74 First published Paper on Electricity 



in proportion to its surface ; though it does not appear from this reasoning 

 whether the waste of electricity from the whole cone should be greater or 

 less than from a cylinder of the same base and altitude*. 



All which has been here said relating to the flowing out of electricity 

 from overcharged bodies, holds equally true with regard to the flowing in 

 of electricity into undercharged bodies. 



125] But a circumstance which I believe contributes as much as any 

 thing to the quick discharge of electricity from points, is the swift current 

 of air caused by them, and taken notice of by Mr Wilson and Dr Priestly 

 (vide Priestly, pp. 117 and 591) ; and which is produced in this manner. 



If a globular body ABD is overcharged, the air close to it, all round 

 its surface, is rendered overcharged by the electric fluid which flows into 

 it from the body ; it will therefore be repelled by the body ; but as the air 

 all round the body is repelled with the same force, it is in equilibrio, and 

 has no tendency to fly off from it. If now the conical point ACB be made 

 to stand out from the globe, as the fluid will escape much faster in pro- 

 portion to the surface from the end of the point than from the rest of the 

 body, the air close to it will be much more overcharged than that close 

 to the rest of the body ; it will therefore be repelled with much more force ; 

 and consequently a current of air will flow along the sides of the cone, 

 from B towards C; by which means there is a continual supply of fresh air, 

 not much overcharged, brought in contact with the point; whereas other- 

 wise the air adjoining to it would be so much overcharged, that the 

 electricity would have but little disposition to flow from the point into it. 



The same current of air is produced in a less degree, without the help 

 of the point, if the body, instead of being globular, is oblong or flat, or 

 has knobs on it, or is otherwise formed in such manner as to make the 

 electricity escape faster from some parts of it than the rest. 



In like manner, if the body ABD be undercharged, the air adjoining 

 to it will also be undercharged, and will therefore be repelled by it; but 

 as the air close to the end of the point will be more undercharged than 

 that close to the rest of the body, it will be repelled with much more force ; 

 which will cause exactly the same current of air, flowing the same way, 

 as if the body was overcharged ; and consequently the velocity with which 

 the electric fluid flows into the body, will be very much increased. I 

 believe indeed that it may be laid down as a constant rule, that the faster 

 the electric fluid escapes from any body when overcharged, the faster will 

 it run into that body when undercharged. 



Points are not the only bodies which cause a quick discharge of 

 electricity ; in particular, it. escapes very fast from the ends of long slender 

 cylinders; and a swift current of air is caused to flow from the middle of 

 the cylinder towards the end: this will easily appear by considering that 



[* Note 9, p. 374.] 



