ELECTRICITY. 



Pl ATE 

 fig. 6. 



On ota 



in which 



budu*rr- 



ctdee- 



t:'vit\ !-!:* 



or nut 



with il to 

 the r. 





Expimntinn 

 tbe iaflu. 

 tow of 



In like manner, if any part of the surface it under- 

 charged. the fluid will b*ve a tendency to nin in t 

 part t'r.ini tin- air. The trutli of this it soroewli.n 

 confirmed by the 2d problem ; as in all the case* of that 

 problem, the fluid wu shown to have a tendency to run 

 out of the space* AD ami LI1, nt ..n\ rarface which 

 wa overcharged, and to run in at any which was un- 

 dercharged. 



/ 1 . It' any body at a distance from other over 



or under ch.-irged Uxlies, be positively electrified, the 



rluid will (gradually run out of it from all parts of ita 



i 1 mi. i tlie adjoiningair; as it is plain that all parts 



Muiace of that lx>dy will he overcharged .- and if 



the body is negatively electrilied, the fluid will gradu- 



ally run into it at all parts of its surface from the ad- 



joining air. 



>/. 2. Let the body A, Fig. 6, insulated, and 

 containing just fluid enough to saturate it, be brought 

 near the overcharged body li ; that part of die surface 

 of A which is turned towards B will, by Prop. 2, be 

 Tendered undercharged, and will therefore imbibe elec- 

 tricity from the air ; and at the opposite surface HS, the 

 fluid will run out of the body into the air. 



Coral. 3. If we now suppose that A is not insula- 

 ted, but communicates with the ground, and conse- 

 quently that it contained just fluid enough to saturate 

 it before the approach of B, it is plain that the surface 

 MX will be more undercharged than before; and there- 

 fore the fluid will run in there with more force than 

 before ; but it can hardly have any disposition to run 

 out at the opposite surface US ; for if the canal by 

 which A communicates with the ground is placed oj>- 

 posite to B, as in Fig. 5, then the fluid will run out 

 through that canal, till it has no longer any tendency 

 to run out at HS ; and by the remarks at the end of 

 Prop. 25, it seems probable that the fluid in A will be 

 nearly in the same quantity, and disposed nearly in the 

 Maine manner, into whatever part of A the canal is in- 

 serted, by which it communicates witli the ground. 



Carol. 4. If B is undercharged, the case will be re- 

 versed ; that is, it will run out where it before ran in, 

 and will run in where it before ran out. 



These corollaries seem conformable to experiment : 

 thus tar is certain, that bodies at a distance from other 

 electrified bodies receive electricity from the air, if ne- 

 gatively electrified, and part with some to it if positive- 

 ly electrified : and a body not electrified, and not insu- 

 lated, receives electricity from the air if brought near 

 an overcharged body, and loses sonic when brought near 

 an undercharged body : and a body insulated and con- 

 taining its natural quantity of fluid, in some cases re- 

 ceives, ami in others loses electricity, when brought 

 near an over or under charged body. 



* The well-known effects of points in causing a quick 

 discharge of electricity, seem to agree very well with 

 this theory. 



It appears from the 18th Proposition, that if two si- 

 milar bodies of different sizes are placed at a very great 

 distance from each other, and connected by n slender 

 canal, and overcharged, the force with which a parti- 

 cle of fluid, placed close to corresponding parts of their 

 surface, is repelled from them, is inversely us the cor- 

 responding diameters of the bodies. If the distance 

 f the two bodies is small, there is not so much differ- 

 ence in the force with which the particle is repelled by 

 the two bodies ; lint still, if the diameters of the two 

 bodies are very different, tlie particle will be repelled 

 with much more force from the smaller body than from 

 he larger. It is true, indeed, that a particle placed 

 at a certain distance from the smaller body, will be 



repelled with less force tluui if it be placed at tlu- 

 tame di.-t:nuv from the greater body : but this dis- 

 tance is. in most cases, pretty considerable; if tlie bo- 

 die* are spherical, and the repulsion inversely as the 

 square of the distance, a particle placed at any distance 

 from the surface nf the smaller body, less than a mean 

 pro|H>rtional between the radii of the two bodies, will 

 M rcp.-lli-d I'rom it with more force than if it be placed 

 at the same distance from tlie larger body. 



Hence, if two similar bodies are connected together 

 by a slender canal, and are overcharged, the fluid must 

 escape fit-lvr from a smaller body than from an equal 

 surface of the larger ; but as the surface of the larger 

 Ikxly is greatest, it does not appear which body ought 

 to lose most electricity in the same time ; and indeed 

 it seems impossible to determine positively from this 

 theory which should, as it depends in great measure on 

 the manner in which the air opposes the entrance of the 

 electric fluid into it. Perhaps in some degrees of elec- 

 trification tlie smaller body may lose most, and in others 

 tlie larger. 



Let now ACB, Fig. 18, l>e a conical point, standing 

 on any body DAB, C being the vertex of the cone; 

 and let DAB be overcharged : A particle of fluid 

 placed close to the surface of the cone, any where 

 between b and C, must be repelled with at least as 

 much, if not more force, than it would, if the part 

 A a bR of the cone was taken away, and the part aCft 

 connected to DAB by a slender canal ; and conse- 

 quently, from what has been said before, it seems rea- 

 sonable to suppose, that the waste of electricity from 

 the end of the cone must be very great in proportion 

 to its surface ; though it does not appear from this rea- 

 soning, whether the waste of electricity from the whole 

 cone, should be greater or less than from a cylinder of 

 the s ime base and altitude. Alf that has been here 

 said relating to the flowing out of electricity from over- 

 charged bodies, holds equally true with regard to the 

 flowing in of electricity into undercharged bodies. 



But a circumstance which 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 Priestley, (see 

 Priestley, p. 117 and 591); and which is produced 

 in this manner. If a globular body ABD is over- 

 charged, the air close to it, all round its surface, is ren- 

 dered overcharged by the electric fluid, which Hows 

 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 ten- 

 dency 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 proportion 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 n 

 current of air will flow along the sides of the cone, 

 from B towards (.' ; by which means there is a conti- 

 nual supply of fresh air. not much overcharged, brought 

 in contact with the point; whereas otherwise the air 

 adjoining to it would be so much overcharged, that the 

 rUctiirit .ild have but little disposition to flow from 

 the point into it. 



Tlie same current of air is produced in u less degree, 

 without the help of the point, if tlie body, instead of 

 being globular, is oblong or flat, or has knobs on it, or 

 is otherwise formed in such a manner as to make the 

 electricity escape faster from some parts of it than the. 

 rest. 





1 \pLttjintior. 

 i .1 the iiiflu. 

 nice of 

 points. 



PI.ATI. 



cc;.i. 



Fig. 18. 



On the cur- 

 rent of air 

 caused by 

 points. 



