44 



ON THE RATIO OF THE ELECTROMAGNETIC TO THE 

 ELECTROSTATIC UNIT OF ELECTRICITY 



Br flcxKr A. ROWLAXD, with the *UUnc<r of E. H. BALL mud L. B. FLETCMEK 



(PkitMipktrml MmpuiHe [5J, XXVIII. 304-315, 1889; 4wrfe SOWTM/ / &* [S], 



JTJTJT K///, 299-998, IMf] 



The determination described below was made in the laboratory of 

 the Johns Hopkins University about ten years ago, and was laid aside 

 for further experiment before publication. The time never arrived to 

 complete it, and I now seize the opportunity of the publication of a 

 determination of the ratio by Mr. ROM in which the same standard 

 condenser was used, to publish it. Mr. Rosa has used the method of 

 getting the ratio in terms of a resistance. Ten years ago the absolute 

 resistance of a wire was a very uncertain quantity and, therefore, I 

 adopted the method of measuring a quantity of electricity electro- 

 statically and then, by passing it through a galvanometer, measuring it 

 electromagnet ically. 



The method consisted, then, in charging a standard condenser, whose 

 geometrical form was accurately known, to a given potential as meas- 

 ured by a very accurate absolute electrometer, and then passing it 

 through a galvanometer whose constant was accurately known, and 

 measuring the swing of the needle. 



DESCBIPTIOX OF IXSTBCTCEXTS 



Ekctrt/rnetT. This was a very fine instrument made partly according 

 to my design by Edelmann, of Munich, As first made, it had many 

 faults which were, however, corrected here. It is on Thomson's guard 

 ring principle with the movable plate attached to the arm of a balance 

 and capable of accurate adjustment. The disc is 10-18 cm. diameter 

 in an opening of 10-38 cm. and the guard plates about 33-0 cm. diam- 

 eter. All the surfaces are nickel plated and ground and polished to 

 optical surfaces and capable of accurate adjustment so that the dis- 

 tance between the plates can be very accurately determined. The 

 balance is sensitive to a mg. or less and the exact position of the beam 



