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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 another conductor replace it, the latter will become charged by repeating the operation: 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 bin like manner electrised by the other surface.

Then when A is presented to B, or a to b, repulsion takes place as before described; but when A is presented to a, or B 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 meJium, 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 distribution 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.

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.

This 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 maWhen electricity is generated by the friction of two sub-terially to facilitate the actual calculation of the distribustances, one acquires positive, the other negative electri- tion of electricity on two electrised surfaces in presence of city, but it is difficult to judge à priori, from the nature of each other. 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 positive; 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 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. From 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

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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 quantities 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 cal! 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=z.

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 ƒ 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-Xƒ(z)

C= X'ƒ(a+z)
D= Efa+c+2).

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 Our first equation of condition must therefore bewhich they are connected. We shall observe a considerable Xƒ (z)+X'ƒ (a+z)−Eƒ (c−z) +E'ƒ (a+c+z)=0...(1); divergence in the pair suspended nearest the influencing body, because they are strongly charged with an electricity and if we consider in precisely the same way the equilibrium of a contrary nature to that of the body: going along the of a point Q within the second plate and in its axis, we obcylinder, the divergence diminishes, and at a point not as tain (putting CQ=z')remote as the middle of the cylinder there will be no di- X'f(z)+Xƒ (a+z′)−E'ƒ (c− z')+Eƒ(a+c+z')=0.....(2). vergence. Beyond this neutral line the cylinder has an elec- The equations (1) and (2) must hold true for all values of tricity of the same kind as the influencing body, increasing z and 2 between o and c, and they serve to determine the in intensity towards its farthest extremity, and therefore the form of the function and the values of X, X'. strings commence to diverge more and more as we approach If the bases were infinite, ƒ(z) would be constant. (Printhat end. In making this experiment a single pair of pith-cipia, book xiv.)

P. C., No. 573.

VOL. IX.-2 X

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tricities would penetrate it, and unite; but in chemical +&c. by Maclau-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.]

for z being very small, we reject the powers higher than
f'(0)
the first, and put for abridgment, instead of ; c is
f(0)

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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, cæteris paribus, to their surfaces, it is usual to employ several narrow cylindrical conductors placed parallel to each other; the total surface in this case

Since -' may be positive, negative, or zero, and yet this being the same as that of a single cylinder of the same equation always true, we must have separately

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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.

Adding now the two equations, we find

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First, that the greater the extent of the plates, the less n will be, being zero when that extent is infinite; there fore 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.

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 conducted 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 subThese results are perfectly accordant with experience. ject of controversy among physical philosophers; but the In practice, the conducting plates are generally separated opinion most generally received is, that it is the effect of by a plate of glass or a cover of varnish, the latter being the compression of the traversed medium, which, under used when the electrical charge is feeble; for the attractive such circumstances, would give out light and heat. Mr. forces of the two opposite electricities X, X' would be too Wheatstone has recently exhibited some ingenious experipowerful for such an obstacle if E were great, and the elec-ments to show the velocity of the electric fluid, an account

of which may be seen in a paper communicated to the Royal | Society. Its immense velocity has been demonstrated long since by the instantaneousness of its arrival at different parts of long metallic tubes, or a series of them, such as are used in pipes for conducting gas. The chemical and other effects of electricity will be found under their proper heads.

(Biot's Physique, tome ii.; Pouillet, Elémens de Physique; Murphy's Electricity; Papers by Mr. Snow Harris in the Philosophical Transactions; Turner's Chemistry, fifth ed., &c.)

ELECTRICITY, LATENT. [MOLECULARITY.] ELECTRICITY, Medical application of. A supposed analogy between electricity and the nervous power has led to the employment of this agent, particularly in diseases connected with defective nervous energy, and also in cases of defective secretion, perhaps originating in a similar cause. The influence of electricity on the human system differs much according to the manner in which it is applied, the length of time during which it is continued, and the degree of intensity. It also differs in its action according as it is abstracted from, or communicated to, the individual. When applied in a moderate degree of intensity, it occasions an increase of nervous action, of sensibility and irritability, more vigorous circulation of the blood, augmented warmth, and secretion, especially cutaneous transpiration: even the exhalation of plants is much increased by electricity. When the electric principle is more intense, all these actions are heightened, often to a painful degree; while such a degree of concentration as occurs during certain atmospheric changes can occasion instant death. Death occasioned by this means is always followed by rapid decomposition of the body. The diseased states in which electricity has been found most useful are-in asphyxia, from any cause (except organic disease of the heart), but particularly from exposure to irrespirable gases; in certain asthmatic diseases; and dyspepsia, dependent on irregular or defective supply of nervous energy to the lungs and stomach. It is however much inferior to galvanism as a remedial agent in these diseases. (Wilson Philip on the Vital Functions.) local paralytic affections, when of a chronic character, electricity, duly persevered with, has been found very useful: in a case of dysphagia, from paralysis of the esophagus, the patient could only swallow when placed on a seat resting on nonconductors and electrified. In deafness and loss of sight, when directed by a competent judge, it has restored the functions of seeing and hearing. Lastly, in defective secretion, especially amenorrhoea, it has proved of service.

In

ELECTRO-CHEMISTRY. The effects of electricity, whether produced by the common electrical machine or the galvanic apparatus, when applied to substances with sufficient intensity, are recognized in their composition or decomposition, their fusion, phosphorescence, &c.; this class of phenomena have been successively investigated by Franklin, Priestley, Cavendish, Wollaston, Cuthbertson, Davy, Van Marum, Gay-Lussac, and Thénard, Becquerel, Faraday, &c., and form an important branch of science closely connected with the nature of chemical affinities, and

called electro-chemistry.

Though the decomposition of water by common electricity was effected before Wollaston, yet the remarkable simplifications which this celebrated man introduced into every chemical subject with which he was connected accompanied his electro-chemical researches, we shall therefore confine ourselves to the description of the mode he adopted to decompose water.

Finely-pointed wires of gold or platina are introduced into capillary tubes, the glass is then heated by a lamp until it becomes soft and completely covers the metal, the uncovered part of the wire is then cut off with a sharp instrument, or else the glass may be ground away until the very point of the wire commences to project; sometimes the platina is first silvered, and when the uncovered part has been cut away, the whole is plunged in nitric acid, which dissolves the silver envelope, leaving only a very fine point of platina.

Two wires thus prepared are placed in a vessel containing water, bringing the metallic points very close to each other; one wire is now put in communication with the ground, the other with one of the conductors of an electrical machine; when the electricity of the machine is evolved, a series of sparks passes through the water between the metallic

points, the water becomes decomposed into its constituent gases at the points of the wires, and being collected in glass vessels filled with water, they are found in the ratio of 2 to I in volume, which is the known proportion of hydrogen and oxygen combined to form water: the finer the metallic points, the greater will be this decomposition; some of those used by Wollaston were of only the 1500th of an inch in diameter.

The decomposition of æther, alcohol, oils, &c., have also been effected in nearly a similar manner, by the electrical spark, and the correct proportions of the constituent gases have invariably been obtained; the oxygen in such cases is always found at the negative pole, and the same result is obtained if we use only a single wire, and do not insulate the vessel containing the fluid operated on.

In metallic solutions the precipitate on the electrised wire shows visibly the decomposition; thus in a solution of copper, if we employ a silver wire, protected by an envelope of sealing-wax, when the electrical discharge is communicated we find the copper precipitated on the negative wire. When solid substances are inclosed in tubes of glass, such as the oxides of gold, tin, &c., and a strong electrical discharge passed through them, a similar decomposition takes place, the products being deposited on the sides of the glass.

When a gas is thus to be decomposed, a glass vessel is filled with mercury, and the wires introduced; the vessel is then inverted in a reservoir of mercury, and the vacuum is sufficiently filled with the gas that the wire points project above the surface of the mercury; the discharge is then effected, and the required decomposition is produced. Example.-Sulphuretted hydrogen. Hydrogen is disengaged, the sulphur deposited, and the volume remains unaltered.

Conversely, the composition of compound bodies from their elements in many cases is effected by the electrical spark, two volumes of hydrogen, and one of oxygen gas being introduced into a stout glass tube filled with mercury, and an electrical spark passed through them by means of the wires, water is formed, accompanied with a loud detonation, and the rising of the mercury in the tube in consequence of the diminution of volume. The instruments used for the combustion of gas are called eudiometers, the principle of their construction, particularly that of Mitscherlisch, is particularly simple; the object being generally to measure the quantity of oxygen contained in airs.

The electrical spark will re-light a candle which has been just blown out by the carburetted hydrogen of the smoke with the oxygen of the air, which are then easily inflamed.

When electrical sparks are transmitted incessantly through a small given portion of atmospheric air, its volume becomes compressed, and nitric acid formed. (Phil. Trans., vol. 75.) By adding oxygen, we can decompose gaseous bodies (by means of the spark) which contain hydrogen: however, it has been found that there is a limit in such mixtures, beyond which the burning will no longer take place. A table of these limits is given by Turner.

Table of mixed Gases and their Products when decomposed.

Names of the gases.

Deuto-carbonate of hydrogen

Olefeant gas
Gas ammoniacal

Phosphoretted hydrogen

Carbonic acid gas
Hydro-chloric acid gas

Azote-protoxide
Nitrous gas

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Products.

Carbon and hydrogen; volume of hydrogen double Carbon and hydrogen Hydrogen and azote; volume doubled

Phosphorus precipitated; volume of hydrogen unaltered

Imperfect decomposition
Hydrogen and

chlorine

(Henry) Oxygen and azote Nitric acid and azote

A species of phosphorescence is produced in different bodies by subjecting them to the action of a powerful electrical machine. Calcareous spar, carbonate of barytes, Derbyshire bitumen, &c. become luminous by the shock, white other substances give great sparks, and do not become luminous, as mica, dry peat, plumbago, &c.

We shall now consider the electro-chemical effects of the voltaic pile. In general, if two rods of platina in communication with the poles of a voltaic battery be immersed in

2 X 2

other, and in point of fact we find that it neither will attract nor repel D. We have here a positive current of electricity issuing from A and a negative from B, and no effect of attraction or repulsion is produced on an electrised point as in statical electricity. How then is its state recognized? First by touch; for if we touch the rod ACB, a series of shocks is felt, the interval between two succeeding ones being inappreciable; and secondly, powerful chemical decomposition may be effected. [GALVANISM, ELECTRO-CHEMISTRY.] But thirdly, we may recognise it mechanically by presenting to AB another rod A'B' under exactly similar circumstances,

a saline solution, it will cause a separation of its constituent | parts; when the current passes through solutions of neutral salt, coloured with vegetable blues, the part of the liquid at the negative pole is green, at the positive, red. When the water contains a solution of a metallic salt, as of zinc, lead, or copper, the negative wire receives a coating of the particular metal; and in the general electro-chemical decompositions, oxygen and the oxides are found at the positive pole, hydrogen and the bases at the negative. Bodies thus susceptible of decomposition by the pile have been lately distinguished by the term electrolytes (nλEKTρov Xvw): thus hydro-chloric acid, water, &c. are electrolytes; boric acid is not such; substances are said to be electrolysable when capable of being thus decomposed. These terms have been introduced by Mr. Farady.

This is the place to notice the decomposition of the alcalis by means of a powerful galvanic battery. These substances had been previously taken as simple or elementary, but upon being introduced into the circuit of the battery, at the positive pole oxygen was disengaged, while at the negative pole was found the metallic base of the alcali, as sodium or potassium, according to the nature of the alcali employed; these substances burn at the temperature of the air, in oxygen or air, and are even capable of being inflamed in water. To preserve them therefore from the contact of the air, Seabeck adopted a process by which they are made to combine with mercury, in proportion as they become disengaged; the amalgam thus produced is afterwards separated by the evaporation of the mercury.

Gold-leaf, carbon, &c. placed in the voltaic current between the points of the positive and negative wires of a pile become inflamed, yielding a light of the greatest brilliancy often so intense as to be painful to the eyes.

All the chemical effects of voltaic electricity are cæteris paribus proportional to the extent of surface of the plates employed, and are also increased by augmenting the number of plates. (For further information on this subject, the reader is referred to the actual memoirs of Davy, Wollaston, and Faraday, in the Transactions of the Royal Society; to the Annales de Chimie; and to the Traité de l'Electricité, par Becquerel. Several isolated and interesting facts on the same subject will be found in the Annals of Philosophy, and the Edinburgh Journal of Science. See also GAL

VANISM.)

ELECTRO-DYNAMICS. In ordinary electricity, that fluid when developed takes a position of equilibrium, dependent on the conducting power of the medium on which it is disposed, on the non-conducting power of the medium by which it is enveloped, and on the law of force, whether of attraction or repulsion, between the elementary portions of electricty. The motions of electrised bodies are only results of the statical equilibrium of this fluid, and do not therefore belong to electro-dynamics. The mode of calculating such effects may be found under the head ELECTRICITY. These effects are moreover of the same nature whether the source of electricity be by means of friction, or by chemical action, as in the voltaic pile, the nature of the electricities in these cases differing from each other only in the mode of their production; but when the contrary, electricities are no sooner produced than re-combined, again reproduced and again re-combined, a new class of phænomena is produced belonging to electricity as it were in motion. Suppose, for example, that the plate A is a constant source of positive i

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electricity, the plate B in like manner a constant source of negative electricity of equal intensity; that AC, BC are two conducting rods communicating with each; the electricities immediately combine when the conductors are made to touch at C, and for an instant the whole may be conceived to be in the neutral state, but A being the next instant replenished with positive and B with negative electricity, the same combination takes place over again, the same neutrality succeeds, and so on indefinitely. The rod ACB is in a different condition from one in its natural state, since electrical charges are continually pouring through it from A and B; and again it is in a different condition from an electrised rod, since we cannot at any moment say that it is charged positively rather than negatively. Hence we cannot infer that it should attract rather than repel an electrised ball D, since there is as much reason for one event as the

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when the effects of the currents in AB, A'B' will be recognized by the visible motions of the rods, provided they be free to move while their communication with the proper sources of electricity remains unbroken: for example, if their extremities be immersed in cups of mercury communicating with the constant sources of the positive and negative electricities. The laws of the mutual action of electrical currents constitute the science of electro-dynamics; and previous to its study it would be desirable that the reader should be acquainted with the construction and applications of the galvanic apparatus, the opposite poles of which afford the two constant sources A, B of electricity which we have supposed. These will be found under the head GAL

VANISM.

To discover the laws of the mutual actions of electrical currents we must have recourse to experiment. An apparatus similar to that employed by Ampère will be found in Professor Cumming's translation of Dumonferrand's treatise on this subject; together with a description of the mode of performing the various experiments by which these laws have become known. The term direction of a current is convenient when speaking of more than one; for instance, the zinc end of the pile being a constant source of positive electricity and the copper end of negative, a rod communicating with wires connected respectively with these extremities will have a current of positive electricity from the zinc to the copper, and a negative current in a contrary direction; but as it is simultaneously permeated by both, when we speak of the direction of a current we shall understand that of the positive current to avoid ambiguity.

Two parallel currents which are directed in the same way attract each other, but when directed in opposite ways, they repel.

When rectilineal currents form mutually an angle, the species of action which they exercise may be thus defined Two portions of currents will attract if they are both approaching or both receding from the vertex of the angle which they form; but when one approaches and the other recedes from that angle, then they repel:' the same law holds in the limiting case of parallelism.

B

Let two currents cross each other, as AEB, CED, and suppose the directions of the currents to be those indicated by the arrows in the figure; then, according to this law, the force between CE, EB, is repulsive; and that between CE, EA, attractive; if, therefore, AB is fixed, and CED moveable, we ought to have CE tending towards AE, and for the same reason, DE tending towards EB; the rod CED, therefore, has a rotatory motion impressed on it until it is placed parallel with AB: this is confirmed by experiment.

If we now consider two currents to form a very obtuse angle, one of them approaching, and the other receding

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