Abbildungen der Seite
PDF
EPUB

electrified body will discharge the atmosphere | made of several thin sheets of clothier's pasteof that body or communicate it farthest to an- board, formed into a tube, near ten feet long other body, so the point of an unelectrified and a foot diamater. It is covered with body will draw off the electrical atmosphere Dutch embossed paper, almost totally gilt. This from an electrified body, farther than a blunter large metallic surface supports a much greater part of the same unelectrified body will do. electrical atmosphere than a rod of iron of 50 Thus, a pin held by the head, and the point times the weight would do. It is suspended presented to an electrified body, will draw off by silk lines, and when charged will strike, its atmosphere at a foot distance; where, if at near two inches distance, a pretty hard the head were presented instead of the point, stroke, so as to make ones knuckle ache. no such effect would follow. To understand Let a person standing on the floor present the this, we may consider, that if a person stand-point of a needle at 12 or more inches dising on the floor would draw off the electrical tance from it, and while the needle is so preatmosphere from an electrified body, an iron sented, the conductor cannot be charged, the crow and a blunt knitting-needle held alter-point drawing off the fire as fast as it is thrown nately in his hand, and presented for that pur- on by the electrical globe. Let it be chargpose, do not draw with different forces in pro-ed, and then present the point at the same portion to their different masses. For the man, and what he holds in his hand, be it large or small, are connected with the common mass of unelectrified matter; and the force with which he draws is the same in both cases, it consisting in the different proportion of electricity in the electrified body, and that common mass. But the force with which the electrified body retains its atmosphere by at-iron punch, inch thick is what I use) and tracting it, is proportioned to the surface over which the particles are placed; i. e. four square inches of that surface retain their atmosphere with four times the force that one square inch retains its atmosphere. And as in plucking the hairs from the horse's tail, a degree of strength not sufficient to pull away a handful at once, could yet easily strip it hair by hair so a blunt body presented cannot draw off a number of particles at once, but a pointed one, with no greater force, takes them away easily, particle by particle.

18. These explanations of the power and operation of points, when they first occurred to me, and while they first floated in my mind, appeared perfectly satisfactory; but now I have written them, and considered them more closely, I must own I have some doubts about them; yet, as I have at present nothing better to offer in their stead, I do not cross them out for, even a bad solution read, and its faults discovered, has often given rise to a good one, in the mind of an ingenious reader.

19. Nor is it of much importance to us to know the manner in which nature executes her laws; it is enough if we know the laws themselves. It is of real use to know that china left in the air unsupported will fall and break; but how it comes to fall, and why it breaks, are matters of speculation. It is a pleasure indeed to know them, but we can preserve our china without it.

20. Thus in the present case, to know this power of points may possibly be of some use to mankind, though we should never be able to explain it. The following experiments, as well as those in my first paper, show this power. I have a large prime conductor,

distance, and it will suddenly be discharged. In the dark you may see the light on the point, when the experiment is made. And if the person holding the point stands upon wax, he will be electrified by receiving the fire at that distance. Attempt to draw off the electricity with a blunt body, as a bolt of iron round at the end, and smooth (a silversmith's

you must bring it within the distance of three inches before you can do it, and then it is done with a stroke and crack. As the pasteboard tube hangs loose on silk lines, when you approach it with the punch-iron, it likewise will move towards the punch, being attracted while it is charged; but if, at the same instant, a point be presented as before, it retires again, for the point discharges it. Take a pair of large brass scales, of two or more feet beam, the cords of the scales being silk. Suspend the beam by a packthread from the ceiling, so that the bottom of the scales may be about a foot from the floor; the scales will move round in a circle by the untwisting of the packthread. Set the iron punch on the end upon the floor, in such a place as that the scales may pass over it in making their circle; then electrify one scale, by applying the wire of a charged phial to it. As they move round, you see that scale draw nigher to the floor, and dip more when it comes over the punch; and if that be placed at a proper distance, the scale will snap and discharge its fire into it. But if a needle be stuck on the end of the punch, its point upwards, the scale, instead of drawing nigh to the punch, and snapping, discharges its fire silently through the point, and rises higher from the punch. Nay, even if the needle be placed upon the floor near the punch, its point upwards, the end of the punch, though so much higher than the needle, will not attract the scale and receive its fire, for the needle will get it and convey it away, before it comes nigh enough for the punch to act. And this is constantly observable in these experiments, that the greater quantity of electricity on the pasteboard tube, the far

ther it strikes or discharges its fire, and the | handle; so the sparks, if the rod is electrified, point likewise will draw it off at a still great- will strike from the rod to the wire, and not er distance. affect him.

between the effects of that, and those of electricity. Lightning has often been known to strike people blind. A pigeon that we struck dead to appearance by the electrical shock, recovering life, drooped about the yard several days, eat nothing, though crumbs were thrown to it, but declined and died. We did not think of its being deprived of sight; but afterwards a pullet, struck dead in like manner, being recovered by repeatedly blowing into its lungs, when set down on the floor, ran headlong against the wall, and on examination appeared perfectly blind. Hence we concluded that the pigeon also had been absolutely blinded by the shock. The biggest animal we have yet killed, or tried to kill, with the electrical stroke, was a well grown pullet.

Now if the fire of electricity and that of 22. Before I leave this subject of lightlightning be the same, as I have endeavour-ning, I may mention some other similarities ed to show at large, in a former paper, this pasteboard tube and these scales may represent electrified clouds. If a tube of only ten feet long will strike and discharge its fire on the punch at two or three inches distance, an electrified cloud of perhaps 10,000 acres may strike and discharge on the earth at a proportionably greater distance. The horizontal motion of the scales over the floor, may represent the motion of the clouds over the earth; and the erect iron punch, a hill or high building; and then we see how electrified clouds passing over hills or high buildings at too great a height to strike, may be attracted lower till within their striking distance. And lastly, if a needle fixed on the punch with its point upright, or even on the floor below the punch, will draw the fire from the scale silently at a much greater than the striking distance, and so prevent its descending towards the punch; or if in its course it would have come nigh enough to strike, yet being first deprived of its fire it cannot, and the punch is thereby secured from the stroke; I say, if these things are so, may not the knowledge of this power of points be of use to mankind, in preserving houses, churches, ships, &c. from the stroke of lightning, by directing us to fix on the highest parts of those edifices, upright rods of iron made sharp as a needle, and gilt to prevent rusting, and from the foot of those rods a wire down the outside of the building into the ground, or down round one of the shrouds of a ship, and down her side till it reaches the water? Would not these pointed rods probably draw the electral fire silently out of a cloud before it came nigh enough to strike, and thereby secure us from that most sudden and terrible mischief?

21. 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 centry-box (as in Fig. 9) 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 20 or 30 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 a 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

23. Reading in the ingenious Dr. Miles's account of the thunder-storm at Streatham, the effect of the lightning in stripping off all the paint that had covered a gilt moulding of a pannel of wainscot, without hurting the rest of the paint, I had a mind to lay a coat of paint over the filleting of gold on the cover of a book, and try the effects of a strong electrical flash sent through that gold from a charged sheet of glass. But having no paint at hand, I pasted a narrow strip of paper over it; and when dry, sent the flash through the gilding, by which the paper was torn off from end to end, with such force, that it was broke in several places, and in others brought away part of the grain of the Turkey-leather in which it was bound; and convinced me, that had it been painted, the paint would have been stripped off in the same manner with that on the wainscot at Streatham.

24. Lightning melts metals, and I hinted in my paper on that subject, that I suspected it to be a cold fusion; I do not mean a fusion by force of cold, but a fusion without heat.* We have also melted gold, silver, and copper, in small quantities, by the electrical flash. The manner is this: take leaf-gold, leaf-silver, or leaf-gilt copper, commonly called leafbrass, or Dutch gold; cut off from the leaf long narrow strips, the breadth of a straw. Place one of these strips between two strips of smooth glass that are about the width of your finger. If one strip of gold, the length of the leaf, be not long enough for the glass, add another to the end of it, so that you may have a little part hanging out loose at each end of the glass. Bind the pieces of glass together from end to end with strong silk thread; then place it so as to be part of an electrical circuit, (the ends of gold hanging out being

See the note in page 257.

of use to join with the other parts of the cir- tricity, that points as they are more or less cuit) and send the flash through it, from a acute, draw on and throw off the electrical large electrified jar or sheet of glass. Then fluid with more or less power, and at greater if your strips of glass remain whole, you will or less distances, and in larger or smaller see that the gold is missing in several places, quantities in the same time we may see how and instead of a metallic stain on both the to account for the situation of the leaf of gold glasses; the stains on the upper and under suspended between two plates, the upper one glass exactly similar in the minutest stroke, continually electrified, the under one in a as may be seen by holding them to the light; person's hand standing on the floor. When the metal appeared to have been not only the upper plate is electrified, the leaf is atmelted, but even vitrified, or otherwise so tracted, and raised towards it, and would fly driven into the pores of the glass, as to be to that plate, were it not for its own points. protected by it from the action of the strong- The corner that happens to be uppermost est aqua fortis, or aqua regia. I send you en- when the leaf is rising, being a sharp point, closed two little pieces of glass with these me- from the extreme thinness of the gold, draws tallic stains upon them, which cannot be re- and receives at a distance a sufficient quantity moved without taking part of the glass with of the electric fluid to give itself an electric them. Sometimes the stain spreads a little atmosphere, by which its progress to the upwider than the breadth of the leaf, and looks per plate is stopped, and it begins to be repelbrighter at the edge, as by inspecting closely led from that plate, and would be driven back you may observe in these. Sometimes the to the under plate, but that its lowest corner glass breaks to pieces; once the upper glass is likewise a point, and throws off or disbroke into a thousand pieces, looking like charges the overplus of the leaf's atmosphere, coarse salt. The pieces I send you were as fast as the upper corner draws it on. Were stained with Dutch gold. True gold makes those two points perfectly equal in acuteness, a darker stain, somewhat reddish; silver, a the leaf would take place exactly in the middle greenish stain. We once took two pieces of space, for its weight is a trifle compared to thick looking-glass, as broad as a Gunter's the power acting on it: but it is generally scale, and six inches long; and placing leaf-nearest the unelectrified plate, because, when gold between them, put them between two the leaf is offered to the electrified plate, ata smoothly-plained pieces of wood, and fixed distance, the sharpest point is commonly first them tight in a bookbinder's small press; yet affected and raised towards it; so that point, though they were so closely confined, the from its greater acuteness, receiving the fluforce of the electrical shock shivered the id faster than its opposite can discharge it at glass into many pieces. The gold was melt-equal distances, it retires from the electrified ed and stained into the glass, as usual. The circumstances of the breaking of the glass differ much in making the experiment, and sometimes it does not break at all: but this is constant, that the stains in the upper and under pieces are exact counterparts of each other. And though I have taken up the pieces of glass between my fingers immediately after this melting, I never could perceive the least warmth in them.

plate, and draws nearer to the unelectrified plate, till it comes to a distance where the discharge can be exactly equal to the receipt, the latter being lessened, and the former increased; and there it remains as long as the globe continues to supply fresh electrical matter. This will appear plain, when the difference of acuteness in the corners is made very great. Cut a piece of Dutch gold, (which is fittest for these experiments on account of its great strength) into the form of Fig. 10, the upper corner a right angle, the two next obtuse angles, and the lowest a very acute one; and bring this on your plate under the electrified plate, in such a manner as that the right-angled part may be first raised (which is done by covering the acute part with the hollow of your hand) and you will see this leaf take place much nearer to the upper than the under plate; because without being nearer, it cannot receive so fast at its right-angled point, as it can discharge at its acute one. Turn this leaf with the acute part uppermost, and then it takes place nearest the unelectrified plate; because, otherwise, it receives faster at its acute point, than it can discharge at its right-angled one. Thus the difference of distance is always proportioned 26. From the before-mentioned law of elec-to the difference of acuteness. Take care in

25. In one of my former papers, I mentioned, that gilding on a book, though at first it communicated the shock perfectly well, yet failed after a few experiments, which we could not account for. We have since found that one strong shock breaks the continuity of the gold in the filletting, and makes it look rather like dust of gold, abundance of its parts being broken and driven off; and it will seldom conduct above one strong shock. Perhaps this may be the reason: when there is not a perfect continuity in the circuit, the fire must leap over the vacancies; there is a certain distance which it is able to leap over according to its strength; if a number of small vacancies, though each be very minute, taken together exceed that distance, it cannot leap over them, and so the shock is prevented.

[merged small][ocr errors]

cutting your leaf, to leave no little ragged particles on the edges, which sometimes form points where you would not have them. You may make this figure so acute below, and blunt above, as to need no under plate, it discharging fast enough into the air. When it is made narrower, as the figure between the pricked lines, we call it the golden fish, from its manner of acting. For if you take it by the tail, and hold it at a foot or greater horizontal distance from the prime conductor, it will, when let go, fly to it with a brisk but wavering motion, like that of an eel through the water; it will then take place under the prime conductor, at perhaps a quarter or half an inch distance, and keep a continual shaking of its tail like a fish, so that it seems animated. Turn its tail towards the prime conductor, and then it flies to your finger, and seems to nibble it. And if you hold a plate under it at six or eight inches distance, and cease turning the globe when the electrical atmosphere of the conductor grows small, it will descend to the plate and swim back again several times with the same fish-like motion, greatly to the entertainment of spectators. By a little practice in blunting or sharpening the heads or tails of these figures, you may make them take place as desired, nearer or farther from the electrified plate.

27. It is said in section 8, of this paper, that all kinds of common matter are supposed not to attract the electrical fluid with equal strength; and that those called electrics per se, as glass, &c. attract and retain it strongest, and contain the greatest quantity. This latter position may seem a paradox to some, being contrary to the hitherto received opinion; and therefore I shall now endeavour to explain it.

28. In order to this, let it first be considered, that we cannot by any means we are yet acquainted with, force the electrical fluid through glass. know it is commonly thought that it easily pervades glass; and the experiment of a feather suspended by a thread in a bottle hermetically sealed, yet moved by bringing a rubbed tube near the outside of the bottle is alleged to prove it. But, if the electrical fluid so easily pervades glass, how does the phial become charged (as we term it) when we hold it in our hands? Would not the fire, thrown in by the wire, pass through to our hands, and so escape into the floor? Would not the bottle in that case be left just as we found it, uncharged, as we know a metal bottle so attempted to be charged would be? Indeed, if there be the least crack, the minutest solution of continuity in the glass, though it remains so tight that nothing else we know of will pass, yet the extremely subtile electric fluid flies through such a crack with the greatest freedom, and such a bottle we know can never be charged: what then

makes the difference between such a bottle and one that is sound, but this, that the fluid can pass through the one, and not through the other?*

29. It is true, there is an experiment that at first sight would be apt to satisfy a light observer, that the fire, thrown into the bottle by the wire, does really pass through the glass. It is this: place the bottle on a glass stand, under the prime conductor, suspend a bullet by a chain from the prime conductor, till it comes within a quarter of an inch right over the wire of the bottle; place your knuckle on the glass stand, at just the same distance from the coating of the bottle, as the bullet is from its wire. Now let the globe be turned, and you see a spark strike from the bullet to the wire of the bottle, and the same instant you see and feel an exactly equal spark striking from the coating on your knuckle, and so on, spark for spark. This looks as if the whole received by the bottle was again discharged from it. And yet the bottle by this means is charged! And therefore the fire that thus leaves the bottle, though the same in quantity, cannot be the very same fire that entered at the wire, for if it were, the bottle would remain uncharged.

30. If the fire that so leaves the bottle be not the same that is thrown in through the wire, it must be fire that subsisted in the bottle (that is, in the glass of the bottle) before the operation began.

31. If so, there must be a great quantity in glass, because a great quantity is thus discharged, even from very thin glass.

32. That this electrical fluid or fire is strongly attracted by glass, we know from the quickness and violence with which it is resumed by the part that had been deprived of it, when there is an opportunity. And by this, that we cannot from a mass of glass draw a quantity of electric fire, or electrify the whole mass minus, as we can a mass of metal. We cannot lessen or increase its whole quantity, for the quantity it has it holds; and it has as much as it can hold. Its pores are filled with it as full as the mutual repellency of the particles will admit; and what is already in, refuses, or strongly repels any additional quantity. Nor have we any way of moving the electrical fluid in glass, but one; that is, by covering part of the two surfaces of thin glass with nonelectrics, and then throwing an additional quantity of this fluid on one surface, which spreading in the non-electric, and being bound by it to that surface, acts by its repelling force on the particles of the electrical fluid contained in the other surface, and drives them out of the glass into the non-electric on that side from whence they are discharged, and then

* See the first sixteen sections of the former paper, called Farther Experiments, &c.

† See sect. 10, of Farther Experiments, &c.

those added on the charged side can enter. But when this is done, there is no more in the glass, nor less than before, just as much having left it on one side as it received on the

other.

The quantities of this fluid in each surface being equal, their repelling action on each other is equal; and therefore those of one surface cannot drive out those of the other; but, if a greater quantity is forced into one surface than the glass would naturally draw in, this increases the repelling power on that side, and overpowering the attraction on the other, drives out part of the fluid that had been imbibed by that surface, if there be any nonelectric ready to receive it: such there is in all cases where glass is electrified to give a shock. The surface that has been thus emptied, by having its electrical fluid driven out, resumes again an equal quantity with violence, as soon as the glass has an opportuni ty to discharge that over quantity more than it could retain by attraction in its other surface, by the additional repellency of which the vacuum had been occasioned. For experiments favouring (if I may not say confirming) this hypothesis, I must, to avoid repetition, beg leave to refer you back to what is said of the electrical phial in my former pages.

33. I feel a want of terms here, and doubt much whether I shall be able to make this part intelligible. By the word surface, in this case, I do not mean mere length and breadth without thickness; but when I speak of the upper or under surface of a piece of glass, the outer or inner surface of the phial, I mean length, breadth, and half the thickness, and beg the favour of being so understood. Now I suppose, that glass in its first principles, and in the furnace, has no more of this electrical fluid than other common matter: that when it is blown, as it cools, and the particles of common fire leave it, its pores become a vacuum: that the component parts of glass are extremely small and fine, I guess from its never showing a rough face when it breaks, but always a polish; and from the smallness of its particles I suppose the pores between them must be exceedingly small, which is 33. Let us now see how it will account for the reason that aqua fortis, nor any other several other appearances.-Glass, a body exmenstruum we have, can enter to separate tremely elastic, (and perhaps its elasticity may them and dissolve the substance; nor is any be owing in some degree to the subsisting of fluid we know of, fine enough to enter, except so great a quantity of this repelling fluid in common fire, and the electric fluid. Now the its pores) must, when rubbed, have its rubbed departing fire, leaving a vacuum, as aforesaid, surface somewhat stretched, or its solid parts between these pores, which air nor water are drawn a little farther asunder, so that the fine enough to enter and fill, the electric fluid vacancies in which the electrical fluid resides, (which is every where ready in what we call become larger, affording room for more of that the non-electrics, and in the non-electric mix- fluid, which is immediately attracted into it tures that are in the air) is attracted in; yet from the cushion or hand rubbing, they being does not become fixed with the substance of the supplied from the common stock. But the glass, but subsists there as water in a porous instant the parts of the glass so opened and stone, retained only by the attraction of the filled, have passed the friction, they close fixed parts, itself still loose and a fluid. But again, and force the additional quantity out I suppose farther, that in the cooling of the upon the surface, where it must rest till that glass, its texture becomes closest in the mid-part comes round to the cushion again, unless dle, and forms a kind of partition, in which the pores are so narrow, that the particles of the electrical fluid, which enter both surfaces at the same time, cannot go through, or pass and repass from one surface to the other, and so mix together; yet, though the particles of electric fluid, imbibed by each surface, cannot themselves pass through to those of the other, their repellency can, and by this means they act on one another. The particles of the electric fluid have a mutual repellency, but by the power of attraction in the glass they are condensed or forced nearer to each other. When the glass has received, and, by its attraction, forced closer together so much of this electric fluid, as that the power of attracting *In the dark the electric fluid may be seen on the and condensing in the one, is equal to the cushion in two semi-circles or half-moons, one on the power of expansion in the other, it can im- fore part, the other on the back part of the cushions just bibe no more, and that remains its constant crescent the fire is passing out of the cushion into the where the globe and cushion separate. In the fore whole quantity; but each surface would re-glass; in the other it is leaving the glass, and returning ceive more, if the repellency of what is in into the back part of the cushion. When the prime the opposite surface did not resist its entrance.orescent disappears. conductor is applied to take it off the glass, the back

some non-electric (as the prime conductor,) first presents to receive it.* But if the inside of the globe be lined with a non-electric, the additional repellency of the electrical fluid, thus collected by friction on the rubbed part of the globe's outer surface, drives an equal quantity out of the inner surface into that nonelectric lining, which receiving it, and carrying it away from the rubbed part into the common mass, through the axis of the globe, and frame of the machine, the new collected electrical fluid can enter and remain in the outer surface, and none of it (or a very little) will be received by the prime conductor. As this charged part of the globe comes round to

« ZurückWeiter »