54 EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 



the horizontal pendulum, read off on the micrometer of the interferometer. 

 The plan was to begin with the plates more than in contact, so that the mov- 

 able disk is carried by the fixed disks until released. In fig. 3 3 A, at b, the 

 plates adhere until at a a release or fall suddenly takes place, the plates being 

 now over i mm. apart. The figure shows that release should have occurred 

 at the position 2.09 cm. In fig. 338, corresponding to the other ("small") 

 side of the Fraunhofer carriage, the plate is released at the position 0.51 cm. 

 and there is no adhesion. In fig. 330, on the original ("large") side, the plate 

 is passed into cohesion and then released with a smaller fall at a. 



TABLE i . Large brass disks, not in metallic contact. Steel horizontal pendulum, m = 468 

 grams; r = 10.2 cm.; = 0.17 cm.; ^ = 0.0108; M=i, 250 grams; A = 8ocm.;.R = iii.3 cm.; 

 F = 



* Mean values. 



Thus the positions 2.00 and 0.65 are guaranteed as free, the space between 

 the reacting plates being over i mm., as compared with the distance 1.4 cm. 

 between the fixed plates. The effects of alternately approaching the opposed 

 fixed plates to the movable disk are shown in fig. 330. They are quite definite, 

 larger in order of value than would be anticipated and constitute repulsions 

 instead of attractions. In fact, figs. 33A and 336 show that in case of attrac- 

 tion or of cohesion, A7V should be too large on the "large " side, and too small 

 on the "small " side of the stationary disk. In fig. 33 D the reverse is the case. 



To explain this repulsion a number of facts have to be taken into ac- 

 count. Both the fixed disks are separate metallic systems, but ultimately 

 anchored into the pier with iron bolts, so that a volta contact force, iron-brass, 

 would be inevitable. The disks are thus carrying charges, depending on the 

 nature of the anchorage in the pier, whether this is moist or quite dry. It seems 

 probable, as will be shown below, that these small potentials are negligible. 

 Again, with the small forces per square centimeter of area in question, the 



