alternately attracted by it and the table, and continue their motion for some time. See fig. 7. An instrument is constructed on purpose for this experiment, by which the dancing of the balls inay be kept up for any length of time, as it may be connected with the conductor.

(9.) Insulate a circular ring of brass so as to stand about an inch and a half from the flat surface of a table; connect the brass ring with the conductor of the electrical machine, and place within it on the table, a very light and round glass ball of two inches diameter; the ball will be attracted by the ring, touch it, and become electrified at the point of contact; this point will then recede and be attracted by the table, whilst another part of the ball is attracted by the ring; and, by the repetition of this process, the ball is made to revolve and travel round the circumference of the ring.

(10.) Fasten a small piece of sealing-wax on the end of a wire, and set fire to it. Then put the electrical machine in motion, and present the wax, just blown out, at the distance of a few inches from the prime conductor. A number of very fine filaments will immediately dart from the sealing-wax to the conductor, on which they will be condensed into a kind of net-work, resembling wool.

If the wire with the sealing-wax be fixed into one of the holes of the conductor, and a piece of paper be presented at a moderate distance to the wax, just after it has been ignited, on putting the machine in motion, a net-work of wax will be formed on the paper.

If the paper on which the wax is thus received be gently warmed, by holding the back of it near the fire, the wax will adhere to it, and thus the result of the experiment will be rendered permanent. A remarkably fine experiment of the same kind may be made with camphor. Let a silver spoon containing a piece of lighted camphor be made to communicate with an electrified body, as the prime conductor of a machine; while the conductor continues electrified, by keeping the machine in motion, the camphor will throw out numerous ramifications, and appear to shoot like a vegetable.

(11.) Take about a dozen of flaxen threads and tie them together at top and bottom; annex them to the conductor of the electrical machine; when electrified the threads will separate from each other, and the knot at the bottom rising they will assume a spheroidal figure, which will continue as long as they are electrified.

(12.) The following experiment we give as being one of the earliest made by Dr. Franklin in illustration of the principle of attraction and repulsion. Fig. 8 represents an electric jar, having a wire CDE fastened on its outside, which is bent so as to have its knob E as high as the knob A. A is a spider made of cork, with a few short threads run through it to represent its legs. It is fastened at the end of a silk thread, proceeding from the ceiling of the room, or from any other support, so that it may hang mid-way between the two knobs A and E, when the jar is not charged. Let the place of the jar upon the table be marked; then charge jar, by bringing its knob A in contact with

the

the prime conductor, and replace it in its marked place. The spider will now begin to move from knob to knob, and continue this motion for a considerable time, sometimes for several hours, The inside of the jar being charged positively, the spider is attracted by the knob A, which communicates to it a small quantity of electricity; the spider then becoming possessed of the same electricity with the knob A, is repelled by it, and runs to the knob E, where it discharges its electricity, and is then attracted by the knob A, and so on. Thus the jar is gradually discharged; and, when the discharge is nearly completed, the spider finishes its motion. EFFECT OF POINTS ON THE ELECTRIC FLUID.

97. The facility with which pointed bodies transmit electricity has given rise to several very delicate and beautiful experiments on the electrical apparatus, of which the following are the most deserving of attention.

(1.) The Electrical Flies.-These flies are composed of small brass wires, fig. 9, fixed into a cap of brass, easily moveable upon an axis of the same metal, and exactly balanced, so that they may turn with the smallest force. The ends, which ought to be very sharp, are all bent one way, with regard to one another, as those belonging to a, b, in the figure; though the two sets of points, constituting the two flies there represented, are contrary to each other; so that the whole flies must have a contrary motion. Fixing the axle with the two flies upon the prime conductor, and working the machine, both will begin to turn very swiftly, each in a direction contrary to that of the points. In this manner, with a powerful machine, several flies may be made to turn either in the same or in contrary directions; and by their gradual increase or decrease in size may represent a cone or other figure; for the course of each will be marked by a line of fire, and thus the whole will exhibit a beautiful appearance in the dark. The light is more brilliant when the ends are slightly covered with sealing-wax, grease, or other electric matter. The flies, in this experiment, turn the same way whether the electricity be positive or negative; the reason of which is that in positive electricity the fluid issues from the body electrified, and that in negative electricity it enters into it. In the former case, the recoil of the fluid, which acts equally on the air and on the point from whence it issues, must continually urge the point the contrary way; and in negative electricity, when the point solicits a continual draught of electric matter from the air, the direct impulse of the former must also produce a motion in the point in the course in which the fluid itself moves. In vacuo no motion is produced; because there is no air on which the fluid may act when it issues from the point.

(2.) The Electrical Orrery, fig. 10, is another instrument frequently used for showing the effect of points in the transmission of the electric fluid. The principle of its action is this: the ball S represents the sun, E the earth, and M the moon, connected by wires ac and bd; b is the centre of gravity between the earth and moon. These three balls and their connecting

wires are hung and supported on the sharp point of a wire A, which is stuck upright in the prime conductor B of the electrical machine; the earth and moon hanging upon the sharp point of the wire cae, in which wire is a pointed short pin, sticking out horizontally at c; and there is just such another pin at d, sticking out in the same manner, in the wire that connects the earth and

the moon.

When the cylinder of the electrical machine is turned, these balls and wires are electrified; and the electrical fire, flying off horizontally from the points c and d, causes S and E to move round their common centre of gravity a, and E and M to move round their common centre of gravity b. And as E and M are light, when compared with S and E, there is much less friction on the point b, than S and E make about the point a. The weights of the balls may be adjusted so, that E and M may go twelve times round b, in the time that S and E go once round a.

(3.) The Electrical Inclined Plane affords another and a still more beautiful illustration of the same thing, showing also that a stream of the electric matter issuing from points possesses force sufficient to counteract the power of gravitation in light bodies. Fig. 11 represents the inclined plane, where A is a board of mahogany, fourteen inches long and four inches broad; BBBB are four glass pillars, threetenths of an inch in thickness; the length of the two longer is seven inches, and that of the two shorter is five inches.

From the longer to the shorter pillars are stretched two fine brass wires, parallel to each other, and tightened by screws which pass through the brass balls which surmount the pillars. On these wires the axis of the fly Crests, the ends of which are formed like a small pulley, having a groove in them to prevent their slipping off the wires, and to guide the fly when in action. It is obvious that, if the fly be placed on the upper part of the wires, it will roll down them by its own gravity; but when it has reached the bottom of the plane, if the upper end of the wires be connected with the machine while in action, the escape of the fluid from the points will cause it to roll very rapidly up the plane till it reach the top of it. These experiments may be varied to a great extent, and models of corn-mills, water-pumps, astronomical clocks, &c., constructed of cork and pasteboard, are readily put in action by directing against their main wheels a stream of electricity from a strong pointed wire inserted into the prime conductor. (4.) By a fine flaxen thread attach a large downy feather to the prime conductor of the machine; turn the cylinder gently round, and the fibres of the feather will repel each other; approach it with a brass ball, or with the closed hand, and it will endeavour to turn itself towards the ball or hand; but present a pointed wire to it, and it will instantly shrink from it back on the conductor, as if animated, which arises from its being suddenly deprived of its electricity by the point. This experiment may be varied by inserting the brass stem of fig. 12, into one of the holes in the prime conductor.

The action of the machine will cause the hairs on the head to diverge from each other, and to stand on end.

98. Such, says Mr. Singer, are the principal phenomena of motion produced by the action of electricity; they are susceptible of almost unlimited variety, but uniformly result from the simples already stated, namely, the attraction of the electric fluid for common matter, its tendency to equal diffusion; and the occasional interruption of these properties by non-conducting power and altered force of attraction.

LUMINOUS EXHIBITION OF ELECTRICITY.

99. It may be necessary to observe here that all experiments made for the purpose of displaying the brilliancy of the electric matter, in passing from one conducting substance to another, should be made in a darkened room, as the presence of either natural or artificial light robs them of more than half of their beauty. The articles of apparatus, too, must be all free from dust, and perfectly dry, besides being a little warm, otherwise the effect expected will not result; we think it particularly necessary to observe that in any experiment requiring the exhaustion of glass vessels the above precautions are peculiarly needful, as we have seen some of the following experiments utterly fail in the hands of public lecturers merely from inattention to them.

100. To render the electrical fluid luminous it must be collected in considerable quantities, and the brilliancy of the display will depend on the particular configuration of the conducting surface over which it is made to pass. The light evolved in ordinary cases, says Mr. Singer, extends only to faint flashes and scintillations, sparks being only produced when these effects are concentrated, as they are in the electrical machine by the action of its conductors.

101. There are three circumstances that influence the electric spark in its passage from one conductor to another, namely, the form of the conductors, their extent, and the nature and density of the medium through which the spark passes. The following remarks on these three circumstances we give nearly in Mr. Singer's own words.

102. The distribution of electricity on conductors has but little relation to their solid contents, and depends almost entirely on extent of surface, for the same effects are produced by the thinnest cylinder or sphere of metal as by the most compact solid body of the same form and dimensions; it is probable that the action of insulated conductors consists in the ready communication of their electric state to the contiguous surface of the extensive stratum of air by which they are surrounded, and to the facility they present to the discharge of that electrified stratum when an uninsulated or differently electrified body is brought near them; for every positively electrified conductor is surrounded by a positive atmosphere, and every negative conductor with. a negative atmosphere whose densities decrease as the square of their increased distance. Hence any insulated electrified body will retain its electrical state until its inter.sity is sufficient to

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 irregular, 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 or angular, 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.

106. 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.

fixed

108. Let, for example, a fine point 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 pass 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 paper.

(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 sharp point at double that distance, and the sparks will immediately cease, the electric matter 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 diameter; 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 receiver 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 appearance and becoming more diffused as the air becomes 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 points 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 slightest degree a luminous 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 composed 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. Fig. 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 under 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. The 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 machine, 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