OP ARTS AND SCIENCES. 



151 



The theory of the process is as follows. The energy of electricity 

 being the product of the potential by the quantity, the energy ex- 

 pended by forcing the quantity of electricity, Q, along a wire of re- 

 sistance, A', in a second of time, must be Q 2 R, and as this must equal 

 the mechauical equivalent of the heat generated, we must have 

 JH= Q' Ht, where // is the heat generated and t is the time the 

 current Q flows. 



The principal difficulty about the determination by this method 

 seems to be that of finding R in absolute measure. A table of the 

 values of the ohm as obtained by different observers, was published by 

 me in my paper on the u Absolute Unit of Electrical Resistance," 

 in the American Journal of Science, Vol. XV., and I hero give it 

 with so:r.c changes. 



TABLE XXXI. 



The ratio of the Siemens unit to the ohm is now generally taken at 

 .953G, though previous to 18G4 there seems to have been some doubt 

 as to the value of the Siemens unit. , 



Since 18G3-4, when units of resistance first began to be made with 

 great accuracy, two determinations of the heat generated have been 

 made. The first by Joule with the ohm, and the second by H. F. 

 Weber, of Zurich, with the Siemens unit. 



Each determination of resistance with each of these experiments 

 gives one value of the mechanical equivalent. As Lorenz's result was 

 only in illustration of a method, I have not included it among the 

 exact determinations. 



The result found by Joule was /= 25187 in absolute measure 



* Given .9912 by mistake in the other tables. 



