1873.] 



Disk by rapid Rotation in vacuo. 



317 



not greatly radiate during rotation of the disk (art. 26). In a paper 

 disk, however, which is formed of a badly conducting material loosely 

 put together, part of the effect does escape as radiation during rotation 

 (art. 27). 



(3) There is a residual effect, which is more deeply seated than the 

 gas-effect. And inasmuch as radiation takes place from a perceptible 

 depth, this effect is much more influential than the gas-effect in increasing 

 radiation after rotation. In the case of a paper disk, this deeply seated 

 effect will be less diminished by radiation during rotation than the gas- 

 effect, and therefore after rotation in such a disk we might expect the 

 gas-effect to be peculiarly small (art. 35). 



47. In the course of these experiments we have endeavoured to prove 

 that this residual effect is not caused by vibration. The radiation-expe- 

 riments with aluminium disks of three different thicknesses went, on the 

 other hand, to show that it was of the nature of a surface-effect. This 

 is confirmed by the results derived from tapping ; for, in the first place, 

 the experiments of art. 45 show that the two effects (the residual and 

 the gas-effect) are probably distributed in the same proportion, going 

 from the centre to the circumference of the disk. Again , taking the two 

 disks of thickness -05 and -025 of an inch, we obtain the following re- 

 sults : — 



Effect for hyd. Effect for ^ hyd. +^ air. 



Thin disk 48 (22 observations). 228 (10 observations). 



Thick disk 29 (20 observations). 108 (10 observations). 



Now, allowing for errors of experiment, we see that the residual, as 

 well as the gas-effect, is reduced to about one half for the thick disk. 



Again, an experiment of a similar nature gave the effect for ^ hyd. in 

 an ebonite disk of ^ in. in thickness =33, against a result =55 for the 

 thin ebonite disk. Unfortunately it was omitted to make a comparison 

 with these two disks for the gas-effect ; nevertheless these results are all 

 in favour of the residual effect being a surface-effect. 



48. It might be well to make one remark regarding these experiments. 

 They are not like the radiation-experiments, which required an extremely 

 delicate instrument in order to give a sensible effect. But, on the other 

 hand, the effect obtained by tapping being that due to the mere surface of 

 the disk, is liable to be altered by any thing which affects the surface of 

 the disk. "We have come to the conclusion that in such experiments it is 

 unadvisable to use a porous hygrometric surface, such as that of paper, 

 not having its pores filled with paraffin or some other similar substance. 

 It is likewise desirable that all parts of the apparatus should be as nearly 

 as possible of the same temperature ; indeed we suspect that some ex- 

 periments made during some very peculiar summer weather were in- 

 fluenced by a want of temperature-equilibrium between the various parts 

 of the apparatus, the result appearing to be that the gas-effect for the 

 ebonite disk was abnormally large, 



