Imágenes de página


as often to make it greater than the striking | every help to obtain, even from broken partial distance. Turning the blunt end of a wire metalline conductors. uppermost (which represents the unpointed It has also been suggested, that from such bar) it appears that the same good effect is electric experiments nothing certain can be not from that to be expected. A long pointed concluded as to the great operations of narod, it is therefore imagined, may prevent some ture ; since it is often seen, that experiments strokes; as well as conduct others that fall which have succeeded in small, in large have upon it, when a great body of cloud comes on failed. It is true that in mechanics this has so heavily that the above repelling operation sometimes happened. But when it is consion fragments cannot take place.

dered that we owe our first knowledge of the

nature and operations of lightning, to observaEXPERIMENT VI.

tions on such small experiments; and that on Opposite the side of the prime conductor, carefully comparing the most accurate acplace separately isolated by wax stems, Mr. counts of former facts, and the exactest relaCanton's two boxes with pith balls suspended tions of those that have occurred since, the by fine linen threads. On each box, lay a effects have surprisingly agreed with the theowire six inches long and one fifth of an inch ry; it is humbly conceived that in natural thick, tapering to a sharp point; but so laid philosophy, in this branch of it at least, the as that four inches of the pointed end of one suggestion has not so much weight; and that wire, and an equal length of the blunt end of the farther new experiments now adduced in the other, may project beyond the ends of the recommendation of long sharp-pointed rods, boxes; and both at eighteen inches distance may have some claim to credit and considerafrom the prime conductor. Then charging tion. the prime conductor by a turn or two of the It has been urged too, that though points globe, the balls of each pair will separate ; may have considerable effects on a small prime those of the box, whence the point projects conductor at small distances; yet on great most, considerably; the others less. Touch clouds and at great distances, nothing is to be the prime conductor, and those of the box with expected from them. To this it is answered, the blunt point will collapse, and join. Those that in those small experiments it is evident connected with the point will at the same the points act at a greater than the striking time approach cach other, till within about an distance; and in the large way, their service inch, and there remain.

is only expected where there is such nearness of the cloud, as to endanger a stroke ; and

there, it cannot be doubted the points must This seems a proof, that though the stall have some effect. And if the quantity dissharpened part of the wire must have had a charged by a single pointed rod may be so less natural quantity in it, before the opera considerable as I have shown it; the quantity tion, than the thick blunt part; yet a greater discharged by a number will be proportionably quantity was driven down from it to the balls. greater. Thence it is again inferred, that the pointed But this part of the theory does not depend rod is rendered more negative : and farther, alone on small experiments. Since the practhat if a stroke must fall from the cloud over tice of erecting pointed rods in America (now a building, furnished with such a rod, it is near twenty years) five of them have been more likely to be drawn to that pointed rod, struck by lightning, viz. Mr. Raven's and than to a blunt one ; as being more strongly Mr. Maine's, in South Carolina ; Mr. Tucknegative, and of course its attraction stronger. er's, in Virginia; Mr. West's and Mr. MoulAnd it seems more eligible, that the lightning der's, in Philadelphia. Possibly there may should fall on the point of the conductor (pro- have been more that have not come to my vided to convey it into the earth) than on any knowledge. But in every one of these, the other part of the building, thence to proceed lightning did not fall upon the body of the to such conductor: which end is also more house, but precisely on the several points of likely to be obtained by the length and lofti- the rods; and, though the conductors were ness of the rod; as protecting more exten- sometimes not sufficiently large and comsively the building under it.

plete, was conveyed into the earth, without It has been objected, that erecting pointed any material damage to the buildings. Facts rods upon edifices, is to invite and draw the then in great, as far as we have them aulightning into them; and therefore dangerous. thenticated, justify the opinion that is drawn Were such rods to be erected on buildings, from the experiments in small as above without continuing the communication quite related. down into the moist earth, this objection might It has also been objected, that unless we then have weight; but when such complete knew the quantity that might possibly be disconductors are made, the lightning is invited charged at one stroke from the clouds, we not into the building, but into the earth, the cannot be sure we have provided sufficient situation it aims at, and which it always seizes I conductors; and therefore cannot depend on


their conveying away all that may fall on power in the clouds of collecting it; yet an their points. Indeed we have nothing to form accumulation and force beyond what mankind a judgment by in this but past facts; and we has hitherto been acquainted with is scarce to know of no instance where a complete con- be expected.*

B. F. ductor to the moist earth has been insuffi August 27, 1772. cient, if half an inch diameter. It is probable that many strokes of lightning have been conveyed through the common leaden pipes af

To Professor Landriani, Italy. fixed to houses to carry down the water from On the Utility of Electrical Conductors. the roof to the ground: and there is no ac

PHILADELPHIA, Oct. 14, 1787. count of such pipes being melted and destroy

I HAVE received the excellent work upon ed, as must sometimes have happened if they the Utility of Electrical Conductors, which had been insufficient. We can then only you had the goodness to send me. I read it judge of the dimensions proper for a conductor with great pleasure, and beg you to accept my conductor of rain, by past observation. And sincere thanks for it. as we think a pipe of three inches bore suf- the number of conductors much increased,

Upon my return to this country, I found ficient to carry off the rain that falls on a square of 20 feet, because we never saw such many, proofs of their efficacy in preserving a pipe glutted by any shower; so we may their utility. Among other instances, my

buildings from lightning having demonstrated judge a conductor of an inch diameter, more than sufficient for any stroke of lightning that own house was one day attacked by lightning, will fall on its point. It is true, that if an- which occasioned the neighbours to run in to other deluge should happen wherein the win. But no damage was done, and my family was

give assistance, in case of its being on fire. dows of heaven are to be opened, such pipes only found a good deal frightened with the viomay be unequal to the falling quantity; and lence of the explosion. if God for our sins should think fit to rain fire upon us, as upon some cities of old, it is not conductor was obliged to be taken down. I

Last year, my house being enlarged, the size, should secure our houses against a mi- found, upon examination, that the pointed racle. Probably as water drawn up into the

termination of copper, which, was originally air and there forming clouds, is disposed to inch in diameter in its thickest part, had

nine inches long, and about one third of an fall again in rain by its natural gravity, as been almost entirely melted; and that its soon as a number of particles sufficient to connexion with the rod of iron below was make a drop can get together; so when the clouds are (by whatever means), over or un- this invention has proved of use to the au

very slight. Thus, in the course of time, dercharged with the electric fluid, to a degree thor of it, and has added this personal adsufficient to attract them towards the earth, the equilibrium is restored, before the differ- vantage to the pleasure he before received,

from having been useful to others. ence becomes great beyond that degree.

Mr. Rittenhouse, our astronomer, has inMr. Lane's electrometer, for limiting precise formed me, that having observed with his ly the quantity of a shock that is to be ad- excellent telescope, many conductors that are ministered in a medical view, may serve to within the field of his view, he has remarkmake this more easily intelligible. The dis- ed in various instances, that the points were charging knob does by a screw approach the melted in like manner. There is no example conductor to the distance intended, but there of a house, provided with a perfect conductremains fixed. Whatever power there may be in the glass globe to collect the fulminat

* The immediate occasion of the dispute concerning ing fluid, and whatever capacity of receiving the preference between pointed and blunt conductors and accumulating it there may be in the bottle of lightning, arose as follows:-A powder-mill having or glass jar; yet neither the accumulation with lightning, the English board of ordinance applied nor the discharge ever exceeds the destined to their painter, Mr. Wilson, then of some note as an quantity. Thus, were the clouds always at electrician, for a method to prevent the like accident to a certain fixed distance from the earth, all vised a blunt conductor, and it being understood that discharges would be made when the quantity Dr. Franklin's opinion formed upon the spot, was for a accumulated was equal to the distance: but Royal Society, and by them as usual, to a committee, there is a circumstance which by occasionally who, after consultation, prescribed a method conform lessening the distance, lessens the discharge; lightning, having under particular circumstances, fallen to wit, the moveableness of the clouds, and upon one of the buildings and its apparatus in May their being drawn nearer to the earth by at- 1177; the subject came again into violent agitation, traction when electrified; so that discharges again referred to a new committee, which committee are thereby rendered more frequent and of confirmed the decision of the first committee; it procourse less violent. Hence whatever the duced an acrimonious controversy in the Royal Society,

and a series of pamphlets; which, however, ended in quantity may be in nature, and whatever the I the triumph of the Franklinian theory.


or, which has suffered any considerable da- | been accompanied with proper medicine and mage; and even those which are without regimen, under the direction of a skilful phythem have suffered little, since conductors sician. It may be, too, thata few great strokes, have become common in this city.

as given in my method, may not be so proper B. FRANKLIN. as many small ones; since by the account

from Scotland of a case, in which two hun

dred shocks from a phial were given daily, it John Pringle, M. D.

seems, that a perfect cure has been made. As On the Efects of Electricity in Paralytic Cases. to any uncommon strength supposed to be in

CRAVEX-STREET, Dec. 21, 1757. the machine used in that case, I imagine it In compliance with your request, I send could have no share in the effect produced; you the following account of what I can at since the strength of the shock from charged present recollect relating to the effects of elec- glass, is in proportion to the quantity of surtricity in paralytic cases, which have fallen face of the glass coated : so that my shock under my observation.

from those large jars, must have been much Some years since, when the newspapers greater than any that could be received from made mention of great cures performed in Ita- a phial held in the hand. ly and Germany, by means of electricity, a

B. FRANKLIN. number of paralytics were brought to me from different parts of Pennsylvania, and the neigh

Electrical Experiments on Amber. bouring provinces, to be electrised, which I

Saturday, July 3, 1762. did for them at their request. My method To try, at the request of a friend, whether was, to place the patient first in a chair, on amber finely powdered might be melted and an electric stool, and draw a number of large run together again by means of the electric strong sparks from all parts of the affected Auid, † took a piece of small glass tube, about limb or side. Then I fully charged two six- two inches and a half long, the bore about one gallon glass jars, each of which had about twelfth of an inch diameter, the glass itself three square feet of surface coated; and sent about the same thickness; I introduced into this the united shock of these through the affect- tube some powder of amber, and with two ed limb or limbs, repeating the stroke com- pieces of wire nearly fitting the bore, one inmonly three times each day. The first thing serted at one end, the other at the other, I observed, was an immediate greater sensible rammed the powder hard between them in warmth in the lame limbs that had received the middle of the tube, where it stuck fast, the stroke, than in the others; and the next and was in length about half an inch. Then morning the patients usually related, that leaving the wires in the tube, I made them they had in the night felt a pricking sensa- part of the electric circuit, and discharged tion in the flesh of the paralytic limbs ; and through them three rows of my case of botwould sometimes show a number of small tles. The event was, that the glass was red spots, which they supposed were occasion- broke into very small pieces, and those dis ed by those prickings. The limbs, too, were persed with violence in all directions. As I found more capable of voluntary motion, and did not expect this, I had not, as in other exseemed to receive strength. A man, for in- periments, laid thick paper over the glass to stance, who could not the first day lift the lame save my eyes, so several of the pieces struck hand from off his knee, would the next day my face smartly, and one of them cut my lip raise it four or five inches, the third day higher; a little so as to make it bleed. I could find and on the fifth day was able, but with a fee- no part of the amber; but the table where ble languid motion, to take off his hat. These the tube lay was stained very black in spots, appearances gave great spirits to the patients, such as might be made by a thick smoke and made them hope a perfect cure; but I do forced on it by a blast, and the air was filled not remember that I ever saw any amendment with a strong smell, somewhat like that from after the fifth day; which the patients per- burnt gunpowder. Whence I imagined, that ceiving, and finding the shocks pretty severe, the amber was burnt, and had exploded as they became discouraged, went home, and in gunpowder would have done in the same cira short time relapsed, so that I never knew cumstances, any advantage from electricity in palsies that That I might better see the effect on the was permanent. And how far the apparent amber, I made the next experiment in a tube temporary advantage might arise from the ex- formed of a card rolled up and bound strongly ercise in the patients' journey, and coming with packthread. Its bore was about one daily to my house, or from the spirits given eighth of an inch diameter. I rammed powby the hope of success, enabling them to ex- der of amber into this as I had done in the ert more strength in moving their limbs, I other, and as the quantity of amber was will not pretend to say.

greater, I increased the quantity of electric Perhaps some permanent advantage might Avid by discharging through it at once five have been obtained, if the electric shocks had rows of my bottles. On opening the tube, I

found that some of the powder had exploded, at about the distance of three or four inches an impression was made on the tube, though from that part which is most distant from the it was not hurt, and most of the 'powder re- balls, and you will see the balls maining was turned black, which I suppose each other, being positively electrified by the

separate from might be by the smoke forced through it from natural portion of electricity, which was in the burned part: some of it was hard; but as the box, and which is driven to the further part it powdered again when pressed by the fingers, of it by the repulsive power of the atmosphere I suppose that hardness not to arise from melt- in the excited glass. Touch the box near the ing any parts in it, but merely from my ram- little balls (the excited glass continuing in the ming the powder when I charged the tube. same state) and the balls will again unite;


the quantity of electricity which had been driven to this part being drawn off by your

finger. Withdraw then both your finger and To Thomas Ronayne, Esq. Cork, Ireland. the glass at the same instant, and the quantity On the Electricity of the Fogs in Ireland.

of electricity which remained in the box,

uniformly diffusing itself, the balls will again LONDON, April 20. 1766. be separated; being now in a negative state. I HAVE received your very obliging and very While things are in this situation, begin once ingenious letter by captain Kearney. Your more to excite your glass, and hold it above observations upon the electricity of fogs, and the box, but not too near, and you will find, the air in Ireland, and upon different circum- that when brought within a certain distance, stances of storms, appear to me very curious, the balls will at first approach each other, beand I thank you for them. There is not, in ing then in a natural state. In proportion my opinion, any part of the earth whatever, as the glass is brought nearer, they will which is, or can be, naturally in a state of ne- again separate, being positive. When the gative electricity: and though different cir- glass is moved beyond them, and at some litcumstances may occasion an inequality in the tle further distance, they will unite again, bedistribution of the fluid, the equilibrium is im- ing in a natural state. When it is entirely mediately restored by means of its extreme removed, they will separate again, being then subtlety, and of the excellent conductors with made negative. The excited glass in this which the humid earth is amply provided. I experiment may represent a cloud positively am of opinion, however, that when a cloud, charged, which you see is capable of producwell charged positively, passes near the earth, ing in this manner all the different changes it repels and forces down into the earth, that in the apparatus, without the least neces

cessity natural portion of electricity, which exists for supposing any negative cloud. near its surface, and in buildings, trees, &c. I am nevertheless fully convinced, that so as actually to reduce them to a negative there are negative clouds; because they somestate before it strikes them. I am of opinion times absorb, through the medium of the aptoo, that the negative state in which you have paratus, the positive electricity of a large jar, frequently found the balls, which are suspend- the hundredth part of which the apparatus ed from your apparatus, is not always occa- itself would have not been able to receive or sioned by clouds in a negative state; but contain at once. In fact, it is not difficult to more commonly by clouds positively electri- conceive, that a large cloud, highly charged fied, which have passed over them, and which positively, may reduce smaller clouds to a in their passage have repelled and driven off a negative state, when it passes above or near part of the electrical matter, which naturally them, by forcing a part of their natural porexisted in the apparatus; so that what remain-tion of the fluid either to their inferior sured after the passing of the clouds, diffusing faces, whence it may strike into the earth, or itself uniformly through the apparatus, the to the opposite side, whence it may strike inwhole became reduced to a negative state. to the adjacent clouds ; so that when the large

If you have read my experiments made in cloud has passed off to a distance, the small continuation of those of Mr. Canton, you will clouds shall remain in a negative state, exreadily understand this, but you may easily actly like the apparatus; the former (like the make a few experiments, which will clearly latter) being frequently insulated bodies, havdemonstrate it. Let a common glass be warm- ing communication neither with the earth nor ed before the fire that it may continue very with other clouds. Upon the same principle dry for some time; set it upon a table, and it may easily be conceived, in what manner place upon it the small box made use of by a large negative cloud may render others po Mr. Canton, so that the balls may hang a lit- sitive. tle beyond the edge of the table. Rub ano The experiment which you mention, of ther glass, which has previously been warm- filling your glass, is analogous to one which ed in a similar manner, with a piece of black I made in 1751 or 1752. I had supposed in silk or silk handkerchief, in order to electrify my preceding letters, that the pores of glass it. Hold then the glass above the little box, I were smaller in the interior parts than near

the surface, and that' on this account they plate touches the upper part of the fish, with a prevented the passage of the electrical fluid. metal rod : then observe, if the force of the To prove whether this was actually the case shock be the same as to all the persons formor not, I ground one of my phials in a part ing the circle, or is stronger than before. where it was extremely thin, grinding it con Repeat this experiment with this differsiderably beyond the middle, and very near ence: let two or three of the persons forming to the opposite superfices, as I found, upon the circle, instead of holding by the hand, breaking it after the experiment. It was hold each an uncharged electrical bottle, so charged nevertheless after being ground, that the little balls at the end of the wires equally well as before, which convinced me, may touch, and observe, after the shock, if that my hypothesis on this subject was erro- these wires will attract and repel light bodies, neous. It is difficult to conceive where the and if a ball of cork, suspended by a long silk immense superfluous quantity of electricity on string between the wires, a little distance from the charged side of a glass is deposited. the bottles, will be alternately attracted and

I send you my paper concerning meteors, repelled by them.
which was lately published here in the Phi-
losophical Transactions, immediately after a
paper by Mr. Hamilton on the same subject.

T6 M. Dubourg,

On the Analogy between Magnetism and Elec

tricity. Mode of ascertaining, whether the Power,

LONDON, March 10, 1773. giving a Shock to those who touch either

As to the magnetism, which seeins prothe Surinam Eel, or the Torpedo, be elec- duced by electricity, my real opinion is that trical.

these two powers of nature have no affinity

with each other, and that the apparent pro1. Touch the fish with a stick of dry seal- duction of magnetism is purely accidental. ing-wax, or a glass rod, and observe if the The matter may be explained thus: shock be communicated by means of those

1st, The earth is a great magnet. bodies.

2dly, There is a subtle fluid, called the Touch the same fish with an iron, or other magnetic fluid, which exists in all ferrugimetalline rod.

nous bodies, equally attracted by all their parts, If the shock be communicated by the latter and equally diffused through their whole subbody, and not by the others, it is probably not stance; at least where the equilibrium is not the mechanical effect, as has been supposed, of disturbed by a power superior to the attracsome muscular action in the fish, but of a sub- tion of the iron. tle fluid, in this respect analogous at least to

3dly, This natural quantity of the magnetic the electric fluid.

fluid, which is contained in a given piece of 2. Observe farther, whether the shock can iron, may be put in motion so as to be more be conveyed without the metal being actually rarefied in one part and more condensed in in contact with the fish, and if it can, whe- another; but it cannot be withdrawn by any ther, in the space between, any light appear, force that we are yet made acquainted with, and a slight noise or crackling be heard.

so as to leave the whole in a negative state, If so, these also are properties common to at least relatively to its natural quantity; the electric fluid.

neither can it be introduced so as to put the 3. Lastly, touch the fish with the wire of iron into a positive state, or render it plus. a small Leyden bottle, and if the shock can be In this respect, therefore, magnetism differs received across, observe whether the wire will from electricity. attract and repel light bodies, and you feel a Athly, A piece of soft iron allows the magshock, while holding the bottle in one hand, netic Suid which it contains to be put in moand touching the wire with the other.

tion by a moderate force, so that being placed If so, the fluid, capable of producing such in a line with the magnetic pole of the earth, effects, seems to have all the known properties it immediately acquires the properties of a of the electric fluid.

magnet; its magnetic fluid being drawn or

forced from one extremity to the other; and Addition, 12th of August, 1772,

this effect continues as long as it remains in In consequence of the Experiments and Disco- the same position, one of its extremities beveries made in France by Mr. Walsh, and coming positively magnetised, and the other communicated by him to Dr. Franklin.

negatively. This temporary magnetism Let several persons, standing on the floor, ceases as soon as the iron is turned east and hold hands, and let one of them touch the fish, west, the fluid immediately diffusing itself so as to receive a shock. If the shock be felt equally through the whole iron, as in its naby all, place the fish flat on a plate of metal, tural state. and let one of the persons holding hands touch 5thly, The magnetic fluid in hard iron, or his plate, while the person farthest from the steel, is put in motion with more difficulty,

« AnteriorContinuar »