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Overcome the resistance of the air, and the 108. Let, for example, a ine point 'e fixed greater or less interval through which the spark in the axis of a large brass ball, from beneath passes is called the striking distance.

the surface of which it may be protruded more 103. When the surface of the conductor is or less by the action of a fine screw, the effect uniform, the re-action of the air around it is also of a ball of any size may be obtained; when uniform; but if the surface of the conductor be beneath the surface of the ball the point does not irregular, the tendency of the electric fluid to act, but in proportion as it is protruded it inescape or enter it will be greatest at the most creases the transmitting power, and, if projected prominent parts, and most of all when these far enough, at length entirely overcomes the inare angular or pointed. To understand this it is fuence of the ball. only necessary to recollect that every electrified 109. The same writer gives the following exconductor is surrounded by an atmosphere of its periments, among others, for illustrating the inown figure, the contiguous surface of which is fluence of the form and extent of the conductor similarly electrified: and that electricity is not on the appearance of the electric spark. transmitted through air, but by the motion of its (1.) Present a brass ball of about three inches particles.

in diameter to the positive conductor of a pow104. For this inotion of particles is resisted erful electrical machine; sparks of brilliant white by a uniform surface from the similar action of light will pass between them, accompanied by a the air around it, which is all equally capable loud snapping noise: to produce these sparks in of rece.ving electricity, and cannot tend to dis- rapid succession the ball must be brought near tribute it in one direction more than another; the conductor, and they then appear perfectly the immediate electrical atmosphere of the con- straight. ductor will be therefore resisted in any attempt (2.) Annex a ball of an inch and a half or two to recede from it by a column of air which is inches diameter to the conductor, so as to project equally opposed in every part; but if there be three or four inches from it; present the large any prominent point on the conductor projecting ball to this, and much longer sparks will be obinto the atmosphere, it will facilitate the re- 'tained than from the conductor itself, but they Cession of the electrified particles opposite to it will be less brilliant and of a zigzag form. by removing them farther from the electrified (3.) Substitute a small ball for that used in the surface, and opposing them to a greater number former experiment; the fluid will now pass to a of such as are unelectrified.

greater distance, but in the form of a divided 105. The action then, of bodies that are brush of rays, faintly luminous, and producing pointed or angular, appears to consist in promo- little noise; this brush will even occur with ting the recession of the particles of electrified larger balls, if the machine be very powerful; it air, by protruding a part of the electrical atmos- is most perfect when procured by presenting a phere of the conductor into a situation more ex- flat imperfect conductor, as a piece of wood or posed to the action of the ambient unelectrified paper. medium, and thereby producing a current of air (4.) Whilst a current of sparks is passing befrom the electrified point to the nearest uninsu- 'tween a large ball and the conductor, present, at lating body. Hence the most prominent and the the distance of about an inch and a half, a sharp most pointed bodies are such as transmit electri- point at double that distance, and the sparks will city with the greatest facility, for with them this immediately cease, the electric matter being condition is most perfectly obtained.

silently drawn off by the point. 106. A spherical surface is that which, con 110. The brilliancy of the electric spark is sidered with regard to its surrounding atmosphere, always in proportion to the conducting power of is most uniform; hence balls, or cylinders, with the bodies between which it passes; hence metals rounded ends, are naturally employed for insu- are almost exclusively employed for this purpose, lated conductors, and their magnitude is propor- as wood and other imperfect conductors produce tioned to the intensity of the electrical state they only faint red streams; yet these substances act are intended to retain; for a point is but a bail as points with some efficacy, and the particles of indefinite diameter, and will act as such on of dust which collect around the apparatus are very small quantities of electricity; and a ball often troublesome to electricians from the same of moderate size may also be made to act as a point by electrifying it strongly.

111. The nature and density of the medium 107. If two spheres of equal size are con- through which the electric spark passes has also nected together by a long wire and electrified, a powerful influence on its character. Dr. Watson their atmospheres will extend to the same dis- seems to have been the first who made experitance, and they will of course have respectively ments on this subject; these he conducted on a the same intensity; but if the spheres be of un- very large scale, and he describes the results as equal size, the atmosphere of the smallest will having been very beautiful; they will be noticed extend furthest, and it will necessarily have the in another part of this article. greatest intensity; so that a longer spark can be 112. The following is a description of the simdrawn from a small ball annexed to the side of ple apparatus used by Mr. Singer for showing a conductor than from the conductor itself, and the effect of different gaseous mediums on the longer in proportion as the ball projects farther passage of electricity. It consists of a glass from the side. Hence the finer the point, and globe, fig. 1, plate II., of about four inches diathe more freely it projects beyond any part of meter, having two necks capped with brass ; to tbe conductor to which it is annexed, the more one of the necks a stop-cock is screwed, with a rapidly will it receive or transmit electricity. wire and ball projecting into the globe; another

cause.

ball is attached to a wire that slides through a green; a long spark taken over powdered charcollar of leathers screwed to the opposite cap, so coal is yellow; and the sparks from imperfect that the balls may be set at any required dis- conductors have a purple hue. The quantity of tance from each other within the globe. This air through which these sparks are seen also apparatus may be exhausted by connecting the influences their appearance; for the green spark stop-cock with an air-pump, and various gases in the vapor of ether appears white when the eye may be introduced into it, or the air it contains is placed close to the tube, and reddish when it may be rarefied or condensed, and the effect of is viewed from a considerable distance. these processes on the form of the spark exa 114. When metallic conductors are of sufmined. In condensed air the light is white and ficient size and perfectly continuous, they transbrilliant; in rarefied air, divided and faint; and mit electricity without any luminous appearance; in highly rarefied air, of a dilute red or purple but if the continuity be interrupted in the slightcolor. The effect of gases seems to be propor est degree a luminous effect is produced, a bright tioned to their density; in carbonic acid gas the spark occurring at every separation. Various spark is white and vivid, in hydrogen gas it is articles of apparatus are used for the exhibition red and faint.

of this effect, according to the faney of the ope113. The brilliancy of the electric spark seems rator; the following are those used in general by to be in proportion to the density of the medium public lecturers:-1. The spiral tube: this inthrough which it is made to pass. This is strument is represented by fig. 2, and is comproved by the following experiments :-1. Fix posed of two glass tubes C), one within another, with cement a short iron or platina wire within and closed with two knobbed brass caps A and B. one end of a glass tube thirty inches long, so The innermost of these has a spiral row of small that the wire may project a little way within the round pieces of tin-foil stuck upon its outside tube, and fix a small brass ball on the outer ex- surface, and lying at about one-thirtieth of an tremity of the wire. Fill the tube with mercury, inch from each other. If this instrument be held and at the open end place a drop of ether, which by one of the extremities, and its other extremity secure by the point of the finger while the tube is be presented to the prime conductor, every spark inverted in a vessel of mercury, so as to form a that it receives from the prime conductor will Torricellian vacuum in the upper part. The cause small sparks to appear between all the ether will rise to the top; and upon the removal round pieces of tin-foil stuck upon the innermost of the finger and the fall of the mercury, will ex tube; which in the dark affords a beautiful specpand into vapor. If now electricity be trans- tacle, the tube appearing encompassed by a spiral mitted through this vapor, it will be rendered line of fire. fig. 3 represents several spiral luminous, and assume various hues according to tubes placed round a board, in the middle of its strength. When the spark is strong, and has which is screwed a glass pillar, and on the top of to pass through some inches of the expanded this pillar is cemented a brass cap with a fine vapor, the light is usi ally of a beautiful green steel point. In this a brass wire turns, having a color. 2. Take an air-pump receiver twelve or brass ball at each end, nicely balanced on the fourteen inches high, and six or seven inches in wire. To make use of this apparatus, place the diameter; adapt a wire, pointed at its lower ex- middle of the turning wire under a ball proceedtremity, to the top of the receiver, letting the ing from the conductor, so that it may receive a point project about two inches into its inside; succession of sparks from the ball; then push the place the receiver on the plate of the air-pump, wire gently round; and the balls in their relative and electrify the wire at its top positively; motions will give a spark to each tube, and whilst the air remains in the receiver, a brush of thereby illum ate them down to the board, light of very limited size only will be seen, but which from its brilliancy and rapid motion, in proportion as the air is withdrawn by the ac affords a most beautiful and pleasing sight. tion of the pump it will enlarge, varying its ap Fig. 4 is another instrument for showing the pearance and becoming more diffused as the air same effect in a diversified form: the action becomes more rarefied; until at length the whole being in this case the same as in the preceding, of the receiver is filled by a beautiful blush of no further explanation is necessary. The beauty light, changing its color with the intensity of the of this kind of exhibition is sometimes much intransmitted electricity. 3. Into a piece of soft creased by laying down the devices on glass deal about three inches long and an inch and a stained of different colors. There are other half square, insert two pointed wires obliquely methods of rendering the electric fluid visible in into its surface at nearly an inch and a half dis a very pleasing manner, some of which we shall tance from each other, and to the depth of an here enumerate. eighth of an inch; the wires should incline in 115. The luminous conductor, as representeil opposite directions, and the track between the at fig. 5, consists of a glass tube about eighteen points be in that of the fibres; a spark in pass- inches long, and four in diameter, tò the ing from one point to another through the wood ends of which are cemented the hollow brass will assume different colors in proportion as it pieces DF, E B, the former having a point, C, passes more or less below the surface; and by for receiving electricity from the electrical mainserting one point lower than the other, so that chine, while the other has a wire terminating in the spark may pass obliquely through different a ball, G, from which a strong spark may be depths, all the prismatic colers may be made to drawn. From each piece a knobbed wire proappear at once. Sparks taken through balls of ceeds within the cavity of the glass tube, One wood or ivory appear of a crimson color; those of these brass pieces is composed of two parts, from the surface of silvered leather are of a bright in one of which is a valve covering a hole by

which the tube may be exhausted of its air. The subject of experiment: but such is not the case, whole is supported on two glass pillars fixed in since the extension of the surface of any conducta wooden frame. When this tube is exhausted ing body diminishes its intensity. This fact is of its air, and the point C set near the machine, admirably illustrated by Mr. Singer in the folthis point will appear illuminated with a star, lowing experiment. while the glass tube will exhibit a weak light on 119. Insulate a flat metal plate with smooth its inside; and, from the knobs within the glass, rounded edges, and connect with it a pith-ball the appearance of positive and negative light electrometer ; electrify the plate, and the balls will be evident, as the knob at D will show a will diverge : bring a similar plate uninsulated bright pencil of rays, and the opposite knob a near that which is electrified, keeping the r flat round star. If the point C, instead of being surfaces parallel and opposite to each other; the presented to the cylinder, or the positive con- balls of the electrometer gradually collapse as ductor, be placed near the rubber or negative the plates approach, and, when they are within conductor, the appearance of the light from the about half an inch of each other, the insulated internal knobs will be reversed.

plate appears unelectrified; but, on the removal 116. The visible electrical atmosphere is ex- of the uninsulated plate, the original divergence hibited by the apparatus represented at fig. 6, is restored. See fig. 8. where G I represents the receiver with the plate 120. When the insulated conductor, he adds, of an air-pump. In the middle of the plate IF is electrified, its pith-balls separate, because they a short rod is fixed, having at its top a ball B, are in a different electrical state to the air by whose diameter is nearly two inches. From the which they are surrounded, the fluid of wh ch top of the receiver another rod AD with a like they attract; but all unelectrified bodies have hall A proceeds, and is cemented air-tight into the same relation to the electrified balls like the the neck C; the distance of the balls from one ambient air, and such as are conductors and conanother being about four inches. If, when the nected with the ground present a more ampe receiver is exhausted of air, the ball A be elec- source of matter and electricity; consequently, trified positively, by touching the top D of the if such bodies are brought near the electrirod AD) with the prime conductor, or an excited fied conductor, its attraction is exerted on them, glass tube, a lucid atmosphere appears about it, and the influence of the surrounding air is prowhich, although it consists of a feeble light, is portionably diminished; and if the proximity yet very conspicuous, and very well defined; be sufficient, the attraction of the electrified surat the same time the ball B has the least face will be so exclusively exerted in that direclight. The atmosphere does not exist all round tion as to be imperceptible in any other. the ball A, but reaches from about the lower half 121. In the above experiment the bodies are of it. If the rod, with the ball A, be electrified not brought in contact, but only near each other, Degatively, then a lucid atmosphere, like the and consequently there is no communication or above described, will appear upon the ball B, loss of electricity, but merely a compensat.on of reaching from its middle to a small distance be- its attractive power; hence, when the uninsuyond that side of it that is towards the ball A; lated plate is removed, the divergence of the at the same time the negatively electrified ball electrometer is restored. A remains without any light.

122. The grand instrument used by electri117. Fig. 7 represents a mahogany stand, so cians for the accumulation of electricity is denoconstructed as to hold three eggs at a greater or minated the Leyden jar, or phial: its construcsmaller distance, according to the position of tion has already been in some measure described, the sliding pieces. A chain C is placed at the but it may be necessary still further to explain it, bottom, in such a manner as to touch the bottom and to make some remarks 0. the pr.ncipe of of the egg at B with one end, and with its other its action. the outside coating of a charged jar. The sliding 123. The Leyden Jar in whatever form it may wire A at the top is made to touch the upper egg; be constructed is nothing more than an electric and the distance of the eggs asunder should not placed between two non-electrics. The following exceed a quarter of the eighth part of an inch. description of this remarkable instrument is The electric spark, being made to pass down by from Mr. Cavallo's treatise on electricity: If, means of the discharging rod through the wire says he, to cne side of an electric, sufficiently and ball at A, will, in a darkened room, render thin, as for instance a pane of glass, a piece of the eggs very luminous.

sealing-wax, &c., be communicated one electriACCUMULATION OF ELECTRICITY.

city, and to the opposite side the contrary,

that plate in that case is said to be charged; and 118. Although the electricity we have already the two electricities can never come together described be sufficient for the performance of except a communication of conducting substances many very fine experiments, and for enabling be made between both sides, or the electric be us to investigate the nature and properties of broken by the power of electric attraction. When the electric fluid ; yet the full energy of this the two electricities of a charged electric are hy wonderful agent can only be displayed when any means united, and therefore their power it is collected in great quantities, and made to destroyed, that electric is then said to be risoperate on substances in a strongly concentrated charged and the act of union of these two optostate. This, it might be supposed, would be site powers is called the electric discharge. best effected by diffusing the electrical matter 124. To avoid the difficulty of communicating over very extensive conductors, and at once dis- electricity to an electric plate, it is customary to charging the quantity thus accumulated, on the coat the sides of it with some conducting subVol. VIII.

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stance, as tin-foil, gilt paper, &c., by which means and forming the line of communication between the charging and discharging becomes very easy; the coatings of the jar. In this way the abbé for when the electricity is communicated to one Nollet succeeded in giving the shock to 180 of part of the coating, it is immediately spread the French Guards in the king's presence. through all the parts of the electric that are in 129. When it is wished to discharge the jar contact with that coating; and, when the electric without allowing the charge to pass through the is to be discharged, it is sufficient to make a body, an instrument is used called the discharging conducting communication between the coatings rod, which is composed of a bent wire or two of both sides, to discharge entirely the electrici- branches, connected by a joint, and furnished with ties of that electric.

a glass handle. The extremities of the rod or 125. When plates of glass are thus coated, it branches are pointed, but have screws, by means is of essential importance that the glass should of which they are fitted with balls. In disextend two or three inches beyond the metal charging a jar with this instrument, it is held by coatings; for, although they do not absolutely the glass handle, and, while one end is applied to touch one another, yet, when they are electrified, the outer coating of the jar, the other is made to the electricity will easily force a passage through approach the ball of its wire, and thus the electhe air, and by passing over the surface of the tricity passes through the metallic part of the electric, from one coating to the other, render it discharger from the one coating to the other of incapable of receiving any charge.

the jar. If the extremities be without their balls 126. If a glass plate be properly coated on the discharge is effected without noise, but otherboth sides with a conducting substance, and if to wise there takes place an explosion, more or less one of these coatings be communicated some loud according as the jar is more or less charged. electricity, the other coating, while communica- The neatest form of the discharging rod is that ting with the earth, or with other conducting represented at fig. 10; where AA is the glass bodies, acquires by itself an equal quantity of handle by which it is held, and BC are the two the contrary electricity; but if, while one side branches with their balls. is acquiring electricity, the opposite side does not 130. When the accumulation of great quancommunicate with the earth, or the conducting tities of electricity is required, the instrument substances, the glass cannot be charged. The then made use of is termed the electrical battery, reason of this is founded on the property of and is composed of a number of Leyden jars bodies to acquire an electricity, contrary to that connected together and placed in an appropriate possessed by a contiguous electrified body; and box. The most modern construction of the ihe cause that hinders these two electricities from battery, is represented at fig. 11. It consists of mixing, is the interposition of the glass plate twelve jars placed in a mahogany box, the bottom which is impermeable to electricity. Although of which is covered with tin-foil, for the purpose if the glass be too thin, or the charge too high, of connecting together all the outside coatings; the strong attraction, between the positive and the inside coatings being connected together by negative electricities, forces a passage through the wires and balls that rise from their centres, the glass and discharges it.

and are united together at the top. On one 127. The most usual, and by far the most side of the box there is a small brass hook A for convenient form of the Leyden jar is that repre- the purpose of connecting the battery by means sented at fig. 9. It is coated on the inside and of a chain with any substance through which the also on the outside with tin-foil to within two discharge is to be made; this hook passes through jaches and a half of the top. With the inside the box and is fixed in contact with the tincoating a wire is connected which rises through foil which connects the exterior coating of the jars. a lid of baked wood neatly fitted into the mouth 131. A battery may also be constructed by a of the jar, and terminating in a smooth brass combination of panes of glass properly coated. ball. The uncoated part of the jar must be kept Dr. Franklin formed a battery of this kind with perfectly clean and dry, otherwise the action will eleven panes of common window-glass, and with be very incomplete. The coating is best fas- it he made the greater part of his experiments. tened on with very strong gum water, but some 132. In whatever form batteries are constructed clectricians use common paste; and in some in- they are charged and discharged in the same stances the tin-foil is first pasted upon paper, manner as a single jar. If one of the knobs of and afterwards on the glass: this is considered the battery communicate with the prime conducan improvement both as it respects the facility tor of the machine in a state of action, it will of drying the gum or paste, and also the strength- soon be charged ; and the discharge may be (ning of the jar.

effected by making a communication between 128. If a jar thus constructed be held in one the coatings, by means of a discharging rod, or hand by the lower part, and the knob applied to any other conductor. the prime conductor when the machine is in ac 133. Batteries of great size have been contion, it will become charged in a few seconds; and structed by different electricians, so as to accuif then a communication be formed between its mulate an enormous quantity of electricity, outside and inside coatings, by touching the ball capable of melting the hardest metals, and of with the other hand, a smart explosion takes putting an instantaneous termination to animal place, and a peculiar and painful sensation is life. Dr. Priestley constructed a battery consisfelt chiefly at the wrists and elbows, and across ting of sixty-four jars, and containing thirty-two t' e breast: this sensation is called the electric square feet of coated surface. Mr. Cuthbertson shock, and it may be communicated to any number completed, in 1784, for the Teylerian Museum of individuals, holding each other by the hand at Haarlem, a battery of 135 jars and 132 feet

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of coated surface; and in 1789 he completed diameter; they were coated ten inches high, and another battery for the same institution, consis- made of the thickest and strongest glass that ting of 100 jars, and containing 550 feet of coated could be procured, weighing from five pounds surface.

and a half to seven pounds each. In the con134. It is of the utmost importance that a struction of a battery of twenty-seven bottles, he practical electrician should be expert in con- disposed of them in three rows ; nine of the structing batteries, and in coating jars himself, stoutest and best composing the first row, nine not only because of the expense attending the of the next best being disposed in the second, employment of others, but because they may and the third containing the nine weakest. These often be at too great a distance from workmen were all of green glass, but not of the same kind. who are accustomed to operations of this kind. Some of those in the front row were composed of A difference of opinion exists with respect to the a glass like that of which Frontigniac wine botsize of the jars and the kind of glass they are to tles are made; and this kind of glass seemed to be made of. Fine flint or crystal glass may be be by much the best, as being both harder and used with greater advantage than any other; but stronger, and less liable to break by a high the expense becomes a very considerable object, charge. The second and third rows of the batespecially as the jars of a battery are very apt tery consisted of bottles the diameter of which to break by the inequality of their strength; for was from six and a half to ten inches, and which the force of the fluid in a battery is equally dis- were coated from eight and a half to eleven tributed among all the bottles, however their inches high ; none of their mouths being larger capacities may differ. Thus, if we express the than an inch and a half, nor less than three quarquantity of charge which one jar can easily re ters of an inch. The bottles of which Mr. Mor, ceive by the number 10, we ought not to com- gan made use were not coated all over the surbine such a jar in a battery with another whose face usually coated, but slips of tin-foil were capacity is only 8; because the whole force of laid on of about three quarters of an inch in electricity expressed by 10 will be directed also breadth, at the distance of about a slip between against that the quantity of which is only 8; so each : a circumstance which clearly shows that that the latter will be in danger of being broken. a perfectly continuous coating is not of essential It will be proper, therefore, to compare the jars importance. with one another before putting them together in 137. We have already noticed that the una battery.

coated interval of the Leyden jar should be clean 135. Besides the consideration of the absolute and dry; but this must be understood with some capacity which each jar has of receiving a charge, limitation, as if it be perfectly clean and so dry the time which is taken up in charging it must also as to approach to warmness, an explosion will be attended to; and the jars of a battery ought take place between the coatings over the glass, to be as equal as possible in this respect as well and thus occasion a loss of the charge with a as in the former. The thinner a glass is, the more great waste of time. These effects may be prereadily it receives a charge, and vice versa; but vented by breathing on the glass through a piece it does not follow that, on account of its thick- of barometer tube, but much more effectually by 16s, it is capable of containing a greater charge pasting a slip of writing paper, an inch broad, than a thicker one. The reverse is actually the on the inner surface of the jar, close to the upper case : and though a thick glass cannot be charged edge of the coating. By this means the intensity so quickly as a thin one, it is nevertheless capa- of the charge is diminished at the very spot where ble of containing a greater power of electricity. its tendency to explode is the greatest. The

f the thickness of the glass be very great, no breadth, however, of this rim of paper must be charge can, indeed, be given it; but experiments proportioned to the size of the jar. Some prefer have not yet determined how great the thickness varnishing the uncoated part of the jar, in which must be which will prevent any charge. Indeed case the varnish must be of the very finest quait is a fact, that, though a thick glass cannot be lity. charged by a weak electric machine, it may be 138. The precarious process of charging very so by a more powerful one; whence it seems large batteries to a high degree of intensity is reasonable to suppose, that there is no real limit well known to those who have had to make very of this kind; but that if machines could be made powerful experiments with them; and, as frequent sufficiently powerful, glasses of any thickness fractures of jars take place, it may be of impormight be charged.

tance to the practical electrician to know how to 136. The expense of constructing large batte- repair them when they are but slightly injured ; alries is an object of great importance, and has though, at the same time, we would rather purled some electricians to devise a cheaper method suade him to substitute a new jar than to undergo of making them than is commonly used. Among the very troublesome and expensive process of those who have thus labored must be mentioned repairing, by cement, one that has been burst. Mr. Brooke of Norwich, who introduced batteries 139. The following is the method adopted by of green glass bottles, instead of flint glass jars. Mr. Brooke for repairing the bottles of his battery Some of them consisted of nine bottles; but when they become injured. Take, he says, of when a greater power was required more were Spanish white eight ounces ; heat it very hot in added. Jars would have been preferred to bot- an iron ladle, to evaporate all the moisture ; and tles, on account of their being more easily coated; when cool sift it through a lawn sieve; take three but, being less easily procured, he was content ounces of pitch, three qua ers of an ounce of to put up with this inconvenience. The mean resin, and half an ounce of bees'-wax; heat them size of these bottles was about eight inches in all together over a gentle fire, stirring the whole

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