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cushion about a quarter of an inch from its upper edge. It extends over the upper surface of the glass cylinder to within an inch of a row of metallic points, proceeding, like the teeth of a rake, from a horizontal rod, which is fixed to the adjacent side of the opposite conductor. The motion of the cylinder, which is given by a single handle or by a multiplying wheel, must always be given in the direction of the silk flap. That part of the cushion which comes in contact with the glass cylinder, should be coated with an amalgam of tin, zinc and mercury, applied by means of hog's lard. The amalgam should be placed uniformly over the cushion, until level with the line formed by the seam which joins the silk flap to the face of the cushion. No amalgam should be placed over this line, nor on the silk flap; and it is even requisite to wipe the silk flap clean whenever the continued motion of the machine should have soiled it by depositing dust or amalgam on its surface. The best amalgam is formed by melting together one ounce of tin and two ounces of zinc, which are to be mixed, while fluid, with six ounces of mercury, and agitated in an iron or thick wooden box until cold. It is then to be reduced to very fine powder in a mortar, and mixed with a sufficient quantity of hog's lard to form it into a paste. The mode in which the electrical machine just described acts, will readily be understood. The friction of the cushion against the glass cylinder produces a transfer of electric fluid from the former to the latter; that is, the cushion becomes negatively and the glass positively electrified. The fluid, which thus adheres to the glass, is carried round by the revolution of the cylinder, and its escape is at first prevented by the silk flap which covers the cylinder, until it comes to the immediate vicinity of the metallic points, which, being placed at a small distance from the cylinder, absorb nearly the whole of the electricity as it passes near them, and transfers it to the prime conductor. Positive electricity is thus accumulated in the prime conductor, while the conductor connected with the cushion, being deprived of this electricity, is negatively electrified. If both these conductors are insulated, this action will soon have reached its limit; for when the cushion and its conductor have been exhausted of their fluid to a certain degree, they cannot, by the same force of excitation, supply any further quantity to the glass. In order to enable it to do so, we must replenish it, or restore to it a quantity equal to what it
has lost. This is done by destroying tire insulation of the cushion through the means of a metallic chain or wire, extending from it to the earth, which is the great reservoir of the electric fluid. The prime conductor will now be supplied with a constant stream of positive electricity. If it be our object, on the other hand, to accumulate negative electricity by the same instrument, we have only to insulate the conductor to which the cushion is attached, and to connect the prime conductor with the ground, in order to allow the fluid to escape from it as soon as it is collected from the cylinder. The fluid will thus continue to be drawn, without interruption, from the negative conductor, as it now meets with no impediment to its discharge on the opposite side of the machine. That the quantity' of positive electricity produced in one conductor is exactly equal to that of the negative electricity in the other, is proved by the fact, that, if the two conductors are connected by a wire, no signs of electricity are obtained in any of the conductors on turning the machine. A person standing on a stool with glass legs is thereby insulated; and if, in this situation, he touch the prime conductor, either with his hand or through the medium of a metallic rod or chain, ho may be considered as forming part of the. same system of conductors. When the machine is worked, therefore, he will partake, with the conductor, of its charge of electricity, and sparks may be drawn from any part of his body by the knuckle of any other person who is in communication with the ground.
IV. The effects of electrical attraction and repulsion may nowr be exhibited much more distinctly with the aid of those considerable accumulations of electricity which we are enabled to form by the electrical machine. A pith ball, or a fragment of gold leaf, is very strongly and immediately attracted by the electrified conductor; and the instant after it has come into contact with it, it is repelled; but ir is now attracted by the other bodies in ite< neighborhood, to which it communicatee its own electricity, and then is again in a state to be influenced by the conductor, and to be again attracted; and this alternation of effects will continue as long as the conductor remains charged. This alternation of attractions and repulsions accompanying the transferring electricity by movable conductors, is also illustrated by the motions of a ball suspended by a silk thread, and placed between twTo bells, of which the one is electrified, and the other
communicates with the ground. The alternate motion of the ball between the two bells will keep up a continual ringing. This amusing experiment has been applied to give notice of changes taking place in the electrical state of the atmosphere. The mutual repulsion of bodies that are similarly electrified gives rise to many interesting experiments. A small figure in the shape of a human head covered with hair, when placed upon the conductor and electrified, will exhibit the appearance of terror from the bristling up and divergence of the hair. Advantage is taken of the repulsive property of electrified bodies for the construction of an instrument adapted to measure the intensity of the electricity they may contain. This instrument is called an electrometer. That invented by Henley consists of a slender rod of very light wood, serving as an index, terminated by a small pith ball, and suspended from the upper part of a stem of wood, which is fitted to a hole in the upper surface of the conductor. An ivory semicircle or quadrant is affixed to the stem, having its centre coinciding with the axis of motion of the rod, for the purpose of measuring the angle of deviation from the perpendicular, which the repulsion of the ball from the stem produces in the movable rod. The number of degrees which is described by the index affords some evidence of the quantity of electricity with which the apparatus is charged, though the instrument cannot be viewed as affording an exact measure of its intensity. The gold leaf electrometer of Bennet, or rather electroscope, which is one of the most delicate instruments ever invented for detecting the presence of electricity, consists of two narrow slips of gold leaf suspended parallel to each other, in a glass cylinder (which secures them from disturbance by the air), and attached to the end of a small metallic tube, terminating above either in a flat surface of metal or a metallic ball. Two slips of tinfoil are pasted to the inside of the cylinder, on opposite sides, in a vertical position, and so placed as that the gold leaves may come in, contact with these, when their mutual repulsion is sufficiently powerful to make them diverge to that extent. These slips of tin-foil terminate in the foot of the instrument, and thus are in communication with the earth. A very minute charge of electricity, communicated to the upper end of the tube, is immediately transmitted to the gold leaves, which are thus made to repel each other; but if the repulsion is such as to make them
strike against the tin-foil, their insulation ceases, and their electricity is carried off, and becoming neutral, they resume their original position. The most perfect electrometer, however, is that invented by Coulomb, and called by him the torsion balance. It consists of a cylindrical glass jar, covered at the top by a circular glass plate, with a hole in its centre, through which descends nearly to the bottom of the jar, a single fibre of the web of the silkworm, with a needle of gum-lac or a piece of straw coated by sealing-wax, affixed to its lower extremity. The needle is terminated at one end by a small pith ball, and at the other by a disc of varnished paper, to serve as a counterpoise to the ball. The upper end of the silk fibre is attached to a kind of button, having a small index, and capable of being turned round upon a circular plate divided into degrees. One side of the jar is perforated towards its bottom to allow of the insertion of a short horizontal bar, having a small metallic sphere at each of its ends, the one being within, and the other upon the outside of the jar; and the former being so situated as just to allow the ball of the suspended needle to come in contact with it in the course of its revolution. By turning the button or the index, the needle may be brought into this or any other required position with regard to the ball. It is found by experiment, that the angle of torsion of the silk fibre is, within a certain range of distance, very nearly in the direct ratio of the force which acts in producing the torsion; and, therefore, if the two balls be placed in contact by turning the button, and then similarly electrified, the distance to which they are repelled by the angular motion of the suspended ball affords a measure of the repulsive force exerted. In like manner, the distance which the suspended ball is made to move, when it is attracted by the fixed ball, when the two have opposite electricities, gives accurate measures of the attractive forces.
V. It had long been observed, that the quantity of electricity which bodies are capable of receiving, does not follow the proportion of their bulk, but depends chiefly upon the extent of their surface. It was found, for example, that a metallic conductor, in the form of a globe or cylinder, contains just as much electricity when hollow, as it does when solid; from which it was inferred, that electricity does not extend throughout the mass of a body, but resides altogether at its surface. By the application of mathematical calcula
tions to the theory, the most exact information with regard to the distribution of the electric fluid in bodies of different shapes has been obtained; and whenever* a comparison has been instituted, even in the cases of the most complicated kind, between the results of experiment and of theory, the most perfect agreement has been observed between them. For the purpose of measuring the proportional quantities of electricity, with which different parts of the same or of different bodies are charged, no instrument is so well fitted as the balance of Coulomb. Such is its extreme sensibility, that a force only equal to the 270th of a grain is sufficient to make the needle perform an entire revolution; the 360th part of this force, therefore, or less than the 100,000th of a grain, is capable of being estimated by each degree of its angular motion. It would be inconsistent with the limits of the present article to go into a detail of the delicate methods of research adopted in the investigation of this subject. The following are among some of the most interesting results deduced from them. In a solid body, having the form of a perfect sphere, and charged with positive electricity, the whole of the fluid is, in consequence of the repulsion of its own particles, which is every where directed from the centre outwards, accumulated in a thin stratum, at the very surface of the sphere. If the body be charged with negative electricity, the deficiency of fluid will take place only in the superficial stratum of matter. If, instead of being spherical, the body have any other form, the electricity will be chiefly confined to the surface; and if it have an elongated form, there will be a greater charge in the remoter parts than in those nearer to the middle. This result of theory, respecting the limitation of electricity to the mere surface, is confirmed, in the most decisive manner, by the experiments of Coulomb. A conducting body, of a spheroidal shape, with small pits in various parts of its surface, half an inch in diameter, and one tenth of an inch in depth, was electrified, and examined by the torsion balance. The bottoms of these pits afforded no indications of having received any electricity, while the even surface exhibited strong electrical excitement. We may conclude, both from theory and experiment, therefore, that although, strictly speaking, the electricity must reside, within the substance of conducting bodies, it extends, in fact, to a depth so small as to be inappreciable by
any known methods of observation. The effect of an expansion of surface, in lessening the intensity of electricity, while its absolute quantity remains the same, is illustrated by the following experiment: around an insulated cylinder, movable on a horizontal axis, and turned by an insulating handle, is wound a thin lamina of any metal, the end of which is semicircular, and has attached to it a silk thread. The whole apparatus communicates with an electrometer, formed of two linen threads, each terminating in a pith ball. On communicating a charge of electricity to the cylinder, the threads and balls of the electrometer attached to it, diverge. Upon taking hold of the silk thread, and unrolling the metallic lamina from the cylinder, the balls gradually collapse, thus indicating a diminution in the intensity of electrical repulsion. But, on winding up the lamina, by turning the insulating handle, the electricity is restored, and the balls diverge to the same extent as before, allowance being made for the small dissipation of electricity, from the contact of the air during the experiment. In the case of a long and slender lamina of conducting matter, charged with electricity, Coulomb found that its intensity continued nearly uniform, from the middle of the lamina to within a short distance from the ends; at that part it rapidly increased; and at the very extremity, it became twice as much as at the middle part. He also found, that in a cylinder 30 inches long and 2 in diameter, the intensity of the electricity at the ends was to its intensity in the middle, or at any part more than 2 inches from the extremity, as 2.3 to 1. From which instances we infer, that if a conducting substance be drawn out into a point, the intensity of the electricity at that point will be exceedingly great; and that the point will, accordingly, absorb and draw into itself nearly the whole of the electricity that is contained in the body. This great concentration of electricity is found actually to take place in all points that project beyond the general surface. The pressure excited by the electric fluid against a non-conducting medium, such as the air, which opposes an obstacle to its escape, is in a ratio compounded of the repulsive force of its own particles at the surface of the stratum of fluid, and of the thickness of that stratum; but as one of these elements is always proportional to the other, the total pressure must, in every point, be proportional to the square of the thickness. If this pressure be lees
than the resistance, or coercive force, as it has been called, of the air, the electricity is retained ; but the moment it exceeds that force, in any one point, the electricity suddenly escapes, just as a fluid confined in a vessel would rush out, if it were to burst open a hole in the side of the vessel. The irruption of the electric fluid is marked by several very striking phenomena. A sharp snap is heard, accompanied by a vivid spark, and there are evidences of an intense heat being evolved in the line which the electricity takes. Its passage through a perfect conductor is unattended with light. Light appears only where there are obstacles in its path, by the interposition of imperfect conductors; and such is the velocity with which it is transmitted, that the sparks appear to take place at the very same instant, along the whole line of its course. Thus, if a row of small fragments of tin-foil be pasted so as to be nearly in contact, on a piece of glass, and electricity be sent through them, by connecting one of its ends with the conductor of an electrical machine* while the other end communicates with the ground, it will not be possible to detect any difference of time in the occurrence of the light in the different parts. If the tin-foil be arranged so as to represent a chain, it will appear luminous at each link, while conveying a charge of electricity. The longest and most vivid sparks are obtained between two conductors having a rounded form, as may be exemplified in a common electrical machine, by presenting a metallic ball to that side of the prime conductor which is furthest from the cylinder of the machine; a spark is immediately seen, of considerable length, resembling a long streak of fire, extending from the conductor to the ball. Often, when the spark is very long, it is seen to have an angular or zigzag course, exactly like that of a flash of lightning. This irregularity is probably occasioned by the fluid darting obliquely in its course to minute conducting particles, as those of moisture, that are floating in the air, a little removed from the direct line of passage. Electrical light differs in no respect from the light obtained from other sources. Its brilliancy depends upon the conducting power of the bodies between which it passes. When dry wood is employed, it appeare in the form of faint red streams; but metals afford a light of greater brilliancy. Its color is subject to variations, from a great number of different circumstances. Sparks passed through balls of
wood or ivory are of a crimson color; but this depends upon their position with regard to the surface. Electric sparks, passing from one polished metallic surface to another, are white; but if the finger be presented to an electrified conductor, the sparks obtained are violet. They are green, when taken from the surface of silvered leather; yellow, when taken from finely powdered charcoal; and of a purple color, when taken from the greater number of imperfect conductors. In exceedingly rarefied air, the color of the spark is green; in denser air, it acquires a blue tint, and passes to a violet and purple as the condensation of the air is increased. In making these experiments, it is found that in proportion as the medium is more rare, its conducting power increases, and a smaller intensity of electricity is required for the production of light. In the ordinary vacuum of the air-pump, the passage of electricity is rendered sensible by streams or columns of diffused light occasionally varying in their breadth and intensity, and exhibiting movements which give them a marked resemblance to the coruscations of the aurora borealis. It was at first imagined, that the light which appears during the passage of electricity was actually the electric fluid itself, become luminous from its high degree of accumulation. But, since we know that common atmospheric air becomes luminous by violent compression, and we must also presume that electricity exerts a very sudden and powerful pressure upon the air, by its passage through that resisting medium, we are certainly justified in drawing the inference, that the same phenomena proceed, in both cases, from the same cause. The sound, which accompanies the various modes of transferrence, is subject to modifications dependent upon the degree and suddenness of the impulses given to the air. The full, short and undivided spark is attended with a loud explosion; the more lengthened spark, with a sharper snap, which becomes more broken and rattling in proportion to the distance it has to traverse. The great increase of intensity which the electric fluid acquires at the extremities of all elongated conducting bodies, and especially the indefinite augmentation of this intensity at the apex of all projecting points, has been alluded to above. This intensity will necessarily be accompanied with a powerful disposition in the fluid to escape—a circumstance which furnishes a natural and exact explanation of the rapid dissipation of electricity, which
takes place from all bodies of a slender and pointed form. The illustration of these positions is seen in bringing metallic rods of different forms near the prime conductor of a machine charged with either species of electricit}', the conductor being furnished with a pair of pith balls, suspended by a fine wire, whose divergence indicates the presence and degree of the electricity in the conductor: if the metallic rod have a bell at the end which is brought near the charged conductor, the pith balls will be but slightly affected; whereas, if it terminate in a sharp point, and the point be presented to the conductor at the same distance as the ball was in the former case, the divergence of the balls will immediately cease, showing that the electrical charge has wholly disappeared. Currents of air always accompany the discharge of electricity from pointed bodies; for each particle of air, as soon as it has received its electricity from the point, is immediately repelled by the body. Many amusing experiments are founded on this principle. Let two cross wires, bent at right angles near the ends, which terminate in points, and pointing in a similar direction with respect to the axis, be supported at their centre upon a fine point, and electrified by being placed upon the prime conductor of a machine; each of the points will give off a stream of electricity, and the wires will revolve backward with considerable rapidity. An apparatus consisting of wires terminating in points, and having balls annexed to them, to represent the planets, may be constructed so as to revolve when electrified, and thus to imitate the planetary motions. Such an apparatus has been called an electrical orrery. When the transfer of electricity takes place between smooth surfaces of a certain extent, no difference can be perceived in the nature and appearance of the spark, whichever be the position of the negative surface. But in the passage of electricity through points, the effect is considerably modified by the species of electricity with which the bodies are charged; or, in other words, by the direction in which the fluid moves. When the electric fluid is escaping out of a pointed conductor, the luminous appearance is that of diverging streams, forming what is termed a pencil of light, and resembling the filaments of a brush. When, on the contrary, the electric fluid is entering into the pointed body, the light is much more concentrated at the point itself, having a resemblance to a star, in
which, if any streams appear, they are disposed like radii, and equally so in all directions. This difference in these two appearances may be employed, on many occasions, as a useful criterion of die species of electricity, at least, which is passing from one conductor to another, if not of the absolute direction of its motion. For if a needle be presented to an electrified body, the appearance of a star on the needle will show that the electricity of that body is positive; while, on the contrary, a luminous brush on the needle will indicate diat the body is negative. These observations seem to indicate the emanation of some material fluid from the positive, and its reception by the negative point It has, accordingly, been urged, as an argument in favor of the Franklinian theory. The diverging lines on one side, and their inflections on the other, represent exactly the paths of particles flowing out as from a ] >1; >e, and urged forward by a force whkh gives them such a projectile velocity as to prevent their spreading out beyond a certain distance from the direct line of projection. But this very velocity will carry the particles, that happen to have deviated most, somewhat beyond the point to which they are attracted; while the attraction to this latter point will tend to deflect diem from the line of their path, and gradually turn them back, so that they will arrive at the point of attraction by very different paths, and some even by a retrograde motion. Hence, while, in the first case, they form a diverging cone of rays, in die latter they must be distributed on all sides of the point, like the rays of a star.
VI. Active electricity, existing in any substance, tends always to induce the opposite electrical state in the bodies that are near it Now, it is impossible to induce one electrical state in any body, without, at the same time, producing the opposite state in the same body, or in die one which is immediately contiguous. It follows, therefore, that if the bodies subjected to the inductive influence are nonconductors, although the tendency to produce the opposite electricity exists, yet, in consequence of the immobility of the fluid, it can produce no visible change. In proportion as the body opposes less resistance to the passage of electricity, the operation of the disturbing force becomes sensible. For example, in the case of a positively charged electric, acting by induction on an insulated conducting body, the redundant fluid in the former must tend to repel all the fluid contained