52 EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 



29. Observations with small plates. The first experiments were made 

 merely for the purpose of testing the method, using the same heavy horizontal 

 pendulum as in the preceding section. There are two or three objections to 

 this pendulum for the present purposes, of which the first is its weight M; the 

 second is the water damper, which introduces inevitable discrepancies, due to 

 such capillary forces as result from surface viscosity. The third objection is 

 due to the fact that the pendulum is made of light steel tubing and points in 

 the north-south direction. These tubes become weak magnets in the earth's 

 field, and the angle 8 may change with the variations of this field. Finally the 

 inclination a of the axis of the pendulum, due to terrestrial causes, is itself to 

 be considered; this can only be eliminated if the time of observation is reduced. 



The two attracting plates of rolled brass were each 6 inches in diameter 

 and 0.25 inch thick, weighing m'= 1,035 grams. The attracted disk d at- 

 tached to the horizontal pendulum was 4 inches in diameter and 0.125 mcn 

 thick, weighing 227 grams. The distance between the large plates was 2.5 cm. 

 on the micrometer, this being about the limit of the micrometer screw and 

 sufficient for the diminution of the attraction in question to negligible values. 

 The difference of AW for the two extreme positions of the disks was esti- 

 mated above as 0.0024 cm., or 5 drum-parts. It should have been easily 

 detected, if not masked by the incidental disturbances referred to. 



The five series of observations are given in the curves, figs. 31 A, 316, 3iC, 

 32 A and 326. They show both the release of the suspended disks from con- 

 tact with the disk fixed on the micrometer, and the differential effect of the 

 fixed disks on opposite sides of the suspended disk, but near it. 



ABC 



N 



Oi 









-0 



X. 



d- 



A') 



\ 



40 ZO 30 40 -05 <& & 



FIG. 31. 



In fig. 3 1 A, the abscissas are the successive excursions Ax of the micrometer 

 bearing the fixed plates, the ordinates are the corresponding excursions AAf 

 of the suspended plate. Beginning at a, the two plates are nearly in contact, 

 and this contact is made more definite in the direction + x. Hence in the 

 curve from a to d to b, as shown by the arrows, 2A/V = Ax, as it should be. 

 After passing b toward c the suspended plate is released, but released in such 

 a way as to suggest repulsion at 6, whereas the other four points nearer c 



