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IV. Electromagnetic and Calometric Absolute Measurements: 

 the Absolute Value of Siemens' s Unit of Resistance in Electro- 

 magnetic Measure ; the Relation between the Current-work 

 anal the Heat-evolution in stationary Galvanic Currents ; and 

 the Absolute Values of some constant Hydroelectromotive 

 Forces in Electromagnetic Measure. ( Condensed Comparison 

 of the Results of a Series of Investigations.) By H. F. 

 Weber, Professor of Mathematical and Technical Physics 

 at the Federal Polytechnic Academy of Zurich *. 



SINCE Siemens's unit of resistance has been admitted into 

 the department of galvanic measurements, the attempt has 

 been made in four different quarters to fix the absolute value 

 of this empiric unit — that is, to determine in absolute measure 

 that electromotive force which, in a conductor whose resistance 

 is equal to that of Siemens's unit, is capable of calling forth a 

 current of absolute intensity. 



1. The fundamental system of measurement was the elec- 

 tromagnetic. 



In 1862, W. Weber, according to a method devised by 

 himself (Abhandlungen der Gottinger Gesellschaft, Band x.}, 

 found as the absolute value of the Siemens mercury unit : — 



1 S.M.U.=l-0257xl0 



10 



(millimA 

 sec. / 



According to the same method, and by aid of the same in- 

 struments, F. Kohlrausch |? eight years later, repeated the 

 determination, and, from four different measurements, obtained 

 as the mean value 



1 S.M.U. =0-9717 xl0 10 (^^Y 



\ sec. / 



The committee appointed by the British Association for the 

 Advancement of Science to determine upon a suitable unit of 

 resistance, consisting of Messrs. Clerk Maxwell, Balfour 

 Stewart, and Jenkin, in the course of the years 1863 and 1864 

 produced a resistance, the British-Association unit (by English 

 physicists called also the " ohm "), which is said to represent 

 in electromagnetic measure exactly the absolute value 10 10 



( '-). According to the best comparisons, this British 



unit is to the Siemens as 1 : 0'9536 ; consequently, according 

 to the measurements of the English physicists, the absolute 



* Translated from the German original communicated by the Author, 

 t Pogg. Ann. Erg. -Band vi. p. 1. 



