5 tO 



ELECTRICITY. 



' 



ltnei(y. 



sides by the air, ni no fluid can run out of it without 

 running into the air, the fluid will not do so without 

 difficulty. 



Though the terms posilipfly and nrfoth-flif electri- 

 fied are much used, yet the piecwe sense in which they 

 are to be understood, seems not well ascertained ; 

 namely, whether they are to be understood in the 

 same sense with the words over or under charged, 

 or whether, when any number of bodies, insulated and 

 communicating with each other by conducting substan- 

 ce*, are electrified by means of excited gl.iss, they are 

 all to be called potitivriy electrified, {sup|xing. ac- 

 cording to the usual opinion, that excited gl-i" om- 

 t.iinsmore than its natural quantity of electricity) c\en 

 though some of them, by the approach of a stronger 

 electrified Ixxly, are made undercharged. The words are 

 luTi-uscd iii the Litter sense ; but it will l>e proper to 

 ascertain more accurately the sense in which they are 

 employed. In order to judge whether any body, as A, 

 i- positively or negatively electrified, suppose another bo- 

 dy B, of a given shape and size, to be placed at an infinite 

 distance from it, and from any other over or under char- 

 ged body, and let B contain the same quantity of electric 

 fluid, as if it communicated with A by a canal of incom- 

 pressible fluid : then if B is overcharged, A is called po- 

 titi'.-cly electrified, and if it is undercharged, A is called 

 najBaafa electrified ; and the greater the degree in 

 which B is over or under charged, the greater is the 

 <legrce in which A is positively or negatively electri- 

 fied. 



It appears from the corol. to the 20th proposition, 

 that if several bodies are insulated, and connected to- 

 gether by conducting substances, and one of these bo- 

 dies is positively or negatively electrified, all the other 

 bodies must be electrified in the same degree ; for sup- 

 pose a given body B to be placed at an infinite distance 

 from any over or under charged body, and to contain the 

 same quantity of fluid as if it communicated with one 

 of those bodies by a canal of incompressible fluid, all 

 the rest of these bodies must, by that corol. contain 

 the same quantity of fluid as if they communicated with 

 B by canals of incompressible fluid ; but yet it is pos- 

 sible that some of those bodies may be overcharged, and 

 others undercharged : for suppose the bodies to be po- 

 sitively electrified, and let an overcharged body D 

 be brought near one of them, that body will become 

 undercharged, provided D is sufficiently overcharged ; 

 and yet by the definition it will still be positively elec- 

 trified in the same degree as before. 



Besides, if several bodies are insulated and con- 

 nected together by conducting substances, and one of 

 these bodies is electrified by excited glass, there can 

 be no doubt but they will all be positively, electrified ; 

 for if there is no other over or under charged body 

 placed near any of these bodies, the thing is evident ; 

 and though some of these bodies may, by the approach 

 of a sufficiently overcharged body, be rendered under- 

 charged, yet it does not appear possible to prevent a 

 body placed at an infinite distance, and communica- 

 ting with them by a canal of incompressible fluid, from 

 being overcharged. In like manner if one of these 

 Ixxlies is electrified by excited sealing wax, they will all 

 be negatively electrified. 



It is impossible for any body communicating with 

 the ground to be either |>sitively or negatively electri- 

 fied ; for the earth, taking the whole together, contains 

 ju-t fluid enough to saturate it, and consists in general 

 of conducting substances ; and consequently though it 

 is possible for small parts of the surface of the earth to 

 be rendered over or under charged, by the approach of 



clrctritied clouds, or other eau*M ; rrt the bulk of the 

 earth, and especially the interior pnrt*, must be satu- 

 rated with electricity. Therefore assume any part of 

 the earth which is itself saturated, and is at a great 

 distance from any over or under charged part ; any bo- 

 dy communicating with the ground, contains as much 

 electricity as if it communicated with this part by a 

 canal of incompressible fluid, and therefore is not at 

 all electrified. 



If any body A, insulated, and saturated with electri- 

 city, is placed at a great distance from any over or un- 

 der charged body, it is plain that it cannot be electri- 

 fied ; but if an overcharged Ixxly is brought near it, it 

 will l>e )K>sitively electrified ; for supposing A to com- 

 municate with any body B, at an infinite distance, by 

 a canal of incompressible fluid, it is plain that unless B 

 is overcharged, the fluid in the canal could not be in 

 equilibrio, but would run from A to B. For the same 

 reason, a body insulated and saturated with fluid, 

 will be negatively electrified if placed near an under- 

 charged body. 



2. The phenomena of the attraction and repulsion 

 of electrified bodies seem to agree exactly with the the- 

 ory ; as will appear by considering the follow ing cases. 

 Case\. Let two bodies, A and B, both conductors of elec- 

 tricity, and both placed at a great distance from any other 

 electrified bodies, be brought near each other. Let A 

 be insulated, and contain just fluid enough to saturate 

 it ; and let B be positively electrified. They will at- 

 tract each other ; for as B is positively electrified, and 

 at a great distance from any overcharged body, it will 

 be overcharged ; therefore, on approaching A and B 

 to each other, some fluid will be driven from that part 

 of A which is nearest to B to the farther part : but 

 when the fluid in A was spread uniformly, the repul- 

 sion of B on the fluid in A was equal to its attraction on 

 the matter in it ; therefore when some fluid is removed 

 from those part* where the repulsion of B is strongest, 

 to those where it is weaker, B will repel the fluid in A 

 with less force than it attracts the matter ; and conse- 

 quently the Ixxlies will attract each other. 



Case -2. If we now suppose that the fluid is at li- 

 berty to escape out of A, if it has any disposition to 

 do so, the quantity of fluid in it before thr approach of 

 B being still sufficient to saturate it ; that is, it A is not 

 insulated and not electrified, B being still positively 

 electrified, they will attract with more force than be- 

 fore : for in this case, not only some fluid will be dri- 

 ven from that part of A which is nearest to B to the 

 opposite part, but also some fluid will be driven out of 

 A. It must be observed, that if the repulsion of B on 

 a particle at E, Fig. 19, the farthest juirt of A, is very 

 small in respect of its repulsion on an equal particle 

 placed at D, the nearest part of A, the two Inxlies will 

 attract with very nearly the same force, whether A is 

 insulated or not ; but if the repulsion of B, on a particle 

 at D, is very near as great as on one at D, they will 

 attract with very little force if A is insulated. 1'or in- 

 stance, let a small overcharged ball be brought near 

 one end of a long conductor not electrified ; they will 

 attract with very near the same force, whether the 

 conductor be insulated or not ; but if the conductor 

 be overcharged, and brought near a small unelectrified 

 ball, they will not attract with near so much force, if 

 the ball is insulated, as if it is not. 



Case 3. If we now suppose that A is negatively elec- 



trified, and not insulated, it is plain that they will at- 



tract with more force than in the last case ; as A will be 



still more undercharged in tin's case than in the I 



N. B. In these three cases, we have not as yet taken 



tion of at- 







" 



p t AT e 

 rn,l. 

 F'g- 19- 



