« ZurückWeiter »
by his admirable observations on the analogies which the effects of lightning and electricity present, and also of placing before them a plan by which a satisfactory experiment may be made. Dr. Franklin's plan of experimenting bears date 1749, and the following description of it is in his own words :
“ To determine the question whether the clouds that contain lightning are electrified or not, I would propose an experiment to be tried where it may be done conveniently. On the top of some high tower or steeple place a kind of sentry box, as in the figure, big enough to contain a man and an electrical stand. From the middle of the stand let an iron rod rise, and pass bending out of the door, and then upright twenty or thirty feet, pointed very sharp at the end. If the electrical stand be kept clean and dry, a man standing on it when such clouds are passing low, might be electrified and afford sparks, the rod drawing fire to him from the cloud. If any danger to the man should be apprehended (though I think there would be none) let him stand on the floor of his box, and now and then bring near to the rod the loop of a wire that has one end fastened to the leads, he holding it by a wax handle; so the sparks, if the rod is electrified, will strike from the rod to the wire, and not affect him."
The plan for this grand experiment being made generally known throughout Europe and America, many philosophers of both countries made preparations for carrying it into execution. The French were the first in the field on this memorable occasion, and M. Dalibard's apparatus, erected at Marley-la-ville, had the honour of being the first that was visited by the “ ethereal fire,” though that philosopher himself, in consequence of an absence from home at the time, was deprived of the glory of being the first beholder of it, the enviable good fortune falling to the lot of his servant Coiffer, who was left in charge of the apparatus.
Dalibard's apparatus was similar to that proposed by Franklin, which, however, was not placed on a steeple, but on some high ground. It consisted of an iron rod forty feet long, the lower end of which was brought into the sentry box, where the rain could not come. On the outside the rod was fastened by silken cords to three stout wooden posts firmly fixed in the ground. On Wednesday, the 10th of May, 1752, between two and three o'clock in the afternoon, Coiffer saw the first electric spark drawn from the atmosphere ever witnessed by man. He heard a clap of thunder at some distance, and on applying a small Leyden bottle to the iron rod, electric sparks were obtained, and the great question set at rest about one month earlier than Franklin himself had an opportunity of making a satisfactory experiment, which he did by means of an elevated kite in June of the same year, and without having any information of what had been done in France.
Franklin's kite was simply a silk handkerchief stretched diagonally by two sticks, with the usual loop, tail, and hempen string, which was insulated by means of a silken cord at the lower end.
The first indication of electric action observed were the repulsions among the fibres of the string, which stood erect and avoided one another as if attached to the prime conductor of a machine. Shortly after these appearances, Franklin obtained sparks from a key which he had hung at the lower end of the hempen string. A dense cloud was passing over the kite at the time, and in consequence of some rain which fell and wet the kite string, and thus made it a better conductor, the electric fluid was copiously exhibited in various ways. From the time that Franklin's kite experiments became known, to the present, an electric kite has been considered as an indispensable apparatus for explorations of the atmosphere.
The electric kites that I employ differ little from that first used by Franklin, excepting in the manner of applying the string, the tail, and in making them portable and convenient for carriage. • I provide a square of sarsnet, and a pair of stretchers made of light wood, and well varnished for protection against the wet. These stretchers are coupled together by means of a pin which passes through the centre of both, and on which, as a pivot, they can turn and be set to the proper angle for stretching the silk, or they can be brought close and parallel together. One extremity of each stretcher is fastened permanently to the two upper corners of the silk, and each of the other extremities is furnished with a projecting wire loop or staple, which passes through an eye-hole in the lower corner of the silk, the end of the stretcher itself forming a shoulder, which prevents its passing through also. By these means the silk can be stretched or furled at pleasure. The tail consists of several lengths of calico ribands, linked end to end by means of hooks and eyes, its upper end being attached to the middle of a tape, which by means of a hook at each end, can be attached to, or detached from, the lower corners of the kite at pleasure.
In order to relieve the stretchers as much as possible from the strain of the wind's force, the surface of the silk is divided into two equal areas, a central area, and a marginal area, so that the strain on the central and outer parts of the stretchers shall nearly balance one another. This is accomplished by means of four braces, two to each stretcher, and attached at a proper distance from their extremities, to insure the square area within the four points to which the braces are attached, to be equal to the marginal area without. The braces, after passing through small holes in the silk, have their outer extremities united in a brass ring, and to this ring the kite-string, containing a wire strand, is attached by means of a hook. By this
arrangement the wind blows the silk against the stretchers with but little chance of breaking them : and the four braces keep the kite much steadier than by the usual single loop. There are some other peculiarities belonging to this kite, but they are hardly worth describing, excepting, perhaps, the advantage of having the tail in several pieces, which is found convenient to give the proper trim for various forces of the wind.
The complete stock of my atmospheric electrical apparatus, in addition to five silken kites with their wired strings, consists of a reservoir, a Leyden bottle with its discharger, a compass-needle, a soft steel needle enclosed in a spiral wire, and a delicate pith ball electroscope, which, with the exception of the reservoir, are sufficiently portable to be put in the pocket.
The reservoir consists of a hollow tin cylinder, mounted on a stout glass pillar, as represented in the figure opposite. The upper end of the tin cylinder is open for the purpose of lodging in it that part of the string not taken up by the kite, and in order to keep the strain of the string from pulling down the reservoir, the latter is anchored by silken cables, which keep it steady ; the string, also, has a silken cord termination, which reaches over the mouth of the prime conductor, and has its other end anchored in the ground. To the lower part of the reservoir is screwed a Lane's discharger, with a wire for occasional connexions.
The coated bottle is enclosed in a cylindrical brass case, to protect it from breaking. When the cover is taken off the body of the case the neck of the bottle, with its brass ball, is exposed. A small discharging rod, with one metal branch and a short ivory handle, is attached to one end of a copper wire, the other end being connected with the brass case, and consequently with the outer coating of the jar. Within the lid of the cylindrical case is the compass card and its magnetic needle, which being covered with glass, in the usual way of fitting up small compass boxes, is protected from injury, and when removed from the case is placed horizontally on the ground, and the needle takes its proper position. The jar, with its neck and ball exposed, and the discharger applied as in the act of discharging the bottle through the spiral wire, is represented by the figure. The lid of the case in the capacity of a compass-box is seen below the jar.
When the kite is about to be used on a fine cloudless day, for the mere purpose of ascertaining the character of the electricity of the air at a considerable altitude, it may be let fly from the hand in the usual way, by paying out the string as fast as it can be taken up. When sufficiently high, a single hitch is taken round the reel, or stick, if wound on one, to prevent more string leaving it. The silken cord is now fastened to the kite-string, and the other end anchored in the ground. In cases of this kind there is no need of the reservoir. When the kite has been anchored a few minutes, the knuckle may be presented to the string, and probably a spark will be experienced. The bottle is now to be charged by applying its ball to the string, and afterwards discharged by the proper apparatus. If the charge appears high, the spiral with its enclosed needle is to be placed in the circuit as shown in the figure, and a few discharges sent through it. These will magnetize the needle : which, when presented to either pole of the compass needle in the box, will display the character of the pole presented, and this polarity of the magnetized needle will indicate the direction of the electric current through the spiral wire, according to the law of electro-magnetism already explained. If the current traversed the spiral wire from the inside to the outside of the bottle, its inside was electro-positive, and consequently the string was electro-positive, and also the air was in the same state with reference to the ground. This is the usual electric state of the atmosphere when perfectly clear and no appearance of clouds. Such an atmosphere, however, though constantly positive with reference to the ground, is much more powerfully so on some occasions than on others. I have usually found it most powerful during the sharp cold north-east winds in March and the beginning of April.
With about half a mile of string out, during a sharp breeze from the north-east, I have had a series of sparks, too rapid to be counted, through a plate of air between the reservoir and the ball of the discharger, of an inch and a half in thickness. This circumstance occurred at the Military College at Addiscombe, in March, 1824.
During the afternoon of the same day I attached the lower end of the kite-string to the back-band of another kite, and by this means got up about another quarter of a mile of string. The upper kite now floated very high, and being of a light blue colour, nearly corresponding with the colour of the sky, required a good search of the eye to find it. When the lower string was anchored the shocks which it delivered to those who approached it were exceedingly severe. About fifty of the gentlemen cadets received shocks from the string, but not more than two or three of them could be prevailed on to approach the string a second time. I experienced one of these shocks myself, and the blow was tremendous and general throughout the whole system, but most severe in the arm that received the discharge, the chest, thighs, and shin-bones. I next brought out a large Leyden jar of the capacity of three gallons, and applied its ball to the dis charger of the reservoir. The charge of the jar was rapidly accom
plished, with such an intensity as to occasion spontaneous discharges over the top. Under those favourable circumstances I made a great number of experiments. A piece of ordnance was several times fired by discharges from the jar, and twice by sparks from the string; all the sewing needles that could be mustered were magnetized ; copper and silver were revived from their solutions, and water was operated on, but only very slightly decomposed.*
I will just mention in this place, once for all, that I have occasionally found it necessary to elevate the upper end of the string by the assistance of three kites in series, in order to get even a trifling charge of the small experimenting jar.
If the kite experiments be intended to ascertain the electric state of the atmosphere at different altitudes, then it will be necessary to have four or more kites elevated with different lengths of string, in order that they may float at different altitudes. Experiments for this purpose can never give satisfactory results only under a cloudless sky, and then in all cases, as I have before stated, it will be found that the atmosphere is more and more electrical as the strata explored are more elevated.
In hot sultry weather, and especially when hazy, the atmosphere is highly charged with the electric fluid at a very low altitude. I have on some occasions found the shocks from the kite-string quite insupportable when the kite was not higher than a church steeple, and this too when the string was not insulated. Under these circumstances it is impossible to let out much string in the usual way, by paying it through the hand.
When the electric shocks are thus powerful from low altitudes, and it is desirable to get the kite higher, the best method is to bring down the kite, and when down stretch out on the ground the whole length of string intended to go up, with the kite attached at one end and the insulation perfect at the other ; also the spare string in the reservoir and the ball of the discharging piece adjusted to a moderate distance, about an inch and a half from it, with its wire stuck in the ground. Thus prepared the kite will ascend from the hand, and when elevated a while, the apparatus at anchor may be observed. If sparks be seen between the reservoir and the discharging ball the power is great, too great indeed for the operator to approach the string. If no sparks be seen the ball may be pushed a little closer until they appear, which, under the circumstances mentioned, are likely to be copiously produced. In some instances during hot hazy days, I have seen the sparks strike through more than two inches in rapid succession for more than an hour continuously. It is easy to charge a jar on such occasions, and by magnetizing a needle, to ascertain the electric state of the haze, which I have always found to be positive.
• This series of experiments have been regularly alluded to in my lectures, but never before recorded ; nor do I know of any similar series of experiments recorded by any other person.