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sermons for and against the doctrine, may be seen in Watt's Bibliotheca, and Du Pin's Study of Divinity.) ELECTION (law) is when a man is left to his own free will to take or do one thing or another which he pleases (Termes de la Ley); and he who is to do the first act shall have the election. As, if A covenants to pay B a pound of pepper or saffron before Whitsuntide, it is at the election of A at all times before Whitsuntide which of them he will pay; but if he does not pay either before, the time fixed, then it is at the election of B to sue for which he pleases. So, if a man give to another one of his horses, the donee may take which he chooses; but if the donation be that he wili give one of his horses (in the future tense), then the election is in the donor. Courts of equity frequently apply the principle of election in cases where a party has inconsistent rights, and compel him to elect which he will enforce: as, if A by his will assumes to give an estate belonging to B to C, and gives other benefits to B, B cannot obtain the benefits given to him by the will unless he gives effect to the testator's disposition to C. It does not appear to be quite settled whether the party electing to retain his own property in opposition to the instrument is bound to relinquish only so much of the property given to him as will be sufficient to compensate |. disappointed parties, or whether his election will be followed by absolute forfeiture of the whole. The arguments on both sides are ably stated 1 Roper, Husb. and Wife, 566 n. ; 1 Swanst. Reports, 441 ; 2 Coke's Repts, 35 b, Thomas's note. The principle of election is equally recognized in courts of law, though they are seldom called to adjudicate upon it, except where the alternative is very distinct, or the party has already elected. Indeed this principle is of universal application, and prevails in the laws of all countries; it is applicable to all interests, whether of married women or of infants; to interests immediate, remote, or contingent; to copyhold as well as to freehold estates; to personalty as well as to realty; to deeds as well as to wills. Courts of equity also will compel a plaintiff suing at law as well as in equity, or in a foreign court as well as in the court in England, for the same matter, at the same time, to elect in which court he will proceed, and will restrain him from pursuing his rights in all others. There are some exceptions to this doctrine, as in the case of a mortgagee, who may proceed in equity for a foreclosure, and on his bond or covenant at law at the same time; but this arises from the difference of the remedy, and from the original agreement to give the concurrent remedies: and even in such a case a court of equity will restrain a mortgagee from enforcing his judgment at law upon the bond or covenant, if he is not prepared to deliver up the mortgaged property and the title-deeds belonging to it. On Election under a will in the Roman Law see Dig. xxxiii. tit. 5, De Optione yel Electione Legata: and as to the French Law, see the Code Napoléon, art. 1189, &c., Des Obligations Alternatives. ELECTOR. [BoRoughs of ENGLAND; House of..] ELECTRA.. [CELLARIAEA, vol. vi. p. 401.] ELECTRICITY (#Arrpov, amber). The electric phenomena, connected as they are now known to be by certain well-ascertained laws, form together the most complete and important addition to the physical sciences which has been made since the time of Newton. The simplest and most usual mode of producing electricity, is by friction. When any two substances are rubbed against each other briskly it is always produced; but it is oily a particular description of bodies called non-conductors that retain it after it is thus produced so as to exhibit its primary effects of attraction and repulsion. The production of electricity may be observed in a very familiar manner thus:-Tear up a piece of paper into small fragments, and place them on a table; then take a stick of sealing-wax, and rub it briskly with a piece of flannel, or against the sleeve of a cloth coat, o immediately after hold it near the fragments of paper; these small pieces will be soon observed to be agitated and the smaller to fly with considerable velocity to the wax, to which having adhered for some time, some will suddenly jump off, others which have touched the stick edgeways will dangle from it a considerable interval, and then . off by their own gravitation when the electrical force has sufficiently subsided. It was by observing amber produce similar effects after friction


to those we have described that electricity obtained its maine. Glass is now more commonly employed to produce electricity than any wax or gum; and there is a striking difference in the kind of electricity then generated, which we shall afterwards notice more at length. These bodies are non-conductors, as they manifestly retain their electricity beyond the moment of its production, and they are isolators, because a conducting substance will also retain its electricity when communication with the earth, or other conducting substances, is cut off by means of non-conducting supports or envelopes. Water is a conductor of electricity; for if you immerse a conducting isolated and electrised substance in water, it will completely lose its electrical properties. Perfectly dry airs or gases, on the other hand, are non-conductors; for if not, none of the phenomena mentioned could have been observed, as the experiments are not made in vacuo. We are not aware that it has yet been established whether the vapours of all substances are conductors (or all the gases non-conductors). Aqueous vapour certainly is a couductor; and therefore when the air is impregnated with moisture it is difficult to perform any electrical experiments which require duration. Hence a conductor constitutes a channel by which the electricity produced on a non-conductor will gradually escape, and a non-conductor constitutes an isolator by which the electricity communicated to a conductor may be retained. Thus, place an electrified glass tube on a tin stand, and the metals being conductors, the electricity will be gradually dissipated; on the contrary, place an electrised tin cylinder on a glass support in dry air, and the electricity will be oi for a very long time. Nevertheless it is far from improbable that this is a question only of degree; that all substances are conductors; and that the usual terms conductor and non-conductor strictly mean quick conductor and slow conductor. When an electrised body is brought near the skin where the power of touch is delicate, a sensation is produced which has been compared to that produced by the touch of a cobweb; but instruments founded on the laws of electric action have been constructed, which indicate the presence of electricity in its most feeble state, and measure its tension. [ELECTROMETER.] Previous to our study of the properties of electricity, on which the construction of the best machines for procuring it in large quantities depends, it will be useful to describe a simple apparatus, and one easily constructed or procured, by which we may learn the first laws of electrical action.

- * * * * A represents a glass cylinder of which the axis is supported on a frame, and which is pressed against by a cushion stuffed with horse-hair, and covered by an amalgam of zinc and mercury spread over its surface; this cushion is attached to a conducting bar C, such as a lamina of iron connected with the frame, and with it communicating with a table or the ground.

At one extremity of the axis of the cylinder is attached a handle by which it may be rapidly turned round, and the friction which is generated against the cushion will produce electricity on the surface of the glass, to guard which against the action of the moisture of the air, the upper side of the cylinder may be lapped over by a piece of glazed taffeta in the direction of the rotatory motion of the cylinder. B is a long narrow and hollow metallic cylinder standing on a glass support, and having at the extremity near A a small cross bar garnished with points or teeth presented towards the cylinder. B is called the conductor in this *PWoo, - hen the cylinder A is turned briskly round, the motion will be accompanied by a crackling noise, and if in the dark, streams of light of a blueish hue will be perceived directed to the several points on the projecting bar of the conductor. B may be thus charged with electricity, and when renoved from the presence of the cylinder (taking it away by its isolating support), it will retain its electrical proper: ties (the longer as the air is more free from moisture), and will by simple contact communicate a portion of its electricity to another isolated conducting substance, or be discharged by touching one not isolated: if, with a feeble charge, it is touched by the finger, a sensation like the pricking of a needle is felt, accompanied by a faint spark o penetrating the fingers. t is useful to cover the glass supports at their points of contact with gum-lac, which is an excellent isolator. Glass plates are now in more general use than cylinders for the production of electricity by friction. It would be impossible here to describe the varieties of electrical machines which have been constructed. Perhaps the most perfect apparatus for producing electricity, and also measuring its quantity, is that employed by Mr. Show Harris, anol which is described in his paper on Electricity in the Philosophical Transactions; a valuable memoir, which deserves also to be consulted for the electrical data, which are there established with much accuracy, in the mode of performing the experiments on which they are founded; though we do not acquiesce with some of the inferences drawn by the author. We shall now observe, as our first phenomenon, that neither the cylinder which has generated and given out electricity, nor the conductor which has acquired it, exhibits the least alteration of weight, nor will the greatest possible accumulation of electricity produce the least perceptible alteration in this way. Those who suppose that electricity is a distinct species of matter, an all-pervading fluid, have therefore denominated it imponderable. Facts do not, however, authorize us yet to take this view of it, or to regard it as essentially different from the forces which the molecules of matter exercise, which, though neutralized for external bodies when these molecules are in positions of equilibrium or stability relative to their mean places, may beeome sensible by impressed forces, such as friction, which would alter the position of their poles, or by sudden concussion forcibly altering their relative situations. There is a convenience of language, however, in speaking of it as a fluid, which can lead to no error by its adoption until the phenomena of molecular actions are more studied, and the views of Mosotti and many other natural philosophers with respect to the identity of these forces better established. Adopting, therefore, this hypothesis, the sense in which we speak of its quantity, its accumulation, its density, or its partition between bodies, 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 notice is the limit to the quantity of electricity 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 we 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 it traverses. This spark travels with immense yelocity, and is accompanied by a very audible sound; and if received by the body of a man or animal, it produces through a part 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, as deafness, though its use has become by no means general. Two points 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 of electricity of which they are capabič are proportional, not to their solid content or weight, but to their surfaces; that is, to the square of their linear dimensions. 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. Mr. 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 proportionality to the surfaces, and not to the content or weight of the body, will not excite surprise when we state that no developed electricity exists within a body, at least to any appreciable depth below its surface. This fact has been repeatedly confirmed experimentally; and in conse. quence of this law we see the advantage of using a long cylindrical form for a conductor, and perceive the propor tions 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 de veloped 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 suspend it from the hand by a fine silk thread, which is a good non-conductor; 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 slant direction, until the obliquity is sufficiently great for its own gravity and the tension of the string to counterpoise the repulsion of the electricity on the conductor Let two pith balls be suspended by rallel strings

so as to touch each other, as A, B, and if a portion of elez tricity be communicated to them by temporary contact with a body already charged, the strings will diverge in conse. quence of the mutual repulsion, and the balls will come into the positions a, b, where, 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 towards each other as they lose their electricity from the contact of the surrounding medium. That the law of force in this case is, as in gravitation, expressed by the inverse square of the distance, was satisfactorily demonstrated by Coulomb by means of his torsion balance, the principle of which we have before explained [ELAsticity], and has been lately confirmed by Mr. Harris's experiments: and a remarkable fact arises from it, namely, if the electrised pith-ball A be placed any where within the concavity of a spherical shell, it will not be moved in any di rection by the repulsion of the electricity on the surface of the shell; for the electricity being then uniformly dis tributed, the intensity of the force of any small portion DC is at its extent, or proportional to the square of the arc DC, and is therefore destroyed by the action of an opposite por: tion dc, cut off by a conical surface having A as vertex, and DC as base; for D C*: d co: : D A*: c A*, that is, what the portion gains in extent it loses in its 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 same mutual pestruction of forces occur (Laplace, Mécanique Céleste, tom..., liv.2; Murphy's Electricity, chap. iii.); and Poisson, from other considerations which we shall afterwards notice, made this condition, for 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 an: other conductor replace it, the latter will become charged by repeating the . thus the cylinder and every substance is an inexhaustible source of electricity. We have supposed the cushion by which the cylinder is rubbed to be in communication with the ground by a conductor; but if two substances both isolated be electrised by friction, and when separated the electricities belonging to each surface be examined, we find the following results:— Let two isolated pith-balls A and B, as before, be electrised by communication with one of the surfaces, and two other balls a b in like manner electrised by the other surface. Then when A is presented to B, or a to b, repulsion takes lace as before described ; but when A is presented to a, or }. to b, they will attract each other; and if A, a have equal charges from the different surfaces which have been rubbed against each other other, when contact takes place between A and a, all signs of developed electricity will depart from each, and the bodies will take their natural positions, neither attracting nor repelling each other; but if A has a greater electrical charge than a, a surplus of the electricity of A will remain, and will be partly communicated to a when a 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 substance: the pith-ball which receives its electricity from the glass cylinder will attract that which has been in communication with the other machine. Hence arise the terms vitreous electricity and resinous electricity, or, as they are now more usually and properly called, positive electricity and negative; for whatever two substances they may be which are rubbed together when electricity is produced, it will be found positive on one substance, and negative on the other, even if the substances are of the same nature; for instance, both glass. The phenomenon above noticed may be then announced as follows: “Like electricities mutually repel, unlike mutually attract; and the 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 unlike electricities is similar to the addition of quantities with unlike signs in algebra: when equal the sum is zero, when unequal it is the excess, and of the same name as the greater charge. Franklin's theory makes only one electric fluid in excess above its natural state in bodies positively electrised, and in defect in those said to be negatively electrised. Epinus, and most of the continental philosophers after him, suppose two distinct electrical fluids, the particles of each of which repel those of the same kind, but attract those of the contrary, and therefore the opposite electricities always seek combination or neutralization, so that in natural bodies the two fluids exist in equal quantity, by which the presence of neither is indicated. Mosotti has in some degree revived the theory of Franklin in his memoir on the forces which determine the state of bodies. We adopt at present the theory of two fluids, but all the phenomena may be readily expressed also on Franklin's theory. . The pressure of the electricity on the surrounding meSlium, when the body is perfectly conducting, determines the direction of the motion under the influence of foreign


electrised or non-electrised substances, which, by rendering this pressure unequal on the different parts of the surface, produce motion by the unequal reaction of the medium. But imperfectly conducting bodies have in themselves a certain retentive or coercive force, and the electrical particles, instead of then freely obeying the external impressed force by a corresponding law of arrangement or accumulation amongst themselves, communicate the forces impressed to the particles of matter by which they are restrained. In imperfect conductors the force is partially exercised in each of these ways. The circumstances of the motions of electrised substances therefore vary with their conducting faculty. We can now understand the mode in which light substances are attracted to a stick of sealing-wax which has been made electrical by friction: the electricity of the wax is in this case negative; and when brought near a small piece of paper, which is a conductor, it acts upon the neutral fluid of the paper, attracting some of its positive electricity to the side next it, and forcing the negative to the farther surface, which, being in communication with the ground or a conductor, is carried off; so that the paper is thus by influence made positively electrical, which, being of a contrary kind to that of the wax, is attracted by it, and therefore the paper flies to the wax, and having touched it communi cates its positive electricity to it, thereby neutralizing a portion of its free fluid; after which it shares a part of the surplus of negative electricity remaining on the wax, when it is of course repelled; and if it become neutral by again touching the ground, and the electrical force has sufficient energy, it will again fly to the wax and the same results will be repeated. When a body is of an irregular figure, and is electrised, the electricity of its surface will be differently accumulated at the different parts, projecting points having the most, and portions of small curvature the least in convex surfaces; and it is a mathematical problem of considerable difficulty in some cases “to find the law of the distribution of free electricity on the surface of a perfectly conducting body of a given form.” The datum for the solution is, that the whole action of the electric envelope on any point interior to the body is zero: we have shown that it would be so in the case of a sphere by a uniform distribution on the surface; but in other bodies this distribution cannot be uniform to produce the same effect. The next case in the order of simplicity is the spheroid, or more generally the ellipsoid, for a spheroidal shell, bounded by two similar and concentric spheroidical surfaces, and attracting by the law of the inverse square of the distance, will exercise no action on an internal point ; hence the accumulation of electricity on the surface of a spheroid at any point is proportional to the normal breadth of the stratum at that point, which it may be easily proved is proportional to the perpendicular drawn from the centre on the tangent plane, or inversely as the diameter parallel to the tangent at that point. Hence we see why the accumulation of electricity at points is so great, which are therefore part of the armature of prime conductors; for if we conceive the axis minor of an ellipse to diminish indefinitely, while the axis major remains invariable, the breadth of the spheroid generated will be correspondingly diminished while the length remains the same, and ultimately it will approximate to the form of a needle pointed at the extremities of its axis major, the breadth of the electricity at the point is then to that at the middle of the needle as the length of the needle to its greatest breadth. Now, in consequence of the law of force being the inverse square of the distance, we find the pressure against the air is as the square of the accumulation, and consequently is very much greater at either, extremity of the needle than at or towards the middle; and therefore, on being overcharged, the electrical spark is given from the extremity, when not otherwise determined by the influence of external bodies. Moreover, when several conducting bodies, some or all of which are electrised, are placed near each other, a new dis; tribution of electricity takes place on their surfaces, caused by the decomposition of the neutral fluid of each by the action of the extraneous substances: thus, the principle for calculating the distribution in this case on every body is to suppose it such that the total action on any point within each of the conductors shall be zero; for if not, the neutral fluid at that point would be decomposed, and the separated fluids proceeding to the surface of the body would alter the distribution When the distribution is ascertained, then the motions of the bodies may be calculated according to the laws of dynamics, the pressure against the surrounding medium being as the square of the accumulation. Two spheres placed in contact and electrised will have the point of contact neutral. This result of theory (founded on the principles above detailed), with many others, has been fully confirmed by experiment. Those who wish to follow up the mathematical principle here noticed, may see Poisson's Memoirs on Electricity (Mémoires de l’Institut), and an English treatise expressly on this subject by Mr. Murphy of Cambridge. - - When electricity is generated by the friction of two substances, one acquires positive, the other negative electricity, but it is difficult to judge à priori, from the nature of the substances employed, the character of the electricity which each will take; and though most treatises contain tables of substances in which each is positive to that which precedes and negative to the succeeding, yet the nature of the electricity is so liable to alteration, from very minute circumstances of the friction, that it is better, even in each case, to try direct experiment. The friction produced by liquids also produces electricity, the electric light, when a barometer well freed from air is first filled with mercury, having been remarked from the earliest dates of the use of that instrument; and when a current of air is directed against a plate of glass the latter will acquire positive electricity, and therefore the air negative, and the rapid agitation of a piece of silk in the air communicates to the latter positive electricity while the silk acquires negative. The difference of temperature of a substance often determines the species of electricity it acquires by friction. Generally an increase of temperature disposes to negative electricity, and polish or smoothness to {j ; pressure on many crystals will produce opposite electricities, as will also heat (as in tourmaline), and even the slight adherence which a piece of glazed taffeta would have to an isolated metallic plate which it covers is sufficient to give the plate negative electricity, which is the more remarkable from the fact that the friction of the two would have made the taffeta negative and the plate positive. Moreover, both the electricities are produced in most of the chemical compositions and decompositions, in the sudden fracture of substances, in evaporations, &c.; and the higher couches of the air are in a state of positive electricity when unoccupied by clouds, which are found indifferently charged with either. When a body is positively electrised, we can procure the negative electrisation of another conducting substance by the influence of the former on the neutral electricity of the latter. Let the conductor be placed in the vicinity of the influencing body, but not so close as to receive any positive electricity by sparks or other direct communication. The natural electricities of the conductor will be then separated by the influence of the positively electrised body, towards which the negative electricity must be attracted and the positive repelled; the part of the conductor nearest the influencing body must therefore be covered with negative electricity, and that more remote with positive. If now this end of the conductor be made to communicate with the ground, the positive electricity will escape into this great reservoir, and moreover sufficient negative electricity will be communicated from the ground to the conductor to render the point of contact neutral: thus the conductor acquires a double change of negative electricity, and when isolated will be found negatively electrised after it has been removed from the vicinity of the isolating body. The effects of influence, as above described, may be easily observed in the following manner: Place a long and narrow isolated conducting cylinder before a body strongly electrised, and from different equi-distant points of the cylinder suspend pairs of pith-balls by cotton threads, which will acquire the electricities of the parts of the cylinder with which they are connected. We shall observe a considerable divergence in the pair suspended nearest the influencing body, because they are strongly charged with an electricity of a contrary nature to that of the body: going along the cylinder, the divergence diminishes, and at a point not as remote as the middle of the cylinder there will be no divergence. Beyond this neutral line the cylinder has an electricity of the same kind as the influencing body, increasing in intensity towards its farthest extremity, and therefore the strings commence to diverge more and more as we approach

balls moved along the cylinder will be sufficient if we secure them from the direct influence of the body by a piece of glass interposed near them. his is the direct influence the electrised body has on a neutral body, but the neutral body must again re-act on the original body, sensibly decomposing its electricity if it be a conductor; and thus the true arrangement of the electricity, in two surfaces influencing each other, although instantaneously effected, may be regarded as the final effect of a succession of direct and reflected influences between the bodies. This principle has been shown by Mr. Murphy materially to facilitate the actual calculation of the distribution of electricity on two electrised surfaces in presence of each other. The effect of the influence of a near electrised cloud has been felt by several persons; among others by the writer; and in many cases fatal results have followed, not from the direct discharge of the electricity or, as it is called, the lightning, but from the sudden reunion of the electricities which had been separated by influence, and which, upon the discharge of the cloud, is effected by means of a corresponding electric charge brought through the body from the ground. rom the power of separation of the neutral fluid in bodies at a distance which is exercised by electricity, an easy means has presented itself by which a much greater quantity of electricity may be collected upon a conducting plate than that which could be directly communicated by a conductor. We shall therefore now endeavour to explain the principle of the condenser, which we think very inaccurately stated in Biot's Physique, in which the subject of electricity is treated, generally speaking, in a very luminous manner. The following investigation the author of this article gives, on his own responsibility, with the desire of placing the power of the condenser on its true basis — A

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Suppose two equal conducting plates, of which the axes are AB, CD, to communicate respectively at A and D with known sources of electricity, and have their opposite faces B,C near to each other and parallel, the whole being surrounded by a non-conducting medium, the known sources of electricity communicate qui #. E, E' of electricity to the bases B, D, and the mutual influences of the system generate other quantities X, X' on the second bases B, C, these quantities are dependent on E, E, on AB, CD, which for simplicity we shall suppose both equal to c, and on the mutual distance B,C of the plates, which we shall call a. Our problem is to find X and X’ from these data. Consider the total action on a point P, taken anywhere within the first plate and on its axis; this must be equal to zero, in order that the neutral electricity at that point may not be further decomposed. Let PB-2. The action arising from the base A and the adjoining portion of the sides of the plate included between A and a parallel drawn through P is Ef (c.—z); the form of the function f is unknown, since it depends on the law of the distribution of the fluid at the different parts of the base and sides. Similarly, the action arising from the base B+ X f(x) C=X' f(a+2) - - - - - - D = Esfia-Hc-Hz). Our first equation of condition must therefore be— X f(z) +X' f(a+z)—Ef (c.—z) +E"f (a+c+z)=o...(1); and if we consider in precisely the same way the equilibrium of a point Q within the second plate and in its axis, we obtain (putting C Q =z')— X'f(z')+X f(a+2')—E' foc-z')+E f(a+c+2')=0......(2). The equations (1) and (2) must hold true for all values of z and 2' between o and c, and they serve to determine the form of the function and the values of X, X'. If the bases were infinite, f(z) would be constant. (Prin

- - - - - - -

that end. In making this experiment a single pair of pithP. C., No. 573. gie p P

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It will be useful to make a few remarks before proceeding further. The expression which we have put for the action of the plane C on P in equation (3) is in reality the action not only of that plane but also of the side of the prism or cylinder, of which the base is C and altitude CP; and a similar remark applies to the action of the plane D; therefore the total action given in that equation is too great by twice the action of the side of the prism or cylinder included between the plates B and C. For the same reasons we have a like excess in the equation (4); wherefore we have subtracted these equations, when that excess disappears; whereas, if we had added them, an error would arise, small with respect to X and X', but comparable to E+E'.

Also, from equation (7), the apparatus would be discharged by making the two plates communicate.

In the actual case the lower plate communicates with the ground; therefore E' → 0.

A. now the two equations, we find—

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n is a very small quantity and negative, since the attractive force diminishes as the point acted on becomes more remote. Hence X is very great and positive compared with E, and it follows— First, that the greater the extent of the plates, the less n will be, being zero when that extent is infinite; therefore the power of the condenser is increased by the extent of the surfaces being enlarged. Second, that another source of increase of the condensing power is the diminution of a, the space occupied by the non-conducting medium interposed between the parallel conducting plates. These results are perfectly accordant with experience. In practice, the conducting plates are generally separated by a plate of glass or a cover of varnish, the latter being used when the electrical charge is feeble; for the attractive forces of the two opposite electricities X, X' would be too powerful for such an obstacle if E were great, and the elec

and subtracting—

tricities would penetrate it, and unite; but in chemical operations, where the electricity developed is of weak tension, the diminution of a is of great advantage, the quantity of electricity acquired by the plates becoming very sensible to the electrometer. [ELECTRoMETER.] The Leyden jar is an instrument founded on these principles. A glass bottle is coated within and without with tinfoil. The conductor of an electrical machine communicates with the foil on the inside by means of a metallic chain, while the outside is in communication with the ground. The opposite electricities are therefore accumulated on the internal and external sides of the glass; hence a flash and a powerful shock is produced, when the two fluids combine, by touching the outside foil with one hand, while the conductor or chain communicating with the inside is touched by the other. It was ascertained by Cavendish that, the quantity of electricity produced in the Leyden jar, with given surfaces, was inversely proportional to the breadth of the glass; this completely corresponds with the results which we have above obtained by theoretical considerations. There seems little doubt, from the experiments of Wollaston, that much of the electricity produced by the common machine is attributable to chemical action; for the best amalgam to use with the rubber is that which oxidizes most readily, such as tin and zinc, and scarcely any quantity of electricity is produced if by the nature of the amalgam, there is no sensible oxidation, or if we envelope the apparatus in a medium which will not communicate oxygen, as carbonic acid gas. As the quantity taken by the conductors is proportional, capteris paribus, to their surfaces, it is usual to employ several narrow cylindrical conductors placed parallel to each other; the total surface in this case being the same as that of a single cylinder of the same length, and of which the radius would be the sum of all their radii. The electrophorus is founded on a principle nearly similar to that of the condenser; but in this case it is the nonisolated body which acquires electricity by the influence of that which is isolated. It should be observed that the non-conducting plates employed in the condenser and Leyden jar have a certain retentive power on the electricity, and which is of the same origin as its non-conducting faculty: hence it will happen generally in experiments that the whole of the electricity will not be discharged at once, when the opposite electricities of the two plates are made to communicate by a conductor, and frequently not after several repetitions. The same principle of the separation of the neutral electricity of remote bodies by influence is only varied in the number of electrical machines which have been at different times constructed, such as electrical batteries, electrical piles, &c. The construction of such apparatus is continually varying, as frequently from caprice as from experience. Those which are most commonly employed in laboratories will be found (by such as cannot actually see them) described in most popular treatises on electricity. In the best ...]"experiments there will be a loss of electricity, arising either from the hygrometric state of the atmosphere or the imperfect insulation of the supporters employed. When, for instance, the moist particles of vapour floating in the air come in contact with the conductor of an electrical machine, they acquire by their own conducting power a small portion of the electricity developed in the conductor; being similarly electrised they are repelled; and new particles of moisture arising, repeat the same process of exhaustion, each tiny robber carrying away as much electricity, not as it can hold, but as it may hold without being itself held. The quantity thus lost in a small given time is proportional to the whole charge, and therefore the latter must diminish in a geometrical progression when the time increases in arithmetical. For atmospheric electricity, see METEoRology. The electrical light produced in a discharge, whether in an artificial vacuum, in air, or in water, which is susceptible of decomposition by the prism, and varies its tint with the substances between which it is discharged, has been a subject of controversy among physical philosophers; but the opinion most generally received is, that it is the effect of the compression of the traversed medium, which, under such circumstances, would give out light and heat. Mr. Wheatstone has recently exhibited some ingenious experiments to show the velocity of the electric fluid, an account

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