272 Prof. A. P. Chattock on the Forces at the 



as the effect o£ the B ions will be shown to be small, we may 

 obtain an idea of its magnitude if we assume r to vary 

 uniformly from its maximum value to zero in passing through 

 the ionizing layer, put 4:7rft = 27rrt, and take for f in the 

 expression for r the value obtained from the pull, viz. 



Estimating the thickness of the glow as 0*005 cm., a 

 number which is certainly too high, and assuming that this 

 represents t, 2i7Tt is about 1 per cent, of f for the sharpest 

 point used and much less for all the others, when the current 

 is 15 microamperes. 



Whether the glow and the ionizing region are exactly equal 

 in extent is, however, doubtful. As already mentioned, f for 

 discharge is proportional to r ' 45 ; the value of f therefore 

 increases rapidly with the curvature of the point, and it is 

 difficult to see why this should be, unless the only effect of the 

 curvature upon the field (viz. the divergence of the lines of 

 force) is able to influence the ionizing process. 



But for this to be, the ionizing region must reach far 

 enough beyond the point to feel the divergence of the lines ; 

 in other w r ords, it seems as though t should be comparable 

 with the radius of the point in spite of the fact that the 

 luminous region is practically confined to the surface of the 

 metal. 



Yet even if t is equal to r , the field of the B ions is 

 less than 3 per cent, of f for the sharpest point and still 

 less for the others ; hence when account is taken of the fact 

 that the ions are newly formed and probably travel much 

 faster than we have supposed, there is not likely to be any 

 serious error introduced if f calculated from the pull be 

 taken as the true field at the point. 



One other effect of the discharge should be mentioned. 

 The field in the discharge area is presumably constant, so 

 that where discharge occurs cr will also be constant, and 

 Young's correction (loc. cit.) for the distribution of a will be 

 reduced. 



When the discharge area is confined to the centre of the 

 point his constant must be used, and 



f =l-085</8P/r ; 



but for the point under discussion glow was visible over the 

 whole hemispherical end of the point from 15 down to 

 1 microampere, and possibly lower. In all cases of positive 

 discharge from this point the values of / have been calculated 

 from the formula 



/„= VSP/r . 



