136 Prof. H. F. Weber on Electromagnetic and 



J =424*50 m.-k., cannot be compared at once with that ob- 

 tained from the behaviour of gases, J = 428*95 m.-k. The 

 two values are referred to quite different units : the basis of 

 the former is ] ° of Joule's mercury thermometer ; that of the 

 latter is 1° of the air thermometer. These two units may pos- 

 sibly differ 1 per cent. Perhaps, on reducing his previous 

 and his recent friction-experiments to the indications of the air 

 thermometer, Dr. Joule obtains a final result as good as iden- 

 tical with the value of J that follows from the behaviour of 

 gases. 



On account of this disturbing circumstance, I hold the value 

 J=428*95, drawn from the behaviour of gases, and immedi- 

 ately comparable with my above result, to be the most certain 

 of those given by purely thermal determinations. Conse- 

 quently, from the heat-evolution of stationary galvanic cur- 

 rents there comes as good as the same mechanical equivalent 

 of the heat-unit as from purely thermal processes*. The hy- 

 pothesis that the entire work consumed in the stationary cur- 

 rent-flow appears in the form of heat has verified itself. 



There still remains to say a few words on the already men- 

 tioned determinations, carried out by Joule and Yon Quintus 

 Icilius, of the mechanical equivalent of the heat-unit by gal- 

 vanic heat-evolution. 



Dr. Joule carried out 45 experiments, in three series f. 

 He regards as the most trustworthy result that of the last 

 series, comprising 30 experiments — J = 429*3 m.-k. In the 

 calculation of this number the specific heat of water at 18°*4 

 was put =1, and it was further assumed that the British unit 



of resistance in fact possesses the asserted value 10 10 ( * ). 



\ sec. / 



According to our results this is not quite exactly the case : if 



the ratio of the British unit to Siemens's is as 1 : 0*9536, then 



10 io / millimA 



V sec. /' 



* The two results, J= 428-15 (derived from the galvanic evolution of 

 heat) and J = 428-95 (determined from the thermal behaviour of the per- 

 manent gases), refer, as was expressly remarked, to two different units of 

 heat : in the former the unit is that quantity which can heat the unit of 

 mass of water from 17°"5 to 18°-5 ,• in the latter it is that which can heat 

 the mass-unit of water from 14° to 15°. Therefore the two results will 

 only then be strictly comparable, when the variation of the specific heat of 

 water at variable temperature is certainly known. The experiments 

 which I have, up to the present, instituted for fixing this hitherto totally 

 uncertain quantity are not yet brought to a perfectly satisfactory conclu- 

 sion. Yet so much can be positively known, that the reduction of the 

 two values of J to the same temperature will bring about only a very 

 slight alteration. 



t Reports of Electrical Standards, edited by Jenkin, p. 175. 



the absolute value of the former is = 1*0014 x 



