balls will now touch one another, the threads hanging perpendicularly, and parallel to each other. But if the cylinder of the machine be turned, by turning the winch, then the pith balls will repel one another, more or less according as the electricity is more or less powerful. If the electrometer be hung to a prime conductor negatively electrified, i. e. connected with the insulated rubber of the machine, the balls will also repel each other. If, in this state of repulsion, the prime conductor is touched with some conducting substance not insulated, the pith balls will immediately come together. But if, instead of the conducting substance, the prime conductor is touched with an electric, as a stick of sealing wax, a piece of glass, &c., then the pith balls will continue to repel each other; because the electric fluid cannot be conducted through that electric. (2.) Take a small downy feather or a pith ball suspended by a thread, and, holding the thread, bring the ball near an electrified conductor, either positive or negative: the ball will be attracted by the electrified conductor, and adhere to it, until its electricity is destroyed. Such bodies as are positively electrified, tend to diffuse their superabundant fluid amongst surrounding substances; and those that are negative, endeavour to acquire electric fluid: hence, either state of electricity will produce attraction; for if light bodies are to be moved, it is indifferent whether the electrified surface attracts their natural electric fluid, or the matter to which it is attached; for the attraction arises only from the different proportions of these in any two bodies, and will of course continue whilst that difference exists. (3.) Repeat the preceding experiment with a ball or feather supported by a silk thread: the light body will first be attracted to the electrified conductor, and will then recede from it; nor can it again be brought in contact until it has touched some conducting substance. Mr. Singer thus explains the cause of this:-The light body is here attracted for the same reason as before, but it is insulated, and consequently receives, by contact with the electrified surface, a similar electric state; it therefore recedes from that surface, being attracted by the ambient air, or other surrounding bodies; for they have their natural portion of electricity, and therefore differ from the light body, which has either more or less; but the electrified surface does not differ from the light body, and consequently cannot attract it, till, by touching some conductor, its natural electric state is restored. (4.) The following is a very pleasing variety of the last-mentioned experiment -Take a glass tube, whether smooth or rough is not material, and, after having rubbed it, let a small light feather be let out of your fingers at the distance of about eight or nine inches from it. This feather will be immediately attracted by the tube, and will stick very close to its surface for about two or three seconds, and sometimes longer; after which it will be repelled; and, if the tube be kept under it, the feather will continue floating in the air at a considerable distance from the tube, without coming near it again, except it first touch some conducting substance; and, if the tube be managed dexterously, you may drive the feather through the room at pleasure. This experiment may be varied as follows: A person may hold in his hand an excited tube of smooth glass, and another may hold an excited rough glass tube, a stick of sealing-wax, or any other electric negatively electrified, at about one foot and a half distance from the smooth glass tube; a feather now may be let go between these two differently excited electrics, and it will leap alternately from one electric to the other. (5.) Place a leaf of gold, silver, or other metal, on the palm of the hand, and bring it within a few inches of an electrified conductor; it will be attracted and continue to move, alternately from the hand to the conductor, as long as the latter is electrified. (6.) Suspend from the conductor, by a brass chain, a circular plate of copper, reaching to within an inch and a half or two inches of the table. Directly under this plate place another of the same form, and a little larger, on the table. Turn the machine, and the fluid will pass from the upper to the lower plate. If now small figures cut out of pasteboard, or pith of elder, be introduced between the plates, they will dance about with apparent vivacity, and sometimes appear to course round the edge of the lower plate. This experiment is represented by fig. 4. (7.) The electrical bells furnish a pleasing illustration of the attraction and repulsion of the electric matter. They are variously constructed, but the form exhibited fig. 5 is the simplest. The two outer bells are suspended by brass chains; the middle bell and the two clappers by fine silk threads. When the bells are attached to the conductor, and the machine is turned very gently, the fluid will pass along the chains to the two outer bells, but will not pass along the silk to the clappers and middle bell. Thus the outer bells being charged with an extra quantity of fluid will attract the clappers, but the moment they touch the bells they become charged, and are repelled with such force as to cause them to strike against the middle bell, on which they deposit their electricity, and are again attracted. By this means a constant ringing is kept up while the machine is turned. From the inside of the middle bell a brass chain passes to the table, for the purpose of conveying away the fluid deposited on it by the clappers. A more elegant form of the electrical bells is thus made :— Fix eight bells near the edge of a circular board supported on four feet, fig. 6, having a glass pillar e, in the centre, terminated by a point g. On this point place the pointed wires used in the last experiment, hanging from one of them, as d, a small glass clapper by a silken thread; and connecting the apparatus by a chain h, proceeding from the prime conductor. On setting the machine in motion, the wire will move round, and the clapper ring the bells. (8.) Place a pointed wire on the machine, electrify the inside of a dry glass tumbler by holding it over the wire whilst the machine is in motion; place some pith balls on the table and, cover them with the electrified glass; they will be 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 may 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 the jar, by bringing its knob A in contact with 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 sinall 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 intersity is sufficient to |