EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 55 



viscosity of air is sufficient to necessitate the lapse of considerable time before 

 a position of equilibrium is assured, even with the disks a millimeter or more 

 apart. In the above experiment sufficient time was allowed until the motion 

 became vibratory, after which the reading was taken; but it is difficult to 

 assert that, even after indefinite waiting, further subsidence would not have 

 taken place. Finally the flexure or tipping of the pier, where long intervals 

 of time are in question, can not be eliminated. There would inevitably be 

 some error on this account. It seems improbable, therefore, that the actual 

 gravitational attraction of metallic disks will be determinable, while a non- 

 metallic system is liable to introduce even greater errors. 







4 



-5 -6 m 1-95 



B C 



FIG. 33. 



31. The same, continued. Metallic contact. The next advance consisted 

 in placing the disks in electrical (metallic) contact, which was easily done by 

 joining the pivots of the horizontal pendulum 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 

 observations is taken in minutes. These results are given in table 2, and are 

 inscribed in figs. 34A and 343. The figures on the curve show the series in 

 question and the plate ("large" or "small" side of the plate micrometer), 

 which is actively repelling. In fig. 34A the alternations are found after long 

 waiting; in fig. 346, however, in time series. When equilibrium is reached, 

 and this is always relatively quickly, the disk oscillates due to incidental 

 causes. It makes no difference from which side the position of equilibrium 

 is approached (series 2, 8, 15). The presence of radium on the plates has no 

 effect, other than the mechanical disturbance given by placing it there; the 

 same position of equilibrium again results. When the disks are jolted by 

 contact (case between series 7 and 8), the equilibrium position may be tern- 



