9 J: 6. Barus — Repulsion of Two Metallic Disks. 



projects. The apparatus was found to work satisfactorily. It 

 is quite possible to rely solely on the effective air damping 

 produced when the disk d is very close to D or D' . In fact, 

 the tin cover in this case was all but superfluous. D could be 

 shifted from end to end of its course without materially inter- 

 fering with the visibility of the ellipses in the spectrum of the 

 interferometer. The occurrences unfavorable to the gravita- 

 tional measurement were all incidental; due either to the 

 change in inclination of the pier, a, or to changes in the mag- 



Fig. 2. 



Fig. 1. 



netic field (inasmuch as the pendulum was preliminarily con- 

 structed on steel tubing), or to the phenomena discussed in this 

 paper. For what was found was not an attraction at all, but a 

 repulsion much larger in absolute value than the attraction 

 anticipated. 



2 Equations. — The chief equations to be used in the pres- 

 ent work have already been given in the former paper.* It is 

 merely necessary to add those which bear upon the sensitive- 

 ness of the method. Since the disk of mass m is added at the 

 mean distance R to the mass of the pendulum M, the force at 

 R from the axis is now 



F\- F R (l+mfi/Mh) 

 * Barus : This Journal, xxxvii, 1914, p. 501, et seq. 



