 

 will cause a flow radially outward in question, and if p is the 

 pressure in the fluid at a distance r from the center, Poiseuille's 

 law would give 



dV _ (Zirryy dp 

 dt ~~ 8 7rri dr 



