Heating of a Disk by rapid Rotation in vacuo. 2 21 



32. These experiments were conducted in the following man- 

 ner: — The disk was first of all tapped before rotation several 

 times ; at each tapping the momentary swing of the needle was re- 

 corded, and the mean of the readings was regarded as indicating the 

 state of the disk with respect to heat. 



The disk was next tapped after rotation, and the difference 

 between the readings before and after was taken as indicating the 

 change in the state of the disk produced by rotation. In the later 

 experiments the disk before rotation was kept in slight motion in 

 order to equalize any tendency to unequal heating of its various 

 parts; but this was not done in the experiments of June 1871. 

 In these experiments (June 1871) the disk was of brown paper, 

 and the results obtained were as follows : — For a carbonic-acid va- 

 cuum (pressure 0-065 in.) a swing of 307 divisions was recorded, 

 while for a hydrogen vacuum (pressure 0*150 in.) a swing of 281 

 divisions was recorded. Each result was the mean of three rotations. 



33. The next experiments were made in January 1872. The gal- 

 vanometer was one of Thomson's, but more adapted for battery 

 currents, and hence not in, a very delicate state for these experi- 

 ments ; the time of vibration of the needle was 3 seconds. The 

 disk used was of ebonite (thickness about -g^ in.). The ebonite of 

 this disk was completely black ; and in this respect, as well as in 

 being thinner, it differed from the ebonite disk first used in the 

 radiation-experiments (art. 17). In the present experiments the 

 centre of the pile was made to tap the disk at a distance of 1*5 

 in. from the rim. The amount and velocity of rotation were re- 

 presented by 30 turns of the handle, or 3750 turns of the disk, in 

 about 40 seconds. The following results were obtained, a result 

 representing on an average somewhat more than four rotations : — 



Pressure of Heat-indication for 



residual gas, ,. * N 



in inches. Dry hydrogen. Dry air. Dry carbonic acid. 



(A) fa 11-5 35-0 33-0 



(B) fa 7-5 15-0 15-0 



It would thus appear that the results derived by tapping are 

 very different from the radiation-results, inasmuch as in the former 

 the effect of the pressure and quality of the residual air is very ap- 

 parent, while in the radiation-results it is hardly perceptible. A 

 probable explanation of this will be given afterwards (art. 46) ; but 

 in the mean time, in view of these results, it will be expedient to 

 discuss them quite independently and by themselves, with the view 

 of ascertaining whether they can best be explained by a gas-effect 

 alone, or whether they likewise indicate a residual effect indepen- 

 dent of gas. 



34. With this object let us take \ — -^- as representing" the 



whole effect at a pressure of fa in., due to whatever cause or causes. 

 We thus obtain 



Dry hydrogen. Dry air. Dry carbonic acid. 



Whole effect at fa . . 9-5 25-0 24-0 



