[Proc. Roy. Soc. Victoria, 32 (N.S ), Pt. II., 1920]. 



Aht. X. — A New Method of DeterminiTig the Mechanical 

 Equivalent of Heat 



BY 



Professor T. H. LABY, M.A. 



AND 



J. K. ROBERTS, B Sc. 



(Natural Philosophy Department, University of Melbourne). 



(With Plates X. and XI. and 1 Text Figure.) 

 [Read 11th September, 1919.] 



Introduction. 



It -appeared to the writers' that an additional direct determina- 

 tion of the mechanical equivalent of heat would be of value, and 

 might, in view of the progress of precise measurement, attain 

 higher accuracy than that previously realised, and, incidentally, 

 information might be obtained as to the accuracy of the electrical 

 units. 



After a long series of preliminary experiments, a method, in 

 ^vhich a copper cylinder is heated by a rotating magnetic field, has 

 been devised, possessing the characteristics — (1) of accurately 

 stationary temperatures, and (2) that a small fraction of the heat 

 developed is lost. This method is being carried out so as to attain, 

 if possible, an accuracy of 1 in 10,000 in J, the mechanical equiva- 

 lent of heat. 



Previous Determinations. 



Therii have been five determinations^ of the mechanical equivalent 

 of heat, to which it is necessary to refer — namely, those made by 

 Joule, Rowland, Misculescu, Reynolds and Moorby and Rispail. 

 Of these experiments only those of Rowland and of Reynolds and 

 Moorby appear to possess high accuracy, and they are not imme- 

 diately comparable, as the values of J, which they give, are in terms 

 of different heat units. Several determinations have been made of 

 what may be called the electrical equivalent of heat. If the 

 mechanical equivalent, J, were known to an accuracy of 1 in 

 10,000 (which should not be beyond attainment), the degree of its 



1. Joule, Phil. Trans., Vol. CXL. p. 61 (1850) ; Rowland. Proc. Amer. 

 Acad., Vol. XV., p. 75 (1879) ; Miculescu. Journ. de Phys., Vol. I., p. 104 

 (1892) ; Reynolds and Moorby, Phil. Tran. Vol. CXC, p. 301 (1897) ; Ris- 

 jiail, Ann. Chim. Phys,, Vol. XX., p. 417, (1910). 



