1866.] 



of a Dish by rapid Rotation in vacuo. 



295 



we suppose the circumstances of the experiment to be equivalent to a tem- 

 perature diiference of 2° Fahr. between the two sides of the washer lasting 

 for one minute, then the quantity of heat conducted across the washer will 

 be a little greater than that observed. But the conductivity of ebonite is 

 no doubt very much less than that of bismuth, and therefore on this 

 account we cannot suppose that the heating effect observed is due to 

 conduction. 



(3) In this investigation no account has been taken of the unequal dis- 

 tribution of temperature from the centre to the circumference of the disk, 

 the tendency of which would be to diminish the effect upon the pile (which 

 was directed to the circumference of the disk) of the heat passing through 

 the washer ; and indeed, when this element is taken into account, it is not 

 surprising to find, as was actually the case, that in some preliminary ex- 

 periments, where the disk was metallically connected with the spindle, the 

 effect was not greater than with the ebonite washer. 



(4) The short time in which the effect attains its maximum value is 

 against the supposition that it is caused by conduction from the bearings. 



(5) The fact that (as we shall afterwards see) the temperature effect in 

 three aluminium disks of different thicknesses is inversely proportional to 

 the thickness, is also against this supposition. 



(6) And so is the fact that a heat-effect obeying apparently the 

 same laws, holds for an ebonite disk in which there is but a very feeble 

 conduction. 



On the whole, therefore, we cannot suppose this effect to be due to 

 conduction, or at least we must conclude that the effect of conduction con- 

 stitutes only an exceedingly small fraction of that observed. 



20. It was suggested to the authors by Professor Stokes and by Mr. Grove, 

 that the effect might be due to vibrations of the disk, the energy of which, 

 owing to the viscosity of the disk for such vibrations, might ultimately 

 become converted into heat ; and it is necessary to examine this question. 



(1) The thickest aluminium disk was found to be out of truth not more 

 than '015 inch on each side. Hence, the thickness of this disk being '05 

 inch, when turned with moderate rapidity, its apparent thickness should be 



•015 + -05-f 015 = -08; 



and experiment showed that when turned very fast, its apparent thickness 

 was no greater. The greatest possible range of vibrations of the disk at 

 its circumference could not, therefore, be more than '015 inch on either 

 side of the position of rest. 



Again, it was ascertained by means of the note given by this disk, that 

 it vibrates about 250 times per second. 



Let us suppose the whole mass to have the same range of excursion 

 (this will of course increase the result), the equation of vibration (not 

 allowing for loss by viscosity) is 



a'm='0l5 cos nt 



