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a highly charged cloud, not only satisfies the curiosity on this point, but demonstrates the fact in the most ample manner. Many are the instances of this that I have witnessed whilst experimenting with an elevated kite.
For the purpose of contemplating lateral discharges on such occasions, I insert a stout brass rod deep in the ground, and bend its upper part so as to lean towards a reservoir, which receives dense sparks, and corresponding lateral discharges take place between that lightning rod and other vicinal conducting bodies, as in the experiment already passed through.
But the most splendid series of experiments in illustration of electric waves, and of the lateral explosion at the same time, were made by Mr. Weekes, of Sandwich, on the 19th of May, 1841. This indefatigable philosopher has a stout copper wire suspended between the steeples of two churches, over a part of the town of Sandwich, and from this wire another descends to his laboratory, and ready to be attached to any piece of apparatus with which he is about to make experiments. In compliance with the rules which I have shown you for illustrating this kind of lateral explosion, Mr. Weekes arrayed his apparatus in the most suitable manner for obtaining brilliant results : which were most amply displayed on the 19th of May last.
A black cloud passed over the town, and flashes of lightning were seen, but no direct discharge ever touched the apparatus; nevertheless,“ a mighty torrent of dense sparks, so vivid as to dazzle the eye of the observer, attended by contemporaneous stunning reports, and fraught with an unusual intensity, rushed from the terminus to the ball in communication with the earth,” though separated to a distance of three and three quarter inches; at the same identical moment “a furious current of lateral sparks takes place between the wire and leaden spout of the pump.” Such is Mr. Weekes's own description of the first part of the electrical drama.
“ But now comes to be described the most resplendent feature of the scene before us; the iron nail, serving to connect the pump machinery, suddenly exhibits the appearance of a magnificent firework, the splendour of which is repeatedly enhanced as waves of electric fluid rush through the arrangement, in obedience to each successive lightning flash from the storm cloud, and this sublime scene, with short intervals of lesser energy in the electric current, continues through the space of one hour and sixteen minutes. The combustion of the iron nail forcibly reminds me of the appearance which that metal exhibits when burnt in oxygen gas, or rather when brought under the influence of the oxy-hydrogen blow-pipe, though the phenomenon was accompanied by a deep red kind of light, which does not belong to either of these comparisons."
The iron nail which deflagrated so splendidly was absolutely in the circuit, and that circuit completely metallic down to the bottom
Annals of Electricity, vol. vi, p. 450.
of the pump pipe in the well of water; the phenomenon was, therefore, a lateral discharge of the same kind as that exhibited by the bent wire and strip of sheet copper. The other lateral discharges which Mr. Weekes observed were of the third kind. These he took from every part of the wire, pump, and other parts of the conducting circuit, many of which were productive of powerful shocks. A young lady, who accidentally stepped on the wire, received a lateral discharge which sent her " reeling across the laboratory.”
Such facts as were developed in this series of experiments, and many others on record, and especially those of M. de Romas, made with an electric kite on the 7th of June, 1753, in which immense torrents of the electric fluid streamed down the string though never struck by lightning,* are not only necessary to be borne in mind by every kite experimenter, but should ever be kept in view by projectors of lightning conductors.
The electro-magnetic action of lightning is also an essential consideration, especially on board of ship; and if it were on no other account, this alone is of sufficient importance to discourage the idea of carrying a lightning conductor through the body of the vessel.
Marine lightning conductors should always be so placed as to carry the lightning over-board, and not so as to entice it into the vessel. Hence it is that even the usual chain conductors, if got to the mast head in time, are a greater protection to the vessel than a conductor that would lead the lightning into the hold.
When Franklin had discovered that tall pointed rods would draw the electric fluid from the air during the transit of an electric cloud, he contrived an ingenious apparatus to give him warning when such clouds were passing over his dwelling. The indication given by this apparatus was a ringing of bells, upon the principles shown in a former lecture.
The cut represents Franklin's atmospherical electric bell apparatus. The outside frame represents a section of the roof and walls of the house, from the top of which rises a pointed rod, which is insulated by passing through a glass tube in the roof. On the floor are placed two small bells, one of which is supported by a glass pillar and the other by a metal one. A wire connects the lightning rod with the insulated bell, and a small metallic ball, suspended by a silken thread, rings the bells when the insulated rod and bell become electrized by a wave which a passing cloud produces.
If we place this model at the distance of a foot from the prime conductor, and put the machine into motion, you will find that the bells begin ringing by the fluid drawn from the air by the pointed
Annals of Electricity, vol. v, p. 63.
rod; which gives a good idea of the indications afforded by Franklin's apparatus when an electric cloud passed over his house.
When lightning happens at a greal distance from the observer, its effects are seen in the horizon amongst the clouds and vapour that are hovering in the atmosphere; it is then called sheet lightning, and by many persons considered to be of a different kind to that in which the eleciric Auid is absolutely seen darting through its zigzag path, and attended with loud thunder. This prevalent error may easily be illustrated by a very beautiful experiment which I will offer to your notice.
1 place on the electrical stool three pieces of tinfoil in the same right line, leaving an opening between the ends of the middle strip and those of the other two. Over each opening I place a large decanter about half filled with water, to represent the clouds illuminated by the discharge of lightning. Having charged a large jar, and connected its outside with one of the outer strips of tiofoil, I apply the discharging rod to the other extreme strip and the ball of the jar. The discharge takes place through the intervals beneath the decanters, and these vessels, with their contents, are highly illuminated. In this experiment I take the precaution to screen the electric fluid from the spectators, so that nothing but its effects are seen in the decanters and water. This is an experiment beautifully illustrative of sheet lightning, which is merely the effect of a distant storin.
In a former lecture, I have stated that the resistance of atmospheric air is niuch abated, when highly attenuated, and now I will solicit your attention to a few illustrations of that fact, and to some jotel'esting phenomena which will attend them.
The instrument represented by the figure is called the luminous conductor, because of its beautifully illuminated interior during
E its electrization.
This apparatus consists of a glass cylinder, about three feet in length and five or sis inches in diameier, terminated at both extremities with hollow brass hemispheres: a point at one end for collecting the electric fluid, and a ball at the other end, from which sparks are taken.
The hemispheres screw on to brass caps, which receive, and are cemented air ight upon the ends of the glass cylinder. A welallic point projects inside from the centre of one cap, and a ball projects inside from the centre of the other; so that when the hemispheres are properly placed, as in the figure, iwo points are connected to one of ihem, and i wo balls to the other. When the hemisphere carrying sudden connection between the ball on the top and the inside of the the ball is removed the cap beneath exposes a stout brass pipe, tapped and furnished with a valve, for the purpose of being screwed to an air pump, and the air within the glass cylinder attenuated to the highest possible degree : which done, the hemisphere is replaced, and the apparatus, thus prepared for experiment, is laid on the two crutches which surmount the glass pillars, as seen in the figure.
If we now present the point of the luminous conductor to the prime conductor, it draws off the fluid and conveys it to the attenuated air inside the glass; and the air being now a tolerable good conductor conveys it onward to the remote brass cap, from the ball of which it may be received in sparks or otherwise as decidedly as from the prime conductor itself. But the beauty of experiments with this apparatus consists in the variegated light which fills the glass cylinder, and shows at first view, that the electric fluid expands when permitted, and occupies every part of the conducting medium. When the remote extremity of the luminous conductor is connected with the floor we behold a steady purple-tinged cylinder of the electric Auid; but if the fluid be taken away in sparks, a momentary darkness succeeds each, causing the light within the conductor to quiver in correspondence with the sparks. A similar agitation of the light is occasioned by removing the exterior metal point, and permitting sparks to pass between the two conductors. Experiments with the luminous conductor are amongst the most interesting in electricity, especially when exhibited in a well darkened room, and with a powerful machine in action.
When the air within a vessel is not too much attenuated, the electric fluid is divided into a cloud of quivering streamlets, intersecting one another in a capricious and most astonishing manner, producing an ever varying fantastic reticulation, &c.
The beautiful variegations exhibited within the luminous conductor are usually resorted to as illustrative of the electric origin of the aurora borealis, or northern lights. This natural phenomenon does certainly, on some occasions, put on precisely the same appearance as that seen within the luminous conductor, and its being displayed in the higher regions of the atmosphere, where the air is much attenuated, is strictly analogous to the conditions of the experiment; and although there are frequent displays of auroral phenomena which have not yet been imitated by electrical experiments, there can be little doubt of the whole of them emanating from an electric source.
In the space intervening the region of the aurora borealis and that of lightning, is another beautiful electrical meteor called the falling star. This phenomenon we also imitate by a beautiful electric experiment. For this purpose 1 einploy a tall glass tube, represented by No. 1, in the following figure. It is furnished with balls, points, and a valve in precisely the same manner as the luminous conductor, but the air within the glass tube is not so highly rarified. I now charge the battery to the highest intensity that i think it will stand; and having the lower brass cap of the apparatus in good metallic connection with the outside coating, I make a
battery, and the discharge takes place through the four feet of attenuated air, the Auid traversing it in a compact mass highly imitative of the meteoric star. When the room is darkened this is an exceedingly beautiful experiment, and gives an opportunity for the eye to follow the electric ball from the top to the bottom of the tube.
Since attenuated air is a good conductor, it may be employed as a coating to glass, in the place of tinfoil. This fact was first shown by the Abbé Nollet, who charged a bottle without any metallic lining, and on discharging it with his hand received a more violent shock than he had been led to expect. The instrument now employed to showth is fact is a Florence flask, having its neck enclosed in a perforated
2 brass cap, furnished with a valve, and tapped for the application of the air pump. A pointed wire projects inwards from the metal cap, and a hollow sperical ball terminates the cap exteriorly. A small portion of the bulb of the flask is covered with tinfoil, but none within. A representation of this apparatus is seen as No. 2, in the cut.
On presenting the ball of the exhausted flask to the prime conductor, whilst holding the coated part of the glass in the hand, a beautiful purple brush of light is seen to issue from the metal point, and spread itself over that part of the interior of the glass whose outer surface is coated with metal. If the charge gets two high for the flask to retain, the fluid will either flow over the top, from the edge of the cap to the metal coating, or it will spring through the solid glass, which it perforates, and thus renders the apparatus useless. If when the charge is high the hand be brougbt to the ball, a shock, even more violent than from an ordinary jar of the same magnitude, will be experienced. A residuary charge however is still left behind, which requires many contacts to dismiss entirely; and as each partial discharge is attended with a display of feeble purple light, the flask exhibits a series of beautiful fashes in a darkened room for a long time after the first discharge.
If, instead of the partially coated Alask used in the last experiments, we were to employ a glass receiver, on the transfer plate of an air pump, as represented in the figure, you will see the electric fluid shooting downwards through the attenuated air in a beautiful divergent brush of light, from the wire that passes through the cover to the metallic pump-plate: and in this case the fluid is conveyed away without charging the glass. But if I touch the side of the glass with my fingers, they become so many patches of coating, and the electric fluid bends towards them in the most fanciful manner, and charges the opposite surface of the glass. By changing the position of