128 Prof. H. F. Weber on Electromagnetic and 



On this hypothesis, consequently, the proportionality-factor of 

 Joule's law of heat-evolution is equal to the reciprocal value 

 of J. Supposing that this assumption, the whole of the me- 

 chanical work consumed by the stationary galvanic current 

 appears in the form of heat, is correct, we have a new definition 

 for the absolute resistance of a conductor : — The absolute re- 

 sistance (measured according to any system) of a conductor 

 is equal to the mechanical value of the amount of heat which 

 is generated in the conductor in the unit of time by the con- 

 stant galvanic current 1 (measured according to the same 

 system of measurement). And a new method for the experi- 

 mental determination of the absolute resistance of a conductor: — 

 Measure the amount of heat, Q, which in the time z is gene- 

 rated by the constant current i measured in absolute measure ; 

 then the absolute value of the resistance (measured in the 

 same system of measurement in which i is measured) is 





,_JQ 



It cannot be maintained that the correctness of the hypo- 

 thesis, "in the stationary galvanic current the entire work of 

 the current is converted into heat," is so far above all doubt 

 that one can without hesitation make use of the heat developed 

 in a conductor by the stationary galvanic current for the abso- 

 lute measurement of the resistance of the conductor. The 

 results of the most exact investigations which have yet been 

 instituted in this direction for testing the fundamental hypo- 

 thesis in question contradict one another. Von Quintus 

 Icilius (Pogg. Ann. vol. ci. 1856), in a carefully executed 

 very extended series of operations, obtained the final result 

 that the stationary galvanic current develops about 7 per 

 cent, more heat than it should according to Thomson's equa- 

 tion ; on the other hand, Joule *, in a comprehensive and very 

 accurately executed investigation, with which he was charged 

 by the British-Association Committee for the production of 

 standards of resistance, found by experiment that in fact almost 

 as much heat is produced in a conductor by the stationary 

 galvanic current as is specified by the above-mentioned law. 

 Yon Quintus Icilius calculates from his experiments the 

 mechanical equivalent of the unit of heat to be 399*7 metre- 

 kilograms ; Joule infers from his the value 429*3 metre-kilo- 

 grams for J (expressed in the usual measure of mechanical 

 work). While the discrepancy between the results of these 

 two series of observations is not cleared up the galvanic heat- 



* Reports of Electrical Standards, edited by Jenkin, p. 165. 



