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overcome the resistance of the air, and the greater or less interval through which the spark passes is called the striking distance. 103. When the surface of the conductor is uniform, the re-action of the air around it is also uniform; but if the surface of the conductor be integular, the tendency of the electric fluid to escape or enter it will be greatest at the most prominent parts, and most of all when these are angular or pointed. To understand this it is only necessary to recollect that every electrified conductor is surrounded by an atmosphere of its own figure, the contiguous surface of which is similarly electrified: and that electricity is not transmitted through air, but by the motion of its particles. 104. For this motion of particles is resisted by a uniform surface from the similar action of the air around it, which is all equally capable of receiving electricity, and cannot tend to distribute it in one direction more than another; the immediate electrical atmosphere of the conductor will be therefore resisted in any attempt to recede from it by a column of air which is equally opposed in every part; but if there be any prominent point on the conductor projecting into the atmosphere, it will facilitate the recession of the electrified particles opposite to it by removing them farther from the electrified surface, and opposing them to a greater number of such as are unelectrified. 105. The action then, of bodies that are pointed orangular, appears to consist in promoting the recession of the particles of electrified air, by protruding a part of the electrical atmosphere of the conductor into a situation more exposed to the action of the ambient unelectrified medium, and thereby producing a current of air from the electrified point to the nearest uninsulating body. Hence the most prominent and the most pointed bodies are such as transmit electricity with the greatest facility, for with them this condition is most perfectly obtained. 105. A spherical surface is that which, considered with regard to its surrounding atmosphere, is most uniform; hence balls, or cylinders, with rounded ends, are naturally employed for insulated conductors, and their magnitude is proportioned to the intensity of the electrical state they are intended to retain; for a point is but a ball of indefinite diameter, and will act as such on very small quantities of electricity; and a ball of moderate size may also be made to act as a point by electrifying it strongly. 107. If two spheres of equal size are connected together by a long wire and electrified, their atmospheres will extend to the same distance, and they will of course have respectively the same intensity; but if the spheres be of unequal size, the atmosphere of the smallest will extend furthest, and it will necessarily have the greatest intensity; so that a longer spark can be drawn from a small ball annexed to the side of a conductor than from the conductor itself, and longer in proportion as the ball projects farther from the side. Hence the finer the point, and the more freely it projects beyond any part of the conductor to which it is annexed, the more rapidly will it receive or transmit electricity.
108. Let, for example, a fine point he fixed in the axis of a large brass ball, from beneath the surface of which it may be protruded more or less by the action of a fine screw, the effect of a ball of any size may be obtained; when beneath the surface of the ball the point does not act, but in proportion as it is protruded it increases the transmitting power, and, if projected far enough, at length entirely overcomes the influence of the ball. 109. The same writer gives the following experiments, among others, for illustrating the influence of the form and extent of the conductor on the appearance of the electric spark. (1) Present a brass ball of about three inches in diameter to the positive conductor of a powerful electrical machine; sparks of brilliant white light will pass between them, accompanied by a loud snapping noise: to produce these sparks in rapid succession the ball must be brought near the conductor, and they then appear perfectly straight. (2.) Annex a ball of an inch and a half or two inches diameter to the conductor, so as to project three or four inches from it; present the large ball to this, and much longer sparks will be obtained than from the conductor itself, but they will be less brilliant and of a zigzag form. (3.) Substitute a small ball for that used in the former experiment; the fluid will now to a greater distance, but in the form of a divided brush of rays, faintly luminous, and producing little noise; this brush will even occur with larger balls, if the machine be very powerful; it is most perfect when procured by presenting a flat imperfect conductor, as a piece of wood or Daper. p (4.) Whilst a current of sparks is passing between a large ball and the conductor, present, at the distance of about an inch and a half, a shar point at double that distance, and the sparks will immediately cease, the electric Platter being silently drawn off by the point. 110. The brilliancy of the electric spark is always in proportion to the conducting power of the bodies between which it passes; hence metals are almost exclusively employed for this purpose, as wood and other imperfect conductors produce only faint red streams; yet these substances act as points with some efficacy, and the particles of dust which collect around the apparatus are often troublesome to electricians from the same cause. 111. The nature and density of the medium through which the electric spark passes has also a powerful influence on its character. Dr. Watson seems to have been the first who made experiments on this subject; these he conducted on a very large scale, and he describes the results as having been very beautiful; they will be noticed in another part of this article. 112. The following is a description of the simple apparatus used by Mr. Singer for showing the effect of different gaseous mediums on the passage of electricity. . It consists of a glass globe, fig. 1, plate II., of about four inches diameter, having two necks capped with brass; to one of the necks a stop-cock is screwed, with a wire and ball projecting into the globe; another ball is attached to a wire that slides through a collar of leathers screwed to the opposite cap, so that the balls may be set at any required distance from each other within the globe. This apparatus may be exhausted by connecting the stop-cock with an air-pump, and various gases may be introduced into it, or the air it contains may be rarefied or condensed, and the effect of these processes on the form of the spark examined. In condensed air the light is white and brilliant; in rarefied air, divided and faint; and in highly rarefied air, of a dilute red or purple color. The effect of gases seems to be proportioned to their density; in carbonic acid gas the spark is white and vivid, in hydrogen gas it is red and faint. 113. The brilliancy of the electric spark seems to be in proportion to the density of the medium through which it is made to pass. This is proved by the following experiments:–1. Fix with cement a short iron or platina wire within one end of a glass tube thirty inches long, so that the wire may project a little way within the tube, and fix a small brass ball on the outer extremity of the wire. Fill the tube with mercury, and at the open end place a drop of ether, which secure by the point of the finger while the tube is inverted in a vessel of mercury, so as to form a Torricellian vacuum in the upper part. The ether will rise to the top; and upon the removal of the finger, and the fall of the mercury, will expand into vapor. If now electricity be transmitted through this vapor, it will be rendered luminous, and assume various hues according to its strength. When the spark is strong, and has to pass through some inches of the expanded vapor, the light is usually of a beautiful green color. 2. Take an air-pump receiver twelve or fourteen inches high, and six or seven inches in ‘liameter; adapt a wire, pointed at its lower extremity, to the top of the receiver, letting the point project about two inches into its inside; place the reeeiver on the plate of the air-pump, and electrify the wire at its top positively; whilst the air remains in the receiver, a brush of light of very limited size only will be seen, but in proportion as the air is withdrawn by the action of the pump it will enlarge, varying its ap|. and becoming more diffused as the air ecomes more rarefied; until at length the whole of the receiver is filled by a beautiful blush of light, changing its color with the intensity of the transmitted electricity. 3. Into a piece of soft deal about three inches long and an inch and a half square, insert two pointed wires obliquely into its surface at nearly an inch and a half distance from each other, and to the depth of an eighth of an inch; the wires should incline in opposite directions, and the track between the ints be in that of the fibres; a spark in passing from one point to another through the wood will assume different colors in proportion as it passes more or less below the surface; and by inserting one point lower than the other, so that the spark may pass obliquely through different depths, all the prismatic colors may be made to appear at once. Sparks taken through balls of wood or ivory appear of a crimson color; those from the surface of silvered leather are of a bright
green; a long spark taken over powdered charcoal is yellow; and the sparks from imperfect conductors have a purple hue. The quantity of air through which these sparks are seen also influences their appearance; for the green spark in the vapor of ether appears white when the eye is placed close to the tube, and reddish when it is viewed from a considerable distance. 114. When metallic conductors are of sufficient size and perfectly continuous, they transmit electricity without any luminous appearance; but if the continuity be interrupted in the slight
est degree aluminous effect is produced, a bright
spark occurring at every separation. Various articles of apparatus are used for the exhibition of this effect, according to the fancy of the operator; the following are those used in general by public lecturers:–1. The spiral tube: this instrument is represented by fig. 2, and is com— posed of two glass tubes CD, one within another, and closed with two knobbed brass caps A and B. The innermost of these has a spiral row of small round pieces of tin-foil stuck upon its outside surface, and lying at about one-thirtieth of an inch from each other. If this instrument be held by one of the extremities, and its other extremity be presented to the prime conductor, every spark that it receives from the prime conductor will cause small sparks to appear between all the round pieces of tin-foil stuck upon the innermost tube; which in the dark affords a beautiful spectacle, the tube appearing encompassed by a spiral line of fire. F. 3 represents several spiral tubes placed round a board, in the middle of which is screwed a glass pillar, and on the top of this pillar is cemented a brass cap with a fine steel point. In this a brass wire turns, having a brass ball at each end, nicely balanced on the wire. To make use of this apparatus, place the middle of the turning wire to: a ball proceeding from the conductor, so that it may receive a succession of sparks from the ball; then push the wire gently round; and the balls in their relative motions will give a spark to each tube, and thereby illuminate them down to the board, which from its brilliancy and rapid motion, affords a most beautiful and pleasing sight. Fig. 4 is another instrument for showing the same effect in a diversified form : the action being in this case the same as in the preceding, no further explanation is necessary. †. beauty of this kind of exhibition is sometimes much increased by laying down the devices on glass stained of different colors. There are other methods of rendering the electric fluid visible in a very pleasing manner, some of which we shall here enumerate. 115. The luminous conductor, as represented at fig. 5, consists of a glass tube about eighteen inches long, and four in diameter, to the ends of which are cemented the hollow brass pieces DF, EB, the former having a point, C, for receiving electricity from the electrical ma– chine, while the other has a wire terminating in a ball, G, from which a strong spark may be drawn. From each piece a knobbed wire proceeds within the cavity of the glass tube. One of these brass pieces is composed of two parts, in one of which is a valve covering a hole by be exhausted of its air. The whole is suppo on two glass pillars fixed in a wooden frame. When this tube is exhausted of its air, and the point C set near the machine, this point will appear illuminated with a star, while the glass tube will exhibit a weak light on its inside; and, from the knobs within the glass, the appearance of positive and negative light will be evident, as the knob at D will show a bright pencil of rays, and the opposite knob a round star. If the point C, instead of being presented to the cylinder, or the positive conductor, be placed near the rubber or negative conductor, the appearance of the light from the internal knobs will be reversed. 116. The visible electrical atmosphere is exhibited by the apparatus represented at fig. 6, where GI represents the receiver with the plate of an air-pump. In the middle of the plate IF a short rod is fixed, having at its top a ball B, whose diameter is nearly two inches. From the top of the receiver another rod AD with a like ball A proceeds, and is cemented air-tight into the neck C; the distance of the balls from one another being about four inches. If, when the receiver is exhausted of air, the ball A be electrified positively, by touching the top D of the rod AD with the prime conductor, or an excited glass tube, a lucid atmosphere appears about it, which, although it consists of a feeble light, is yet very conspicuous, and very well defined; at the same time the ball B has not the least light. The atmosphere does not exist all round the ball A, but reaches from about the lower half of it. If the rod, with the ball A, be electrified negatively, then a lucid atmosphere, like the above described, will appear upon the ball B, reaching from its middle to a small distance beyond that side of it that is towards the ball A; at the same time the negatively electrified ball A remains without any light. 117. Fig. 7 represents a mahogany stand, so constructed as to hold three eggs at a greater or smaller distance, according to the position of the sliding pieces. A chain C is placed at the bottom, in such a manner as to touch the bottom of the egg at B with one end, and with its other the outside coating of a charged jar. The sliding wire A at the top is made to touch the upper egg; and the distance of the eggs asunder should not exceed a quarter or the eighth part of an inch. The electric spark, being made to pass down by means of the discharging rod through the wire and ball at A, will, in a darkened room, render the eggs very luminous.
which the tube .
Accumulation of Electricity.
118. Although the electricity we have already described be sufficient for the performance of many very fine experiments, and for enabling us to investigate the nature and properties of the electric fluid ; yet the full energy of this wonderful agent can only be displayed when it is collected in great quantities, and made to operate on substances in a strongly concentrated state. This, it might be supposed, would be best effected by diffusing the electrical matter over very extensive conductors, and at once discharging the quantity thus accumulated, on the
subject of experiment: but such is not the case, since the extension of the surface of any conducting body diminishes its intensity. This fact is admirably illustrated by Mr. Singer in the following experiment. 119. Insulate a flat metal plate with smooth rounded edges, and connect with it a pith-ball electrometer; electrify the plate, and the balls will diverge: bring a similar plate uninsulated near that which is electrified, keeping ther flat surfaces parallel and opposite to each other; the balls of the electrometer gradually collapse as the plates approach, and, when they are within about half an inch of each other, the insulated plate appears unelectrified; but, on the removal of the uninsulated plate, the original divergence is restored. See fig. 8. 120. When the insulated conductor, he adds, is electrified, its pith-balls separate, because they are in a different electrical state to the air by which they are surrounded, the fluid of which they attract; but all unelectrified bodies have the same relation to the electrified balls like the ambient air, and such as are conductors and con nected with the ground present a more amoe source of matter and electricity; consequently, if such bodies are brought near the electrified conductor, its attraction is exerted on them, and the influence of the surrounding air is proportionably diminished; and if the proximity be sufficient, the attraction of the electrified surface will be so exclusively exerted in that direction as to be imperceptible in any other. 121. In the above experiment the bodies are not brought in contact, but only near each other, and consequently there is no communication or loss of electricity, but merely a compensation of its attractive power; hence, when the uninsulated plate is removed, the divergence of the electrometer is restored. 122. The grand instrument used by electricians for the accumulation of electricity is denominated the Leyden jar, or phial: its construction has already been in some measure described, but it may be necessary still further to explain it, and to make some remarks on the principle of its action. 123. The Leyden Jar in whatever form it may be constructed is nothing more than an electric placed between two non-electrics. The following description of this remarkable instrument is from Mr. Cavallo's treatise on electricity. If, says he, to one side of an electric, sufficiently thin, as for instance a pane of glass, a piece of sealing-wax, &c., be communicated one electricity, and to the opposite side the contrary; that plate in that case is said to be charged; and the two electricities can never come together except a communication of conducting substances be made between both sides, or the electric be broken by the power of electric attraction. When the two electricities of a charged electric are by any means united, and therefore their power destroyed, that electric is then said to be ‘i:charged and the act of union of these two opiosite powers is called the electric discharge. 124. To avoid the difficulty of communicating electricity to an electric plate, it is customary to coat the sides of it with some conducting subID
stance, as tin-foil, gilt o &c., by which means the charging and discharging becomes very easy; for when the electricity is communicated to one part of the coating, it is immediately spread through all the parts of the electric that are in contact with that coating; and, when the electric is to be discharged, it is sufficient to make a conducting communication between the coatings of both sides, to discharge entirely the electricities of that electric. 125. When plates of glass are thus coated, it is of essential importance that the glass should extend two or three inches beyond the metal coatings; for, although they do not absolutel touch one another, yet, when they are . the electricity will easily force a passage through the air, and by passing over the surface of the electric, from one coating to the other, render it incapable of receiving any charge. 126. If a glass plate be properly coated on both sides with a conducting substance, and if to one of these coatings be communicated some electricity, the other coating, while communicating with the earth, or with other conducting bodies, acquires by itself an equal quantity of the contrary electricity; but if, while one side is acquiring electricity, the opposite side does not communicate with the earth, or the conducting substances, the glass cannot be charged. The reason of this is founded on the property of bodies to acquire an electricity, contrary to that possessed by a contiguous electrified body; and the cause that hinders these two electricities from mixing, is the interposition of the glass plate which is impermeable to electricity. Although if the glass be too thin, or the charge too high, the strong attraction, between the positive and negative electricities, forces a passage through the glass and discharges it. 127. The most usual, and by far the most convenient form of the Leyden jar is that represented at fig. 9. It is coated on the inside and also on the outside with tin-foil to within two inches and a half of the top. With the inside coating a wire is connected which rises through a lid of baked wood neatly fitted into the mouth of the jar, and terminating in a smooth brass ball. The uncoated part of the jar must be kept perfectly clean and dry, otherwise the action will be very incomplete. The coating is best fastened on with very strong gum water, but some electricians use common paste; and in some instances the tin-foil is first pasted upon paper, and afterwards on the glass: this is considered an improvement both as it respects the facility of drying the gum or paste, and also the strengthning of the jar. 128. If a jar thus constructed be held in one hand by the lower part, and the knob applied to the prime conductor when the machine is in action, it will become charged in a few seconds; and if then a communication be formed between its outside and inside coatings, by touching the ball with the other hand, a smart explosion takes Place, and a peculiar and painful sensation is felt chiefly at the wrists and elbows, and across the breast: this sensation is called the electric shock, and it may becommunicated to any number of individuals, holding each other by the hand
and forming the line of communication betweet, the coatings of the jar. In this way the abbé Nollet succeeded in giving the shock to 180 of the French Guards in the king's presence. 129. When it is wished to discharge the jar without allowing the charge to pass through the body, an instrumentisused i. the discharging rod, which is composed of a bent wire or two branches, connected by a joint, and furnished with a glass handle. The extremities of the rod or branches are pointed, but have screws, by means of which they are fitted with balls. In discharging a jar with this instrument, it is held by the glass handle, and, while one end is applied to the outer coating of the jar, the other is made to approach the ball of its wire, and thus the electricity passes through the metallic part of the discharger from the one coating to the other of the jar. If the extremities be without their balls the discharge is effected without noise, but otherwise there takes place an explosion, more or less loud according as the jar is more or less charged. The neatest form of the discharging rod is that represented at fig. 10; where AA is the glass handle by which it is held, and BC are the two branches with their balls. 130. When the accumulation of great quantities of electricity is required, the instrument then made use of is termed the electrical battery, and is composed of a number of Leyden jars connected together and placed in an appropriate box. The most modern construction of the battery, is represented at fig. 11. It consists of twelve jars placed in a mahogany box, the bottom of which is covered with tin-foil, for the purpose of connecting together all the outside coatings; the inside coatings being connected together by the wires and balls that rise from their centres, and are united together at the top. On , one side of the box there is a small brass hook A for the purpose of connecting the battery by means of a chain with any substance through which the discharge is to be made; this hook passes through the box and is fixed in contact with the tinfoil which connects the exterior coating of the jars. 131. A battery may also be constructed by a combination of panes of glass }. coated. Dr. Franklin formed a battery of this kind with eleven panes of common window-glass, and with it he made the greater part of his experiments. 132. In whatever form batteries are constructed they are charged and discharged in the same manner as a single jar. If one of the knobs of the battery communicate with the prime conductor of the machine in a state of action, it will soon be charged; and the discharge may be effected by making a communication between the coatings, by means of a discharging rod, or any other conductor. 133. Batteries of great size have been constructed by different electricians, so as to accumulate an enormous quantity of electricity, capable of melting the hardest metals, and of utting an instantaneous termination to animal ife. Dr. Priestley constructed a battery consisting of sixty-four jars, and containing thirty-two square feet of coated surface. Mr. Cuthbertson completed, in 1784, for the Teylerian Museum at Haarlem, a battery of 135 jars and 132 feet of coated surface; and in 1789 he completed another battery for the same institution, consisting of 100 jars, and containing 550 feet of coated surface. 134. It is of the utmost importance that a practical electrician should be expert in constructing batteries, and in coating jars himself, not only because of the expense attending the employment of others, but because they may often be at too great a distance from workmen who are accustomed to operations of this kind. A difference of opinion exists with respect to the size of the jars and the kind of glass they are to be made of Fine flint or crystal glass may be used with greater advantage than any other; but the expense becomes a very considerable object, especially as the jars of a battery are very apt to break by the inequality of their strength; for the force of the fluid in a battery is equally distributed among all the bottles, however their capacities may differ. Thus, if we express the quantity of charge which one jar can easily receive by the number 10, we ought not to combine such a jar in a battery with another whose capacity is only 8; because the whole force of electricity expressed by 10 will be directed also against that the quantity of which is only 8; so that the latter will be in danger of being broken. It will be proper, therefore, to compare the jars with one another before putting them together in a battery. 135. Besides the consideration of the absolute capacity which each jar has of receiving a charge, the time which is taken up in charging it must also be attended to; and the jars of a battery ought to be as equal as possible in this respect as well as in the former. The thinner a glass is, the more readily it receives a charge, and vice versa; but it does not follow that, on account of its thickress, it is capable of containing a greater charge than a thicker one. The reverse is actually the case: and though a thick glass cannot be charged so quickly as a thin one, it is nevertheless capable of containing a greater power of electricity. f the thickness of the glass be very great, no charge can, indeed, be given it; but experiments have not yet determined how great the thickness must be which will prevent any charge. Indeed it is a fact, that, though a thick glass cannot be charged by a weak electric machine, it may be so by a more powerful one; whence it seems reasonable to suppose, that there is no real limit of this kind; but that if machines could be made sufficiently powerful, glasses of any thickness might be charged. 136. The expense of constructing large batteries is an object of great importance, and has led some electricians to devise a cheaper method of making them than is commonly used. Among those who have thus labored must be mentioned Mr. Brooke of Norwich, who introduced batteries of green glass bottles, instead of flint glass jars. Some of them consisted of nine bottles; but when a greater power was required more were added. Jars would have been preferred to bottles, on account of their being more easily coated; but, being less easily procured, he was content to put up with this inconvenience. The mean size of these bottles was about eight inches in
diameter; they were coated ten inches high, and made of the thickest and strongest glass that could be procured, weighing from five pounds and a half to seven pounds each. In the construction of a battery of twenty-seven bottles, he disposed of them in three rows; nine of the stoutest and best composing the first row, nine of the next best being disposed in the second, and the third containing the nine weakest. These were all of green glass, but not of the same kind. Some of those in the front row were composed of a glass like that of which Frontigniac wine bottles are made; and this kind of glass seemed to be by much the best, as being both harder and stronger, and less liable to break by a high charge. The second and third rows of the battery consisted of bottles the diameter of which was from six and a half to ten inches, and which were coated from eight aud a half to eleven inches high; none of their mouths being larger than an inch and a half, nor less than three quarters of an inch. The bottles of which Mr. Morgan made use were not coated all over the surface usually coated, but slips of tin-foil were laid on of about three quarters of an inch in breadth, at the distance of about a slip between each : a circumstance which clearly shows that a perfectly continuous coating is not of essential importance.
137. We have already noticed that the uncoated interval of the Leyden jar should be clean and dry; but this must be understood with some limitation, as if it be perfectly clean and so dry as to approach to warmness, an explosion will take place between the coatings over the glass, and thus occasion a loss of the charge with a great waste of time. These effects may be prevented by breathing on the glass through a piece of barometer tube, but much more effectually b pasting a slip of writing paper, an inch o, on the inner surface of the jar, close to the upper edge of the coating. By this means the intensity of the charge is diminished at the very spot where its tendency to explode is the greatest. The breadth, however, of this rim of paper must be proportioned to the size of the jar. §. prefer varnishing the uncoated part of the jar, in which case the varnish must be of the very finest quality.
iss. The precarious process of charging very large batteries to a high degree of intensity is well known to those who have had to make very powerful experiments with them; and, as frequent fractures of jars take place, it may be of importance to the practical electrician to know how to repair them when they are but slightly injured; although, at the same time, we would rather pursuade him to substitute a new jar than to undergo the very troublesome and expensive process of repairing, by cement, one that has been burst.
139. The following is the method adopted by Mr. Brooke for repairing the bottles of his battery when they become injured. Take, he says, of Spanish white eight ounces; heat it very hot in an iron ladle, to evaporate all the moisture ; and when cool sift it through a lawn sieve; take three ounces of pitch, three quarters of an ounce of resin, and half an ounce of bees'-wax; heat them all together over a gentle fire, stirring the whole