the Heating of a Disk by rapid Rotation in vacuo. 229 



conducted across such a washer is the same as if for one minute the 

 temperature difference between both sides of the washer were kept at 

 2° Fahr., and if we make allowance for the surface and for the thick- 

 ness of the washer, we obtain the following expression as approxi- 

 mately representing the heat conducted across the washer during 

 the experiment, 



Heat=-011x 2x^X^? = '028unit nearly, 

 144 -2 J 



where the first factor is on account of the double temperature differ- 

 ence, the second on account of the surface, and the third on ac- 

 count of the thickness. 



But a unit of heat in the above expression denotes the amount ne- 

 cessary to raise a cubic foot of water (or nearly 1000 ounces) 1° F. 

 Now the weight of the disk is 10'5 ounces, and its specific heat is 

 0*22. Hence the above amount of heat will raise the disk 



•028 x l ~ X 4h= 12° Fahr. in temperature. 

 10*5 '£6 



Hence we see that if the material of the washer had been of the 

 metal bismuth, of which the conductivity is 7 times less than that of 

 iron, and if we suppose the circumstances of the experiment to be 

 equivalent to a temperature difference 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 

 distribution 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 experiments, where the disk was me- 

 tallically 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 conduc- 

 tion constitutes only an exceedingly small fraction of that observed. 



