1866.] 



of a Disk hy rapid Rotation in vacuo. 



293 



Now 772^= energy required to raise 1 grain of water 1° Falir. 



The disk= — — ^ '22 grain of water, energy corresponding to 1° 



or about -^^270 ^^^"^^^ '^"^ seconds- 



This is when the heat is uniformly distributed through the thickness of 

 the disk, which it will be in less than thirty seconds. If there were no 

 conduction, the rise of temperature at the surface would be about twice the 

 value found above. 



(2) It would appear from the above formula that the heating effect due 

 to this cause should increase with the thickness of the disk. The follow- 

 mg experiments show that, on the contrary, the heating effect, as regards 

 temperature, diminishes as the thickness of the disk increases. 



No. of set. 



No. of ob- 

 co CO servations 

 1 in each set. 



Time at full 

 speed. 



No. of turns 

 of handle 

 at full 

 speed, 



Heat indi- 

 cation, in 

 °Fahr. 



Tension of 

 air, in 

 inches. 





XIII. 

 XX. 



30 

 30 



20 

 21 



0- 8 



1- 7 



0-4 

 0-4 



Aluminium disk inch thick. 

 Aluminium disk -^^ inch thick. 



(3) The heat indication afforded by an ebonite disk is against the con- 

 clusion that this effect is due to rotation under the earth's magnetic force. 



It would therefore appear to be proved that in these experiments the 

 increased temperature of the aluminium disk is not due to rotation under 

 the earth's magnetic force. 



19. It might perhaps be said that the heating of the disk may be due 

 to heat conducted from the bearings into the disk and then distributed 

 outwards ; and this conjecture will require to be examined, since the bear- 

 ings are, no doubt, heated by friction during the motion. This heating 

 effect on the bearings was measured by means of a very delicate tliermo- 

 meter, which was inserted into a small hole in the bush through which oil 

 is supplied to the spindle, and made to be in metallic contact with the 

 sides of this hole ; the mean of three observations made the heating effect 

 at the spindle due to rotation to be 4° Fahr. 



In the next place, the aluminium disk, separated from its metallic 

 spindle by the ebonite washer, and in every respect the same as when 

 made to rotate, had its spindle heated artificially by a mercury bath, gene- 

 rally at least 40° Fahr. above the previous temperature. After the lapse of 

 two minutes tlie effect upon the pile was hardly perceptible — not more 

 than five divisions. 



1) It would appear from this experiment that the heating effect ob- 



32'2 



= 772 x-j^ X 15400, 



^ 12-91 12-91 



or rise of temperature per ^^^^^^^^^i^^QxibAQO^ Tdll^O^O 



degree Fahr. 



1 



