the Heating of a Disk by rapid Rotation in vacuo. 231 



' (2) The thin aluminium disk was out of truth about '02 inch on 

 each side. Its note of vibration was as nearly as possible one octave 

 lower than that of the thick disk, while its coefficient of viscosity was 

 somewhat greater, say in the proportion of 3 to 2, than that of the 

 thick disk. On the supposition that the heat generated is due to 

 vibration, if we call the heat generated during 30 seconds in the thick 

 disk =1, then that generated during the same time in the thin disk 

 ought to be 



1 



x l xC 02 Yx±xz-± 



where the first factor is on account of difference of time of vibration, 

 the second on account of difference of range, the third on account of 

 difference of mass, and the fourth on account of difference of viscosity. 



But the heating effect (as far as quantity of heat is concerned) pro- 

 duced in the thin disk is as nearly as, possible the same as that pro- 

 duced in the thick disk. 



This fact is therefore against the hypothesis that the heating 

 effect is due to vibration. 



(3) In order to estimate the effect (if any) of want of truth in the 

 disk, the thick aluminium disk was purposely put out of truth about 

 3J times its usual amount ; but the heating effect was as nearly as 

 possible the same in both cases, being 32 divisions of the scale in both. 



On all these grounds it would appear that the heating effect cannot 

 be due to vibration of the disk. 



21. It is hardly necessary to mention that the heating effect cannot 

 be due to radiation and convection from the wheelwork, which is 

 no doubt slightly heated during the experiment, for the mass of this 

 metallic matter is so great, that we cannot imagine it to be heated 

 more than 1° Fahr. Now the radiation from this against the back 

 of the disk may certainly be neglected, while the convection must be 

 very small, since in the experiments the pressure of the air was very 

 small. Besides, the heating effect, as will be seen shortly, was found 

 to be independent of the pressure. 



22. It has thus been shown that the disk is heated during the 

 experiment, and that this heating effect — 



(1) Is not due to rotation under the earth's magnetic force ; 



(2) Is not due to conduction of heat from the bearings ; 



(3) Nor to radiation or convection from the wheelwork 



(4) Nor to vibrations of the disk. 



And in view of the large and constant nature of this heating effect 

 it may be asserted that it cannot be sensibly due, either to one of 

 these causes singly, or to their combined effect. 



23. It will now be shown that the heating effect is independent 

 both of the density and chemical constitution of the residual air 

 and vapour around the disk. 



In art. 16, if we compare together the 7th, 8th, 9th, lOtli, 11th, 

 and 12th sets of experiments, we shall see that the heating effect was 

 sensibly the same, whether the residual gas was atmospheric air, 

 or hydrogen or coal-gas. 



