ELECTRICITY, COMMON. 



TltlrlTY. COMMON. 



electricity U a distinct specie* of matter. n all-pervading fluid, hare 

 thgrfori denominate! it impomdmiblt. FscU do not, however, 

 uthorue us yet to Uke tab view of it, or to regard it a* essentially 

 dMfcnmt from the fore*, which the molecule* of matter exercise, 

 which, though neutnliMd for external bodies when theae molecule* 

 are in pojtion of equilibrium or stability relative to their mean 

 placet, mar become Mndble by impnwed force*, tuch u fnction, 

 which would alter the portion of their pole*, or by sudden concussion 

 forcibly altering their relative ntuationa. There U a convenience of 

 language, howerw, in (peaking of electricity a* a fiid, which can lead 

 to no error by iU adoption until the phenomena of molecular action 

 have been more studied, and the view, of Moeotti and many other 

 natural philosophers with respect to the identity of theae force* better 

 tjstabuahed. [ATTRACTION.] Adopting, therefore, this hypotheaii, the 

 *en*e in which we speak of it* quantity, it* accumulation, it* density, 

 or it* partition between bodie*, may be readily translated into the 

 hypothesis founded on the views of molecular action if we should find 

 any occasion for it. 



The next phenomenon to be noticed is the limit to the quantity of 

 electricity which we can communicate to a perfect conductor. If, 

 from different sources of electricity, we charge a metallic ball, and so 

 continue to charge, we shall find that there is a limit beyond which wo 

 cannot communicate more ; for on attempting so to do, the ball will 

 discharge itself through the air into the nearest conducting body, when 

 a spark, describing apparently a zigzag course, will be observed, the 

 colour of the light being dependent on the medium which it traverses. 

 This spark travels with immense velocity (as will be noticed more 

 particularly hereafter), and is accompanied by a very audible sound ; 

 and if received by the body of a man or animal, it produces through a 

 port or the whole of the system an instantaneous muscular contraction, 

 which may be rendered sufficiently strong to cause death, but in more 

 moderation has been used in some diseases, such as deafness, though 

 it* use has become by no means general, and indeed the curative effect 

 is very doubtful. 



Two point* determine this limit, or fix the charge of which a perfect 

 conductor is susceptible. The first is the extent of its surface ; for if 

 two bodies have similar figures, the quantities nf electricity of which 

 they are capable are proportional, not to their solid content or weight, 

 but to then- surfaces ; that is, to the square of their linear dimen- 

 sions. The second is the pressure of the non-conducting medium by 

 which they are encompassed.* The quantity of electricity is then as 

 the square root of the pressure. When placed in an artificial vacuum, 

 an electrical light is observed along the sides of the machine. Harris 

 has shown that dry air, considerably rarefied by the action of the 

 pump, will suffice to retain the electricity of a body for a long time ; 

 but it should be remembered that the square root of two quantities 

 gives a much lower ratio than the quantities themselves; and this 

 ratio cannot be considered very small in any artificially-formed 

 vacuum. 



That the quantities of electricity should follow the law of propor- 

 tionality to the surfaces, and not to the content or weight of the body, 

 will not excite surprise when it is stated that no developed electricity 

 exists within a body, at least to any appreciable depth below its sur- 

 face. This fact has been repeatedly confirmed by experiment ; and in 

 consequence of this law we see the advantage of using a long cylindrical 

 form for a conductor, and perceive the proportions in which the 

 partition of electricity takes place when several similar conductors 

 communicate. 



We come now to the third and very important phenomenon of the 

 mutual repulsion of the different parts of develop] electricity : these 

 parts repel each other with forces which vary inversely as the square 

 of the distance. We may easily witness this repulsion in the following 

 familiar manner. Take a small well-dried ball of elder-pith, and sus- 

 pend it from the hand by a fine silk thread, which is a good non-con- 

 ductor ; then bring it near the conductor of an electrical machine, or 

 to a body to which this conductor has communicated electricity. The 

 ball will at first be attracted to this body ; but when it has touched it 

 and shared a small portion of its electricity, it will be repelled from it 

 and will stretch the string by which it is suspended in a slanting 

 direction, until the obliquity is sufficiently great for its own gravity 

 and the tension of the string to coun- 

 terpoise the repulsion of the electricity 

 on the conductor. Let two pith balls 



\be suspended by parallel strings so as to 

 touch each other, as A, B, and if a por- 

 tion of electricity be communicated to 

 them by temporary contact with a body 

 already charged, the strings will diverge 

 in consequence of the mutual repulsion, 

 and the balls will come into the positions 

 n,l,, when;, notwithstanding their gravi- 

 tation, they will remain a considerable 

 time, if the air be very free from moisture. They will be observed 

 gradually to close in toward* each other as they lose their electricity 

 from the contact of the surrounding medium. 



Tbta Idea of ptrmn hai been coiulderably modified ilnce the sdoptlon of 

 F'Ujr' Ylewi, which will b* rtatad further en. 



That the law of force in thin cue is, as in gravitation, expressed by 

 the inverse square of the distance, wa* satisfactorily demonstrated 

 by Coulomb by mean* of his torsion balance, the principle of which 

 we have before explained [ELASTICITT], and has been confini. 

 Harris's experiment* : and a remarkable fact arises from it, nain 

 the electrised pith-ball A be placed any where within the conc.-i 

 a spherical shell, it will not be moved in any direction by the rep 

 of the electricity on the surface of the shell ; for the electricity being 

 then uniformly distributed, the intensity of the force of any small 

 portion D c is at its extent, or proportional to the square of the arc 

 D c, and is therefore destroyed by the action of an opposite j>orti< >n <l r, 

 cut off by a conical surface having A as vertex, and oca* bane : f. >r 

 DC 1 : dc : : D tf : c A*, that is, what the portion gains in r\t< nt it 

 lose* in it* distance from A. This law was first demonstrated by 

 Newton (' Principia,' book 12), and it was afterwards shown by other 

 analysts, that for no other law of force but that stated could the sainu 

 mutual destruction of forces occur (Laplace/ Mccanique Celeste,' torn. 



i.,liv. 2; Murphy's ' Electricity,' chap, iii.); and Pnisson, from other 

 considerations which we shall afterwards notice, made this condit ioi > . f r 

 a body of any figure, the ground of his calculations on the distribution 

 of electricity over the surfaces of bodies. 



When electricity is produced, as above described, and a conductor 

 charged, if the conductor be removed, and another conductor i 

 it, the latter will become charged by repeating the operation : tlm- 1 1 ., 

 cylinder, and indeed every substance properly mounted, is an 

 haustible source of electricity. 



We have supposed the cushion by which the cylinder is rubbed to 

 be in communication with the ground by means of a conductor ; but 

 if two substances both isolated be electrised by friction, and wh'ii 

 separated the electricities belonging to each surface be examined, we 

 find the following results : 



Let two insulated pith-balls, A and B, as before, be electrised by com- 

 munication with one of the surfaces, and two other balls a i in like 

 manner electrised by the other surface. Then when A is presented to 

 B, or a to 6, repulsion takes place as before described ; but wli- 

 presented to o, or B to 6, they will attract each other ; and if .\, n have 

 equal charges from the different surfaces which have been rubbed 

 against each other, when contact takes place between A and a, all signs 

 of ili veloped electricity will depart from each, and the bodies will 

 take their natural positions, neither attracting nor repelling each 

 but if A has a greater electrical charge than a, a surplus of the elec- 

 tricity of A will remain, and will be partly communicated to a when .1 

 consequent repulsion arises. 



The same results would occur if two machines were used, in one of 

 which the cylinder is glass, and in the other resin or a gummed MI)<- 

 stance : the pith-ball which receives it electricity from the glass 

 cylinder will attract that which has beeu in communication with the 

 other machine. 



Hence arise the terms rilrtmu electricity and retinmu electric! < 

 as they are also called, positive electricity and negative ; for whatever 

 two substances they may be which are rubbed together when elec- 

 tricity is produced, it will be found positive on one substain 

 negative on the other, even if the substances are of the same nature ; 

 for instance, both gloss. [It is necessary, however, that the two sur- 

 faces bo in different physical states, such as one rough and tli. 

 smooth, or heat, not electricity, will be the result of the friction.] 



The phenomenon above noticed may be then announced as follows : 

 "Like electricities mutually repel, unlike mutually attract; " ami tlir 

 law of force between particle and particle is in both cases the inverse 

 square of the distance. 



Moreover, we have seen that the addition of quantities of i 

 electricities is similar to the addition of quantities with unlike -;.-:i- 

 in algebra : when equal the sum is zero, when unequal it is the cxcew, 

 and of the same name as the greater charge. 



Franklin's theory makes only one electric fluid in excess abbve its 

 natural state in bodies positively electrised, and in defect in those 

 said to be negatively electrised. Kpinu. ami most of thi> continental 

 philosophers after him, suppose the existence of two distinct cli 

 fluids, the particles of each of which repel those of the name kind, lint 

 attract the M> of tin- contrary, ami therefore the opposite elect i 

 always seek combination or neutralisation, so that in natural bodies 

 the two fluids exist in equal quantity, by which the presence of 

 is indicated. Mosotti has in some degree revived the theory 

 of Franklin in his memoir on the forces which determine the state of 



