ELECTRICITY. 



intensity at the extremities takes place 

 in bodies of a cylindric or prismatic 

 form ; and the more so as their length 

 bears a greater proportion to their 

 breadth. Coulomb found by experi- 

 ment that, in a cylinder thirty inches 

 long and two inches in diameter, the 

 intensity of the electricity at the ends 

 was to its intensity at the middle, or at 

 any part more than two inches from the 

 extremity, as 2.3 to 1. Pursuing this 

 train of reasoning, it will lead us to a 

 conclusion of some importance, namely, 

 that, if the conducting substance be 

 drawn out into a point, the intensity of 

 the electricity at that point will be ex- 

 ceedingly great ; and that the point will 

 accordingly absorb and draw into] itself 

 nearly the whole of the electricity that is 

 contained in the body. This vast con- 

 centration of electricity is found actually 

 to take place in all points that project 

 beyond the general surface. 



CHAPTER VI. 

 Transference of Electricity* 



(84.) WE are next to consider the 

 condition of bodies during the preva- 

 lence of those forces which tend to over- 

 set the electric equilibrium, over those 

 which tend to preserve it. The pressure 

 exerted by the electric fluid against the 

 non-conducting medium, such as the 

 air, which opposes an obstacle to its 

 escape, is in a ratio compounded of the 

 repulsive force of its own particles at 

 the surface of the stratum of fluid, and 

 of the thickness of that stratum ; but as 

 one of these elements is always propor- 

 tional to the other, the total pressure 

 must, in every point, be proportional to 

 the square of the thickness. If this 

 pressure be less than the resistance, or 

 coercive force, as it has been called, of 

 the air, the electricity is retained ; but 

 the moment it exceeds that force, in 

 any one point, the electricity suddenly 

 escapes, just as a fluid confined in a 

 vessel would rush out if it were to burst 

 open a hole in the side of the vessel. 



(85.) It is only a certain proportion 

 of the whole quantity of electricity in 

 the conducting body that thus suddenly 

 escapes ; but the irruption of it is 

 marked by many very striking pheno- 

 mena, all indicative of the abruptness 

 and violence with which the change is 

 effected. A sharp snap is heard, ac- 

 companied by a vivid spark, and there 

 are evidences of an intense heat being 



evolved in the line which the electricity 

 takes. 



(86.) The passage of the electric fluid 

 through a perfect conductor is unat- 

 tended with light. Light appears only 

 where there are obstacles in its path 

 by the interposition of imperfect con- 

 ductors ; and such is the velocity with 

 which it is transmitted, that the sparks 

 appear to take place at the very same 

 instant along the whole line of its course. 

 Thus, if a row of small fragments of 

 tin-foil be pasted on a piece of glass, 

 fig. 1 9, and electricity be sent through 



Fig. 19. 



them by connecting one of its ends with 

 the conductor of an electrical machine, 

 while the other end communicates with 

 the ground, it will not be possible to 

 detect any difference of time in the oc- 

 currence of the light in the different 

 parts, so that the whole series of lumi- 

 nous points, if sufficiently near, appear, 

 in the dark, like a vivid and continuous 

 line of light. By varying the arrange- 

 ment of the tin-foil, we may distribute 

 the light in any manner we please, so as 

 to exhibit a brilliant delineation of the 

 figure they represent. Even when con- 

 ducting bodies appear to be in contact, 

 if the experiment be made in the dark, a 

 spark is generally seen to pass between 

 them, unless the bodies be pressed toge- 

 ther with considerable force. Hence, 

 a chain appears luminous at each link, 

 while conveying a charge of electricity. 



(87.) The longest and most vivid 

 sparks are obtained between two con- 

 ductors having a rounded form, and the 

 more so in proportion as they are both 

 portions of spheres of large diameter. 

 This may be exemplified in a common 

 electrical machine, by presenting a me- 

 tallic ball of large size to that side of 

 the prime conductor which is furthest 

 from the cylinder of the machine. In 

 such cases, however, the electricity 

 being of weaker intensity, the distance 

 between the conducting bodies requisite 

 for the transfer of electricity through 

 the air, or what is termed the striking 

 distance, is necessarily small. If the 

 ball is of smaller diameter, or the con- 

 ductor of a more elongated shape, the 

 electricity at its surface is of higher 



