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Inder is as free and insulated as if no E. existed within it. This is shown by placing a cylinder near the first, forming a continuation of it, as it were, without touching, when the second cylinder, under the induction of the + E. of the first, is thrown into the same state as the first. This second can induce the same state in a third, and so on. As the excited tube is withdrawn, the whole series return to their natural condition without being in any way permanently affected. The moment, however, it is again brought near, there is manifested at the further termination of the last a+ E., which exerts the same influence there as if a portion of the E. of the tube had been actually communicated or transferred to it.

The air intervening between the tube and the cylinder is termed the dielectric, for it is through it that the electric action is propagated. In proof of this, we have only to place a cake of shellac between the tube and cylinder, when the polarity of the cylinder will rise higher than before, as would be shown by the further divergence of the balls; and if this or a similar experiment be conducted with sufficient care, we find that the inductive action varies in amount for each non-conductor. Induction, therefore, we have reason to conclude, is not the direct action of one body on another, but an action transmitted through, or possibly residing in, the medium between them. In further proof of this, Faraday, who was the first to examine the function of the dielectric in induction, has shown that the action takes place through air in curved as well as in straight lines, which implies the action of an intervening medium. The relative powers of different substances in facilitating induction, are termed by this philosopher their specific inductive capacities. The following table by sir W. S. Harris gives the specific inductive capacities of the more important non-conducting substances, taking that of air as unity: Air, 1.00; resin, 1.77; pitch, 1.80; beeswax, 1.86; glass, 1.90; sulphur, 1.93; shellac, 1.95; india-rubber, 2.8. All gases, whether simple or compound, have the same inductive capacity, and this is not affected by temperature or density. If a large plate of metal be placed between the glass tube and the cylinder, the polarization of the cylinder instantly vanishes, for the induction is diverted by it into the ground.

Theory of Induction.-Faraday, taking for granted that the dielectric is the essential medium of induction, suggests that the molecules of air and other dielectrics are conducting, but that they are insulated from each other. We have already seen that by induction, part of the E. of an insulated body can be in effect transferred to a surface at some distance from it, without any loss experienced by the exciting body. If, now, we could imagine a series of insulated cylinders diverging in all directions from the glass tube, we have reason to expect that the whole of the E. of the tube would be in effect transferred to their outer extremities without loss of E. to the tube. To prove that such would be the case, Faraday took a pewter ice-pail, 10 in. high, and 7 in. in diameter, and insulated it, placing the outside of it in conducting connection with the knob of a gold-leaf electroscope. An insulated ball, charged with E., was then introduced into it without touching. The pail was thus subjected to polarization, the - E. being on the inner, and the + E. on the outer surface. The divergence of the leaves increased as the ball was lowered, until it sunk 3 in. below the opening, when they remained steadily at the same points. The ball was lowered till it touched the bottom, and communicated its charge to the pail, when the leaves remained in the same state as before, showing that the + E. developed by induction on the outer surface was exactly the same in amount as that of the ball itself. He then altered the experiment so as to have four insulated pails inside each other, and the effect on the outmost pail was in no way altered. Here the action of the air between the pails was in effect the same as that of the pails themselves, and if the molecules of air were insulated conductors like these, they would have acted in no way different from what they did. The action of the molecules of air, in certain circumstances, appears to favor the idea that they are individually conducting. The discharge of E. by spark through the air, shows that they can be forced to act as conductors; and the currents which proceed from points highly charged with electricity, appear to indicate that they can be attracted and repelled like the pith-balls of our first experiment.

Conductors, according to this theory, are bodies whose molecules have the power of communicating their electricities to each other with great ease, whilst non-conductors are those whose molecules only acquire this power under great force. Wheatstone has shown, as we shall afterwards see, that facility of discharge is not perfect even in the best conductors, as time is needed for its propagation, and it has been found that the terminal laminæ of non-conductors between two charged plates become penetrated with opposite electricities, which indicates the slow progress of conduction. The molecules of conductors and non-conductors, therefore, have the same power of mutual discharge, but in very different degrees, so that a good non-conductor may be regarded as an excessively slow conductor.

Potential, Density, Tension, Capacity.-Some idea of the meaning of the word potential may be got from the following comparison. Suppose we have a supply of water with a certain head, to fill an elastic bag: when the water is admitted, the bag will swell till the elasticity of the bag is equal to the head of water, and then the flow will cease. The potential is the head of water or elasticity of the bag, so many feet high, or so many pounds per square inch. The capacity of the bag is usually the amount it holds, but capacity in an elastic bag is a shifting quantity, and we must use the term in this wav

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if we wish to compare the capacity of two elastic bags-viz., the ratio of the water it holds to the head that filled it. Thus, a bag holding 10 galls., with a head 1 ft., would have a 10 times greater capacity than a bag holding 10 galls. with a head 10 ft.; for if the first were pressed by a head of 10 ft., it would hold 100 galls., the resistance of the bag being supposed to increase with its contents. Now, let us take a somewhat similar electric problem. An insulated ball is connected with a magazine of energy, ready to make E. flow when occasion offers, such as a galvanic battery. Let the pole of a gigantic battery be connected with the ball, and the other pole with the ground, E. will How to the ball till the air between the ball and the ground presents an electric reaction equal to the potential of the battery. The charge of the ball taken with reference to this potential gives the capacity of the ball. So much, then, for a popular view of these two words. the amount of work that would be expended in bringing a small quantity, a unit of + E., The potential of a body, or any point in the field, is defined thus-viz., from an infinite distance to the body or point. If the body is positive, the work would be expended; if negative, the work would be done on the body and the potential said unit of E. will always move from a point where the potential is high to one where it is lower; in other words, E. will always flow between two points where there is a difference of potential, and will cease to flow when that difference ceases. The charge, V the potential, C the capacity, then C E÷V. From the definition of potential just given, what we have called the potential of the battery in the preceding illustration is in reality its electro-motive force, or the difference of potentials of its poles. If E be As these are alike in power, but different in sign, and as the difference of two quantities. of unlike sign is their sum, the electro-motive force is twice the potential of one pole. If the charging line be withdrawn, the ball will be in all respects as if charged by an electric machine. The battery having, so long as it acts, an unlimited supply of E., its electro-motive force remains the same; but when balls charge one another, the potential falls just as when a limited supply of water has its head reduced when made to run into another vessel. Potential, then, must be estimated by the resistance of the field, or the work value of the unit of charge. The charge being the same, the potential rises with the smallness of the body, or the thickness of the dielectric. electricity on a unit of surface, and tension is the strain which Faraday supposes to exist in the molecules of a dielectric when charged. Tension is commonly used in this counDensity is the quantity of try and abroad for potential, though our best writers never use it now in this sense.

Distribution of Electricity.-We might take it almost as a self-evident truth, that the greater the surface over which E. is diffused, the less is its electric potential at any particular point, and so we are taught by experiment. When two equal balls are insulated, and a charge is given to one of them, and then communicated to the other by contact with the first, it is found that both equally divide the charge, but that the potential of the E. of each is one half of that of the originally charged ball. When a watch guardchain is charged and laid on the plate of an electroscope by means of a glass rod, the gold leaves diverge most when the chain lies in a heap on the plate; and as it is lifted up, the leaves approach each other, showing that as the exposed surface of the chain increases, the electric potential of each part diminishes. The reason of this is obvious. Let us begin with one ball with a certain charge, then take another equal ball and impart half the charge to it by making the two touch. A spark will be seen at the charge of the second ball. The quantity in both is still the same, but energy has been lost by the spark, and the heat generated by the spark is the measure of the loss. If we continue to add ball after ball until we have a very large surface, the quantity is the same as at first, but energy has been squandered in the sparks of each additional ball, and so the potential is lowered.

Experiment teaches us, that E. is exhibited only on the surfaces of conductors. A brass ball is suspended by a silk thread, and covered with two hemispheres, which can be held by insulating handles, and which exactly fit it. A charge is then communicated to the ball so compounded. When the hemispheres are withdrawn, they are found to take away all the E. with them, not the slightest charge being left in the ball. The same fact is exhibited by a hollow ball placed on a glass pillar, with a hole in the toplarge enough to admit a proof plane to the inside. When charged, not the faintest evidence of E. is found on the inner surface, however thin the material of the ball may be. The thinnest metal plate, when under induction, shows opposite electricities on its twofaces. We learn from these and numerous other experiments, that electricity is only found on the outer surfaces of conductors in an envelope of inappreciable thickness. This fact is quite in keeping with Faraday's theory of the action of dielectrics. Within a conducting body we cannot expect E., for the moment it appears in it, the particles communicate their electricities to each other, and the electric state ceases. tric they cannot communicate, and the charge remains. Hence the charge at theconductor only appears at the junction of a conductor and dielectric. In a dielec

We are also taught by experiment that the distribution of E. on the surface of insulated conductors is influenced materially by their form. An electrified ball, for example, exhibits the same density all round, for the resistance is sensibly the same on allsides of it. When, however, a conducting body is made to approach near enough to it, the density of the E. is found to be greater on the side on which the approach is made. This is proved by the aid of a proof plane and an electrometer. drawing away the proof planes from the charged body, its potential, as tested by the When work is done in

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electrometer, is proportional to the density of the charge at the point where it touched The reason of this unequal distribution is obvious, from the fact that the potential of the ball must be the same at every point. If, therefore, the resistance at one side be less than at another, the density there must be greater to maintain equality of potential. The disturbance of equal distribution here spoken of holds true only for short distances; the disturbing body, for instance, in the case under consideration, has to be brought very near before any inequality in the distribution of E. on the ball becomes manifest. It is to this concentration of E. on the side of the approaching conductor that we owe the electric spark; and it is as we near the striking or sparking distance that this disturb ance is revealed. The concentration or fixing of E. on the side of the thinnest and best dielectric, is particularly illustrated in the condenser (q. v.) and Leyden jar, whose action depends upon it; but in these the dielectric must be very thin to secure decided effect. When a conductor somewhat in the form of a prolate spheroid is charged, and the electric density of the several parts tested by the proof plane, it is found to be least at the thickest part, and to increase towards either end; and the difference is found to be all the greater as each end becomes more and more pointed. It is found likewise that the electric density on a point is so great with a considerable charge as to destroy the dielectric condition of the air, the particles of which become electrified, and carry by convection the charge of the point to surrounding conductors. We therefore learn that E. concentrates on points and projections. A similar reasoning with regard to the relations of potential resistance and consequent density bears here as in the previous case. may be here remarked that the density of charge at any point regulates the amount of tension at that point on the molecules of the dielectric. The constraint which they experience in being charged, and which Faraday calls tension, can only be carried to a certain limit. When that is reached, the molecules are forced to be conducting, and the tension ceases.

It

Electrometers and Electroscopes.-These words are generally taken as synonymous; electroscopes, however, should be applied to the instruments which give evidence of electrical potential without giving the exact measure of it; and electrometers to such as show both. Of late years, immense progress has been made in the construction of delicate electrometers, chiefly to meet the demands for such in the working or testing of submarine cables. Sir William Thomson's quadrant electrometer and his absolute electrometer, in point of exactness and delicacy, are a hundred-fold in advance of previous instruments. We shall here, meanwhile, describe the common forms of electric indicators. The quadrant electrometer consists of a conducting-rod, generally of box-wood or brass, with a graduated semicircle attached above, in the center of which is a pivot for the rotation of a straw carrying a pith-ball at its outer end. It is used for a charge of high potential, such as that of the electric machine. When placed on the prime conductor of the machine, the whole becomes charged with E., and the ball is. repelled first by the E. of the rod, and then by that of the prime conductor, the height to which it rises being seen on the semicircle. This is not an electrometer in the strict sense of the word, for although it tells us, by the straw rising and falling, when one potential is greater or less than another, it does not tell us by how much, the conditions of its repulsion being too complicated for simple mathematical expression. It can show us, however, by the indicator standing at the same point, when the electric potential of the machine is the same at one time as another.

The gold-leaf electroscope is a handy instrument for estimating roughly medium potentials. In one of the best of its forms a glass ball, about 4 in. in diameter, rests on a brass tripod, and its neck, about 1 in. in diameter, is inclosed by a brass collar fixed with shellac. A brass plate, with a hole of 4th of an in. in diameter in the middle of it, can be screwed air-tight into the collar. Before it is so fitted, a brass rod, 4th of an in. in diameter, is fixed by shellac or sealing-wax into the hole in the middle, so as to be perfectly insulated from it. The upper end of the rod ends in a brass ball, and the lower end is filled on each side, to allow of two strips of gold-leaf, 1 in. in length, being attached to it. Before the plate and leaves are finally fixed, the interior of the ball is thoroughly dried, by passing hot dry air into it, so that the ball contains no moisture to carry away the charge of the leaves. When the plate is screwed to the collar, there is no communication between the included and external air. The insulation of the leaves is complete; and they keep their charge, in dry weather, for hours together. When the instrument is used, it may be charged directly, by contact being established with the ball and the body whose E. we would examine, or a charge may be carried to it by the proof plane, when the leaves diverge according to the charge communicated. When we would ascertain simply the kind of E. with which a body is charged, we proceed in the following way: A glass tube is rubbed, and brought into the neigh, borhood of the brass knob; the leaves diverge by induction, and, when so diverged, the knob is touched with the finger, and the leaves fall to their original position, for they are then out of the line of action. In this state, E. is fixed by the action of the +E. of the tube on the side of the knob next it, and the corresponding + E. is lost in the ground. When the finger is removed, the + E. is cut off, while the E. remains in the knob; and its presence is manifested by the leaves diverging permanently after the removal of the tube. If, now, a positively electrified body be brought near the knob, it draws away the E. from the leaves, and they consequently fall in; but if a negatively electrified body be brought near, it sends the E. more to the leaves, so that they

Electricity.

diverge further. We are thus enabled to distinguish between a + and a charge. But it may be asked, why not charge the electrometer immediately with the glass? There are two difficulties in the way of this. If the glass is powerfully electrified, it gives too great a charge; and if feebly, contact between the knob and the glass cannot be effected, although its E. acts powerfully by induction. We therefore bring the glass rod near the electrometer, and when the leaves diverge sufficiently, we touch the knob with the finger, and withdraw first the finger, then the rod, and the leaves diverge as before. For the more delicate use of the gold-leaf electroscope, see CONDENSER.

Coulomb's Torsion Balance has played an important part in examining the laws of electric forces. A glass canister is placed on a wooden frame, and is covered above by a plate of glass or wood; in the middle of this plate a round hole is cut, over which is fixed, by wooden fittings, a long glass tube having the graduated rim of a circle attached at its upper end. A circular plate, resting on this rim, closes the upper end of the tube; and when it is turned round, a mark upon it tells the number of degrees through which it has been moved. A cocoon thread or very fine wire is tied to a hook in the center of the lower side of this plate, and thence descends to the body of the canister. It carries below a collar of paper or other light material, in which a needle of shell-lac is adjusted having a disk of gilt paper placed vertically, or a gilt pith-ball at its one end, and a counterpoise at its other. When the plate above is moved through any number of degrees, the needle below, impelled by the torsion of the thread, comes to rest at the same number on the scale below. This last consists of a strip of paper divided into degrees, pasted round the cylinder at the same height as the needle. In the cover of the canister there is another opening, for the admission of a ball insulated at the end of a rod of shell-lac, and which, when supported by the cover, is on a level with the paper disk of the needle. When the instrument is adjusted for observation, the mark on the upper plate and the paper disk stand each at the zero-points of their respective scales, there being of course no torsion in the thread. The ball is removed, to receive a charge from the body under investigation, and is then placed in the cylinder, when the disk is first attracted, then repelled. Suppose that the disk be driven 40°, as shown by the lower scale, from the ball, and that the upper plate has to be moved in the opposite direction, through 160° of the upper scale, to bring it back to 10°, the total degree of torsion is 160° + 40° =200°. If the ball and disk be now discharged, and another charge be given to the ball, which requires 250° of torsion to place the disk at 10°, we have the relation 200 to 250, as that of the repulsive forces of the two charges, for the amount of torsion in degrees is proportional to the twisting force. Without entering further into detail, we may state the two laws that Coulomb established by this instrument: The intensities of the mutual repulsion or attraction of two invariable quantities of electricity of the same or different names, are in the inverse ratio of the squares of the distance at which these act. The intensities of the total repulsive or attractive action of two electrified bodies placed at an invariable distance, are proportional to the products of their electric charges.

Electric Machine.-In the tube of glass and silk rubber of which we have made frequent mention, we have the embryo of the electric machine, viz., a body which, when rubbed, is positively electrified, and its rubber negatively. The first requisite we should expect in a machine of this nature is a large surface, to give a great amount of electricity. But there is another already casually referred to: glass being a non-conductor, the E. formed on its surface has not a combined action, so that some arrangement is necessary

to collect it, and render it available-to act, in fact, as its conducting reservoir. This portion of the machine is denominated the prime conductor. The rubbed surface of the electric machines is either a cylinder or plate of glass; hence we distinguish them into cylinder machines and plate machines. The former, from their more compact form, are the more manageable; and the latter, from both sides of the glass plate being rubbed, are the more powerful forms of the instrument. The description of Winter's plate machine (fig. 1) will be quite sufficient to show the general requirements and construction of electric machines. It is one of the best existing forms of the machine. The glass plate is turned on the axis ab by means of the handle c. The longer end of this axis, consisting of a glass rod, moves in the wooden pillar d, and the other rests in the wooden head of the glass pillar e. The plate is thus completely insulated, and little loss of its E. can take place through its supports. The two rubbers are triangular pieces of wood, covered with a padding of one or two layers of flannel, inclosed in leather, and they present a flat hard surface to the glass, so that friction between it and them takes place in every part. They are placed in a wooden frame on each side of the plate, and the

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pressure is regulated by metal springs, fixed to the outside between them and the frame. Before use they are covered with an amalgam of mercury, zinc, and tin, which is made to adhere with the aid of a little grease, and which increases immensely the production of electricity. The surfaces of the rubbers are therefore conducting, and are made to communicate by strips of tinfoil with the negative conductor, f (fig. 1). To limit the electric field in the neighborhood of the negative conductor, or, which is the same thing, to keep the potential of the glass from rising too high, so as to cause a discharge back into the rubbers, each rubber has a non-conducting wing fastened to it, which is made of several sheets of oiled silk, kept together by shellac varnish, beginning at the rubber with several, and ending with one or two sheets. When the machine is in action, electrical attraction makes them adhere to the plate; but when it is out of action, they may be kept up by a split pin g. As the plate turns, the rubbers are kept in the frame by their 'ledges h. The whole frame-work of the rubbers and negative conductor is supported by the short glass pillar i, so that it can be insulated when required. The prime conductor, k, is a brass ball insulated on the long glass pillar 1, and to prevent the edges of the ball at the junction dissipating the E., the pillar enters the ball by a trumpet-shaped opening. The collection of the E. from the glass is made by a row of points placed in the grooves, inside of two wooden rings, m, m, which are attached on each side of the plate to a piece of brass projecting horizontally from the ball of the conductor. The grooves are covered with tinfoil, which conveys the collected E. to the ball, and the points are kept out of the way of injury by not projecting beyond the grooves.

There are four openings in the prime conductor: the lower one for the head of the supporting pillar; the one at the right for the attachment of the collecting apparatus; the one at the left for the stalk of a small brass ball; and the upper one for admitting the lower end of a large wooden ring, removable at pleasure. This last forms the peculiar feature of Winter's machine. It consists of a bent iron wire carefully covered all round with polished wood, and communicating by a brass pin at the foot of the stalk, on which it stands with the prime conductor. To receive the sparks from the machine, an appendage termed the spark-drawer is provided. This consists of a wooden pillar of the same height as the prime conductor, in the head of which a brass rod slides, with a large flat ball at the one end and a small ball at the other. All the fittings of the machine are of wood, no metal being used but for the prime and negative conductors. On using the machine, it is first necessary to connect the negative conductor by a wire or chain with the ground. As the plate is turned, E. is developed on the rubbers, and led to the negative conductor; and + E. is formed on the glass, which is collected by the points, and transferred to the prime conductor. If the negative conductor be insulated, the electric field would be limited to the space between the negative and prime conductors; but when uninsulated, the floor and walls of the room form part of it, and the field now lies between the prime conductor and any surrounding object. If E. is wanted, the negative conductor is insulated, and the prime conductor connected with the ground, when sparks of E. are given off by the negative conductor.

The various forms of electric discharge through the air, or, as it is termed, disruptive discharge, can be well seen with Winter's machine. The negative conductor being connected with the ground, with a two-foot plate, we may observe them in the following order. On turning the plate once or twice, a faint snapping sound is heard, and, when the room is darkened, a flickering spark is seen to be thrown out from the two-inch ball projecting from the prime conductor, which has the form of a bush, without leaves, with trunk, branches, and twigs, about 10 in. in height. This is one form of what is called the brush discharge. Its general direction is horizontal, or not much inclined from it, but it turns to the hand or other flat conductor brought near it. If it be received on a ball, its various branches concentrate on it. If the brush proceed from the end of a brass rod, instead of from a ball, it becomes very much diminished in size, and resembles a brush of feathers. The brush discharge, though apparently continuous, has been found by Wheatstone to consist of a series of successive brushes. When discharge is effected from a point, a star or glow of light marks its termination, while strong currents of air proceed from it, which are strong enough to blow away the flame of a candle. These currents accompany more or less the various forms of the brush discharge. The particles of air thus carry away the charge from a point to surrounding conductors, and hence a point is said to discharge itself by convection. If we connect the brass rod of the spark-drawer with the ground, or the negative conductor, and bring the flat ball opposite to the small ball on the prime conductor, straight brilliant sparks pass between them so long as the distance does not much exceed 2 inches. Beyond that distance, the sparks become somewhat crooked, and at about 4 in. the discharge begins to take the form of a brush. If, now, the ring be placed in the conductor, the sparks again pass with readiness, and the brush does not again take place till the ball of the spark-drawer is 11 or 12 in. off. The long sparks thus obtained with the aid of the ring are decidedly crooked or forked, with strongly marked lateral branches, which become all the more marked as they lengthen. It would thus seem that the spark has a tendency to break up into branches. When the striking distance is small, this is not perceptible; it is then straight and undivided. As the distance increases, it is crooked, with well-marked offshoots; and when the distance is too great, it splits up entirely into a bush or brush.