Surface of a Needle-Point discharging in Air. 267 



it occurred to Mr. Tyndall that an explanation of some of 

 the results had become possible in the light of modern 

 theories of discharge. We therefore repeated and extended 

 the old work, and an account of what has been done follows 

 the present paper. This has rendered necessary a discussion 

 of the question how far the pull at a discharging point is due 

 to the field at its surface, and how far to purely mechanical 

 forces brought about by the discharge ; and an attempt is 

 here made to estimate the magnitude of these forces, and to 

 show that they may be neglected in the case of our 

 experiments. 



Positive Discharge from a Single Point. 



When a sharp point discharges positive electricity in air 

 at atmospheric pressure it usually becomes capped with a 

 luminous velvety layer, probably not more than one or two 

 hundredths of a millimetre in thickness. This layer and the 

 air near it is presumably the region in which ionization occurs, 

 and from it therefore ions of opposite signs travel towards and 

 away from the point respectively. 



In fig. 1 A represents the surface of the discharging point, 

 much magnified, A and t) the limits of the ionizing layer, and 

 A D the axis of the point. 



Before discharge sets in the field at points 

 ■*H?* 1- along A D will fall off for some distance in 



nearly inverse proportion to the squares of 

 the distances of these points from the centre 

 of curvature of A; but on the occurrence 

 » i of discharge some of the lines of force from 



\ 



\ \ 

 \ \ 

 i i 



i i 



A will end on ions between A and D, say 



-J£ — £-[£ at B, and others beginning on ions of oppo- 



1 i i site sign, say at C, will continue on towards 



/ ,' the right, with the result that the field is 



» ' weakened between B and C. 



i / At the same time changes of pressure are 



j set up in the gas by the moving ions ; those 



at C reducing the pressure between A and C, 

 and those at B raising it upon A, so that B C is a region of 

 low pressure as well as of low field intensity. 



Take first these mechanical effects of the discharge. To 

 simplify the argument, suppose that there is a single layer 

 of negative ions at B and another of positive ions at C ; 

 and let the charge per square centimetre on B be —p and that 

 on A +<t. The pull per square centimetre on A due to the 

 lines of force ending upon its surface is 27r<r 2 ; and if p x be 



T 2 



