THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FIFTH SERIES.] 



JULY 1885. 



I. A Determination of the B.A. Unit in Terms of the Mecha- 

 nical Equivalent of Heat. By La whence B. Fletcher, 

 Ph.D.* 



THE experimental work of the following investigation was 

 completed in 1881, and forms the subject of a thesis 

 submitted to the Johns Hopkins University in that year. In 

 the present paper a more accurate method of calculating the 

 currents from the deflection-curves is used, and some of the 

 other calculations have been revised. The results of the two 

 papers are substantially the same. 



The experiment consisted of simultaneous thermal and 

 electrical measurement of the energy expended by a current 

 in a coil of wire immersed in a calorimeter. The result de- 

 pends upon the values of the mechanical equivalent and the 

 unit of resistance, and gives a determination of either in terms 

 of an assumed value of the other. 



The old determinations of Quintus Icilius and Lens have 

 no value, as the resistance is uncertain, as pointed out by 

 Rowland and H. F. Weber. 



Joulef, in 1867, made a determination of the mechanical 

 equivalent by this method, assuming the B.A. unit as deter- 

 mined by the Committee in 1863-4 to be equal to 10 9 C.G.S. 

 units. The value of the mechanical equivalent thus obtained 

 is more than 1 per cent, greater than Joule's water-friction 



* From an advance proof from Silliman's American Journal, commu- 

 nicated by the Author. 



t Report of British Association Committee on electrical standards, 1873. 

 X Phil. Mag. S. 5. vol. v. pp. 30, 127, 189. 



Phil. Mag. S. 5. Vol. 20. No. 122. July 1885. B 



