ELECTRICITY. 



543 



PLATE 

 F * I8- 



>lu- 

 i produ- 



rheoretkal J n like manner, if the body ABD be undercharged, 

 Electricity. t f, e a ; r ac jj o ining to it will also be undercharged, and 

 """"- 1 ' 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. Mr C. believes, indeed, that 

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

 the electric fluid escapes from any body when over- 

 charged, the faster will it run into that body when un- 

 dercharged. 



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 ap- 

 pear by considering, that the redundant fluid is col- 

 lected in much greater quantity near the ends of the 

 cylinders, than near the middle. The same thing may 

 be said, but he believes in a less degree, of the edges 

 of thin plates. 



What has been just said concerning the current of 

 ajj^ serves -to explain the reason of the revolving mo- 

 t tion ?*" Dr Hamllttm>s and Mr Kinnersley'sbent point- 

 ed wires, (see Phil. Trans, vol. li. p. 905, and vol. liii. 

 p_. 86 ; also Priestley, p. 429, and page 4*0 of this ar- 

 ticle) : for the same repulsion which impels the air from 

 the thick part of the wire towards the point, tends to 

 impel the wire in the contrary direction. 



It is well known, that if a body B is positively elec- 

 trified, and another body A, communicating with the 

 ground, be then brought near it, the electric fluid will 

 escape faster from B, at that part of it which is turned 

 towards A, than before. This is plainly conformable 

 to theory ; for as A is thus rendered undercharged, B 

 will in its turn be made more overcharged, in that part 

 of it which ii turned towards A, than it was before. 

 But it is also well known, that the fluid will escape fast- 

 er from B, if A be pointed, than if it be blunt ; though 

 B v.-Hl be less overcharged in this case than in the other; 

 for the broader the surface of A, which is turned to- 

 wards B, the more effect will it have in increasing the 

 overcharge of B. The cause of this phenomenon is as 

 follows : 



If A is pointed, and the pointed end turned towards 

 B, the air close to the point will be '-cry much under- 

 charged, and therefore will ho strongly repelled by A, 

 and attracted by B, which will cause a swift current of 

 arr to flow from it towards B, by which means a con- 

 stant supply of undercharged air will be brought in con- 

 tact with B, which will accelerate the discharge of elec- 

 tricity from it in a very great degree : and, moreover, 

 the more pointed A is, the swifter will be this cur- 

 rent. If, on the other hand, that end of A which is 

 turned towards B is so blunt, that the electricity is 

 not disposed to run into A faster than it is to run out 

 of B, the air adjoining to B may be u much over- 

 charged as that adjoining to A is undercharged ; and 

 therefore may, by the joint repulsion of B and attrac- 

 tion of A, be impelled from B to A, with as much or 

 more force than the air adjoining to A is impelled in 

 the contrary direction ; so that what little current of 

 air there is, may flow in the contrary direction. 



It is easy to jipplv what lias been here said to the Theoretical 

 case irt which B is negatively electrified. Klectriotj^ 



. r >. In the paper .of Mr Canton's, quoted in the 2d ""Y* 

 section, and in a piiper of Dr Franklin's (Phil. Trans. 

 1775, p. 300, and Franklin's Letters, p. 155,) are some 

 remarkable experiments, showing that when an over- 

 charged body is brought near another body, some fluid 

 is driven to the farther end of this body, and also some 

 driven out of it, if it is not insulated. The experiments 

 are all strictly conformable to the 9th, 10th,and 1 1th pro- 

 positions : but it is needless to point out the agreements 

 the explanation given by the authors does it sufficiently. 



6. On the Leyden Phial. The shock produced by Explana- 

 the Leyden phial seems owing only to the great quan- ^ on f tke 

 tity of redundant fluid collected on its positive side, j^* 

 and the great deficiency on its negative side ; so that if 

 a conductor was prepared of so great a size, as to be 

 able to receive as much additional fluid by the same 

 degree of electrification, as the positive side of a Ley den 

 phial, and was positively electrified in the same degree 

 as the phial, he does not doubt but what as great a 

 shock would be produced by making a communication 

 between this conductor and the ground, as between the 

 two surfaces of the Leyden phial, supposing both com- 

 munications to be made by canals of the same length 

 and same kind. 



It appears plainly from the experiments which have 

 been made on this subject, that the electric fluid is not 

 able to pass through the glass ; but yet it seems as if 

 it was able to penetrate, without much difficulty, to a 

 certain small depth, perhaps he might say an imper- 

 ceptible drpth, within the glass ; as Dr Franklin's ana- 

 lysis of the I. eyden phial shows that its electricity is 

 contained chiefly in the glass itself, and that the coat- 

 ing is not greatly over or under charged. 



It is well known that glass is not .the only substance 

 which can be charged in the manner of the Leyden 

 phial ; but that the same effect may be produced by 

 any other electric. 



Hence * the phenomena of the phial seem easily ex- PLATE 

 plicable by means of the 20th proposition. For let CCLI. 

 ACGM, Fig. 20, represent a flat plate of glass, or any Fi & * 0> 

 other substance which will not suffer the electric fluid 

 to pass through it, seen edgewise ; and let BWD, and 

 EffF, or Brf and E/J as they may be called for shortneis, 

 be two plates of conducting matter of the same size, pla- 

 ced in contact with the glass .opposite to each other ; and 

 let Btf be positively electrified ; and let F^/ communi- 

 cate with the ground ; and let the fluid be supposed 

 either able to enter a little way into the glass, but not 

 to pass through it, or uruiMc to cuter it at all ; and if it 

 is able to enter a little way into it, let b/s2ti, or b), as it 

 may be called, represent that part of the glass into which 

 the fluid can enter from the plate He/, and eQ that which 

 the fluid from K/' can enter. By the above mentioned 

 proposition, if be, the thickness of the glass, is very 

 small in respect of brf, the diameter of the plates, the 

 quantity of redundant fluid forced into the space B il, or 

 B 3, that is, into the plate B d, if the fluid is unable to 

 penetrate at all into the glasw, or into the plate U il, and 

 the space b 3 together, if the fluid is able to penetrate 

 into the glass, will be many times greater than what 

 would be forced into it by the same degree of electri- 

 fication, if it had been placed by itself; and the quan- 

 tity of fluid driven out of E<p, will be nearly equal to the 

 redundant fluid in B 3. 



The following explication is strictly applicable only to that tort of Leyden phial, which consists of a fiat plate of glats or other matter. 

 It u evident, however, thai the result must be nearly f the tame kind, though the glass is made into the shape of a bottle, as usual, or into 

 OBJ other form. ;l 



