Ratio of the Electromagnetic to the Electrostatic Unit. 315 



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, 

 however, it may possess some interest for the scientific 

 world. 



XXXIX. Determination 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 Labora- 

 tory of Johns Hopkins University during the months of 

 March to June 1889, under the direction of Associate Pro- 

 fessor A. L. Kimball. The writer takes great pleasure in 

 acknowledging his obligations to Dr. Kimball for valuable 

 advice and encouragement 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 determi- 

 nation of v, published in the ' Philosophical Transactions ' for 

 1883. The following is substantially his description of the 

 method. In a Wheatstone bridge, ABCD (fig. 1), the circuit 



B D is not closed, but the points D and B are joined to two 

 poles R and S 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 momentary current through 



* Communicated by Prof. Rowland, 



