Heating of a Disk by rapid Rotation in vacuo. 219 



1868, by means of a direct comparison of electrostatic attraction 

 with electromagnetic repulsion. His experiments gave v = 288 x 10 8 

 centims. per second. 



The value of v given by the experiments here described is 

 293 x 10 s centims. per second. The method employed was that 

 of obtaining an absolute electrostatic and an absolute electromag- 

 netic measurement of the same electromotive force, v is defined 

 as the ratio of the units of quantity in the two systems ; but it 

 follows from the definition of electromotive force, that v is also the 

 ratio of the units of electromotive force in the two systems. 



The electromotive force, or the difference of potentials between 

 the two poles of a constant Daniell's battery, was measured elec- 

 trostatically by means of Sir William Thomson's absolute electro- 

 meter. The absolute electromagnetic value of this electromotive 

 force was given by the effect of the current which it maintained 

 in the circuit of an electrodynamometer. The determination of 

 this value depended on the resistance of the electrodynamometer- 

 circuit, which was reckoned in terms of the absolute value of the 

 British-Association standard unit of resistance. Any correction 

 which may hereafter be found to be applicable to the absolute 

 value of this standard coil, as measured at King's College by Pro- 

 fessors Clerk Maxwell, Balfour Stewart, and Fleeining Jenkin, must 

 be applied to the value of v given above. 



The comparisons made in 1867 and 1868 by Mr. King gave as 

 the mean value of v, 284*6 x 10 8 centims. per second. 



The experiments made in 1870 with the new absolute electro- 

 meter gave as the mean result f=294-5 x 10 8 centims. per second. 

 The result of later observations made under much more favourable 

 circumstances was v =292-4 x 10 8 centims. per second. 



The latest observations (1872) furnish the most probable value 

 of v, 293 x 10 8 centims. per second. 



" On the Heating of a Disk by rapid Rotation in vacuo." By 

 Balfour Stewart, M.A., F.R.S., and P. G. Tait, M.A. 



26. In two previous communications (Proc. Roy. Soc. June 15, 

 1865, and No. 88, 1866) to this Society, we gave an account of 

 some experiments which we had made upon the heating of a disk 

 through rotation in vacuo. In these experiments the increase of 

 radiation of the heated disk was observed by means of a delicate 

 thermopile and galvanometer. Three aluminium disks of various 

 thicknesses and one ebonite disk were used ; and the results derived 

 from the experiments were as follows : — 



(1) The heating effect observed appeared to be independent of 

 the density, and of the chemical constitution, of the residual air and 

 vapour surrounding the disks. 



(2) The quantity of heat developed under similar circumstances 

 of rotation in three aluminium disks -05, *0375, -025 of an inch in 

 thickness respectively appeared to be the same, inasmuch as the 

 relative thermometric effect for these disks varied inversely as their 

 thickness. 



