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XXXVIXI. On the Ratio of the Electrostatic to the Electro- 

 magnetic Units of Electricity. By Henry A. Rowland, 

 with the assistance ofE. H. Hall and L. B. Fletcher *. 



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 determina- 

 tion of tho ratio by Mr. Rosa, 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 un- 

 certain quantity, and therefore I adopted the method of 

 measuring a quantity of electricity electrostatically, and 

 then, by passing it through a galvanometer, measuring it 

 electromagnetically. 



The method consisted, then, in charging a standard con- 

 denser, whose geometrical form was accurately known, to a 

 given potential as measured 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. 



Description of Instruments. 



Electrometer. — 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 disk is 10-18 centim. diameter 

 in an opening of 10'38 centim., and the guard-plates about 

 330 centim. diameter. All the surfaces are nickel-plated and 

 ground and polished to optical surfaces and capable of 

 accurate adjustment, so that the distance between the plates 

 can be very accurately determined. The balance is sensitive 

 to a millig. or less, and the exact position of the beam is read 

 by a hair moving before a scale and observed by a lens in the 

 manner of Sir W. Thomson. 



The instrument has been tested throughout its entire range 

 by varying the distances and weights to give the constant 

 potential of a standard gauge, and found to give relative 

 readings to about 1 in 400 at least. It is constructed through- 

 out in the most elaborate and careful manner, and the 



* Communicated by the Author. 



