298 E. B. Rosa — Determination of v, the Ratio of the 



Had the correction, A, for the number of discharges been 

 omitted, this difference would have vanished : but the correc- 

 tion seems perfectly certain, and I see no cause for omitting 

 it. Indeed I have failed to find any sufficient cause for this 

 peculiarity which may, after all, be accidental. 



As one of the most accurate determinations by the direct 

 method and made with very elaborate apparatus, I think, how- 

 ever, it may possess some interest for the scientific world. 



Art. 3L~LIl.—l)eterminatio?i of v, the ratio of the Electro- 

 magnetic to the Electrostatic Unit • by Edward B. Rosa, 

 Student in Physics in the Johns Hopkins University. 



This investigation was conducted in the physical laboratory 

 of Johns Hopkins University during the months of March to 

 June, 1889, under the direction of Associate Professor A. L. 

 Kimball. The writer takes great pleasure in acknowledging 

 his obligation to Dr. Kimball for valuable advice and encour- 

 agement throughout the progress of the work. 



The method employed is essentially that given by Maxwell, 

 vol. ii, § 776. It was used by J. J. Thomson in his determina- 

 tion of v, published in the Phil. Trans, for 1883. The follow- 

 ing is substantially his description of the method. In a 

 Wheatstone bridge ABCD (fig. 1), the circuit BD is not closed 

 but the points B and D are joined to two poles S and R of a 

 commutator, between which vibrates the armature P, which is 



connected with the inner shell of a 

 spherical condenser. When P 

 touches S the condenser will be 

 charged, and there will be a mo- 

 mentary current through the vari- 

 ous arms of the bridge, through 

 the galvanometer from D to C 

 When P touches R the two sur- 

 faces of the condenser are con- 

 nected and the latter discharges 

 itself through DR. If now the 

 armature be made to vibrate, con- 

 tinuously there will be a series of 

 " momentary currents through the 

 galvanometer, and by adjusting the resistance a (c and d being 

 large, fixed resistances), these interrupted currents may be 

 exactly counterbalanced by the steady current from C to D, 

 and the resultant deflection of the galvanometer is zero. 

 When this is the case there is a relation between the capacity 



