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XXXVI. On the Electrification of Steel Needle-Points in Air, 

 By A. P. Chattock *. 



TPlate V.] 



§ 1. AS Faraday long ago put it, the discharge of electricity 

 -£*- from a point into a gas may be looked upon as a 

 particular case of sparking between a conductor (the point) 

 and a non-conductor (the surrounding gas). It is, moreover, 

 a particularly interesting form of discharge, as compared 

 with that taking place between two conductors, from the fact 

 that it seems more likely to throw light on the unsymmetrical 

 behaviour of positive and negative electricity. For there is 

 evidence which points to the surface of the electrode as the 

 origin of that want of symmetry ; and by experimenting 

 with one electrode only, one is able to a certain extent to 

 separate its effect on positive electricity from that on negative : 

 such separation being impossible in the case of sparks be- 

 tween two conductors, as both effects are there necessarily 

 present together. 



The phenomena connected with discharge from points group 

 themselves naturally under two heads : — Those occurring be- 

 fore or at the beginning of discharge ; and those occurring 

 during the passage of electricity from the point. What 

 follows refers to the first of these divisions only. 



§ 2. When an earth-connected sewing-needle is placed 

 with its end facing the centre of an insulated metal plate, 

 and the latter is gradually electrified, there comes a point at 

 which the needle begins to discharge onto the plate. This 

 point, for given conditions, is very definite, and corresponds 

 to a breaking down of some resistance between the needle and 

 the plate. It seemed desirable to discover how much of the 

 dielectric was concerned in this process. For this purpose 

 measurements were made of the strength of the electrostatic 

 field at the surface of the needle-point at the instant that dis- 

 charge occurred, the distance of the plate from the needle 

 being varied from O04 centim. to 5 centim., while the needle 

 was suspended in such a manner (§8) that the attraction 

 between it and the plate could be determined. 



Now the attraction is due of course to the tension in the 

 lines of force which end at the surface of the needle ; and 

 since their direction is everywhere normal to the surtace, it 

 follows that if the needle be a true cylinder placed sym- 

 metrically with regard to the plate, the mechanical pulls of 



* Communicated bv the Author, 



