1866.] 



of a Disk by rapid Rotation in vacuo. 



291 



blackened aluminium disk than from an unblackened one does not prove 

 that this heating effect may not be due to air, since the blackened surface 

 may be imagined to lay hold of the air more than the metallic one. But 

 the following sets of experiments prove that the heating effect of the alu- 

 minium disk with both sides blackened is the same as when only one side 

 is blackened. 



No. of set. 



No. of ob- 

 servations 

 in each set. 



Time at full 

 speed, in 

 seconds. 



No. of turns 

 of handle 

 at full 

 speed. 



Heat in- 

 dication, 

 in°Fahr. 



Tension 

 of air. 





X. 



2 



30 



20 



0-9 



1-1 



Disk blackened on one side. 



XIII. 



3 



30 



20 



0-8 



0-4 



Disk blackened on both sides. 



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

 heating effect is due to the increased temperature of the disk. 



1 7. Before proceeding further it may be advisable to detail some experi- 

 ments made with an ebonite disk inch thick. In these experiments 

 care was taken that the ebonite should have the same temperature through- 

 out its thickness, so that there might be no flow of heat from the interior 

 to the surface, or vice versa. The experiments were made with rock-salt 

 in the cone. 



No. of set. 



No. of obser- 1 



vations in 



1 each set. 



Time at full 

 speed, in se- 

 conds. 



No. of turns 

 of handle at 

 full speed. 



Heat indica- 

 tion, m divi- 

 sions. 



Nature of 

 gas. 



Tension 

 of gas, in 

 inches. 



Probable puri- 

 ty of gas (ab- 

 solutely pure 

 gas=100). 



xiy. 





3 





30 



20 



32 



air 



1-1 





XV. 





1 





30 



20 



30 



air 



0-26 





XVI. 





1 





30 



20 



31 



air 



1-1 





XVII. 





2 





30 



20 



28-5 



air 



0-26 





XVIII. 





2 





30 



20 



28-5 



air 



0-25 





XIX. 





3 





SO 



20 



29 jhydrogen 



0-25 



90 



From these experiments it may be taken for granted that the heat indi- 

 cation given by an ebonite disk is, like that from an aluminium disk, inde- 

 pendent both of the density and chemical constitution of the residual gas. 

 It is also highly probable that the unknown cause of the heating effect is 

 the same for both disks. 



IS. To return now to the aluminium disk, it may be shown that the 

 heating of this disk is not caused by revolution under the earth's mag- 

 netic force ; for (1) the following calculation, kindly furnished by Professor 

 Maxwell, shows that the heating effect due to this cause would, for the 

 aluminium disk -^^ of an inch in thickness, amount, under the circum- 

 stances of rotation, only to fi-y^o ^ degree Fahr., whereas the observed 

 effect is more than half a degree. 



An elHpsoid, semiaxes a, h, c, revolves about the axis c with velocity w, 



2 B 2 



