418 HENRY A. EOWLAND 



apparatus results so near those from my elaborate apparatus, which 

 almost grinds out accurate results without labor except in reduction. 

 Indeed, the quantity is the same as I find at about 20 C. 



The experiments of Him of 1860-61 seem to point to a value of the 

 equivalent higher than that found by Joule, but the details of the 

 experiment do not seem to have been published, and they certainly 

 were not reduced to the air thermometer. 



The method used by Violle in 1870 does not seem capable of accur- 

 acy, seeing that the heat lost by a disc in rapid rotation, and while 

 carried to the calorimeter, must have been uncertain. 



The experiments of Him are of much interest from the methods 

 used, but can hardly have weight as accurate determinations. Some 

 of the methods will be again lef erred to when I come to the description 

 of apparatus. 



Method by Heat Generated by Electric Cwrent 



The old experiments of Quintus Icilius or Lenz do not have any 

 except historical value, seeing that Weber's measure of absolute resist- 

 ance was certainly incorrect and we now have no means of finding its 

 error. 



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, R, in a second of time, must be Q Z R, and as this must equal 

 the mechanical equivalent of the heat generated, we must have JH 

 Q z Rt, where H 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 'Absolute Unit of Electrical Besistance/ in 

 the American Journal of Science, Vol. XV, and I give it here with 

 some changes. 



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

 9536, though previous to 1864 there seems to have been some doubt 

 as to the value of the Siemens unit. 



Since 1863-4, 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. 



