C. Barus — Repulsion of Two Metallic Disks. 93 



Art. IV. — The Repulsion of Two Metallic Disks, nearly 

 in Contact ;* by C. Barus. 



1. Apparatus. — The apparatus shown in fig. 1 was originally 

 constructed with the expectation of adapting the horizontal pen- 

 dulum for the measurement of the Newtonian constant ; or 

 conversely to graduate the horizontal pendulum by means of 

 that constant. Here AB suggests the parts of a Fraunhofer 

 slide micrometer, capable of moving the slide about 6 cm and 

 graduated in -0001 cm . On this the two brass disks, DD, origi- 

 nally 15 cm in diameter and about 6 cm thick, are mounted in 

 parallel, rigidly, normally and vertically. To adjust the disks 

 the steel plugs cc are provided, fitting radial holes in the plate. 

 They are further held by the semi-circular frame <?<?, screwed 

 to the slide below and attached above to the disks by aid of 

 the pairs of screws a and b on opposite sides of the diameter. 

 The screw a is sunk into the disk while b presses against its 

 outer surface. As the disks .are to be fitted nearly true to the 

 slide and the frame, but slight adjustment at a and b is needed. 

 The interior of the smaller disk, d, about 10 cm in diameter and 

 *6 cm thick, is suspended vertically by two fine wires/', from the 

 end of the arm RR\ fig. 2, of the horizontal pendulum, just 

 below the grating. The disks D,d,D', are coaxial, while d is 

 relatively stationary ; D or D' may be brought by aid of the 

 slide micrometer as near to d as desirable, the other disk being 

 removed at the same time. 



The method of attachment of the disk d to the horizontal 

 pendulum is shown on a smaller scale in fig. 2. Here G is the 

 grating, secured by three adjustment screws to the table T, 

 the cylindrical stem of which is grasped in a clamp P (open 

 form), of the horizontal pendulum H,PR '. To the bottom of 

 the stem in question, a cross piece, hgh, is screwed and fastened 

 with a lock nut. The two metallic fibers^ which support the 

 disk d are wound above around the screws hh and thus ade- 

 quate vertical adjustment of disk d is available. 



The slide micrometer is attached to the pier by a firm hori- 

 zontal rail capable of adjustment forward and rearward. A 

 strong clamp attaches the base of the slide micrometer to this 

 rail, so that the whole instrument may also be adjusted to the 

 right or left, roughly. The fine adjustment is secured at the 

 slide micrometer itself. 



Finally, a case is provided covering the disks D and d and 

 part of the micrometer, so that only the drum head and scale 



* Abridged from a forthcoming Eeport to the Carnegie Institution of 

 Washington, D. C. See note, this Journal, xxxvii, p. 350. 



