C. Barus — Repulsion of Two Metallic Disks. 99 



discs gradually increasing, as the fixed plate moves from posi- 

 tion 2*15 to position 2*00, i. e., '15 cm . The interval of observa- 

 tion was 48 min.; bat the interval and final reading at 2*15 

 differ in like degree, so that the apparent attraction indicated 

 is merely the result of the shift of the position of equilibrium 

 of the horizontal pendulum. Series 19 and 20 contain similar 

 observations on the " small " side. 



The present and all the following work was done in time 

 series (min.), as shown. Observations were continued until 

 the direction of motion was reversed and the disc reached a 

 stage of slow vibration. Under these conditions the air 

 gradually enters and leaves the fissure between the plates, so 

 that errors due to the viscosity of air are out of the question. 



7. Observation continued. Change of distance apart. — In 

 the next work, the attempt was again made to vary the dis- 

 tances between fixed and movable plates successively; but to 

 determine the micrometer position of contact of the plates by 

 actually pushing them together with a weak spring. 



The results of these laborious experiments, though appar- 

 ently good when taken individually, were not consistent as a 

 whole, because of the incidental flexure of the pier during the 

 long interval of observation. 



It does not seem possible, therefore, to obtain any definite 

 results from methods which consume as much time as the 

 present. 



8. Observations. Long periods and inversion. — The unsat- 

 isfactory results obtained in the last paragraph induced me to 

 give a final trial to the original method of alternating the sign 

 of the repulsion, by moving each of the two fixed plates in 

 turn near the suspended plate. The results are given in tig. 4. 

 In series 1, 2, 3, the equilibrium position is approached from 

 two opposed directions and for two positions on the ''large" 

 side of the micrometer. The same is the case in series 5 and 

 6, while in series 4, 7, 8, 9, similar observations are made on 

 the "small" side. The mean results are (C denotes contact of 

 plates d and D) : 



Fixed discs at, Ace, = 2-10 2*15 -70 2-15 -75 *70 '65 cm. 

 Fixed and movable disc in 



contact, A.Vx 10 3 = 





75- 



-150 



75 



.. 150 



.. cm. 



Movable disc free, 















AiVxlO 3 = 



15 



12 



45 



16 



53 52 



48 cm. 



I0 3 d = 



_. 



31 



97 



30 



.. 101 



_. cm. 



Hence, there can be no further doubt that the repulsions are 

 real, although their nature has not been made out. When the 

 distance between the disks is larger than a millimeter, the air 

 damping is insufficient and the free disk unavoidably oscillates. 



