O Experimento com a Pandorga

Provavelmente o mais famoso experimento a ser feito com os raios é de Benjamin Franklin e sua pandorga.
O que Franklin estava investigando era se o raio era ou não um fenômeno elétrico. Isto parece óbvio para a maioria de nós agora, mas devemos nos lembrar que nos dias de Franklin as maiores centelhas que podiam ser produzidas tinham menos de 3cm de comprimento. Como os raios podem medir vários km, não era tão óbvio que eles tivessem a mesma origem que as centelhas.
Franklin imaginou que se os raios eram eletricidade, então eles deveriam conter uma enorme quantidade dela, e que deveria levar um longo tempo para se acumular na tormenta. Portanto, ele sugeriu, solte a pandorga no início da tormenta, antes que os raios se aproximem de você.
Ele tinha diversas maneiras de mostrar como a eletricidade estava presente - você pode provocar centelhas de uma chave amarrada à corda, ou pode prender a corda a uma garrafa de Leyden. Se a garrafa estava vazia andes de soltar a pandorga e cheia depois, então há boa evidência de que as nuvens que provovam a tormenta contém eletricidade.

Na demonstração ao lado, a pandorga está suspensa por uma vareta plástica (pois não há vento!). A corda cai em uma garrafa (não de Leyden) onde está presa a uma chave. Abaixo da chave existe um espaço entre 2 e 5 cm antes de uma bola de metal aterrada.
Note na figura ao lado (tomada com a máquina funcionando mas não com intensidade suficiente para produzir centelhas) que o rabo da pandorga está sendo puxado para a máquina. Este é um exemplo de atração estática - a mesma coisa que tenta unir suas roupas numa secadora de roupas. Note também que existe uma centelha pulando entre a chave e a bola dentro da garrafa.

Existe um outro efeito que pode ser demosntrado, mas não muito fácil de ser fotografado. É o chamado fogo de São Telmo, ou, para os cientistas, corona. Ele aparece como um brilho púrpura em torno das bordas da pandorga, e foi observado nas cordas e mastros por marinheiros em antigos navios de madeira.
Para os marinheiros, que deram o nome de seu santo padroeiro, era um sinal de proteção, mas pode ser visto na figura o quão pouco ele protege a pandorga! De fato, é um sinal de grande perigo. Então, como os marinheiros puderam se enganar tanto?
A razão provável é que todos os marinheiros em tormentas viam coronas, mas apenas aqueles cujos navios conseguiam sair delas voltavam para contar sobre elas!

Um video clip de uma pandorga sendo atingida por uma centelha (417k, 5 segundos). Rode-o em loop.
Abaixo encontram-se dois artigos escritos por Franklin, obtidos de Stephen Tubbs.


by Benjamin Franklin, The Pennsylvania Gazette, October 19, 1752

As frequent Mention is made in the News Papers from Europe, of the Success of the Philadelphia Experiment for drawing the Electric Fire from Clouds by Means of pointed Rods of Iron erected on high Buildings, etc. it may be agreeable to the Curious to be inform'd, that the same Experiment has succeeded in Philadelphia, tho' made in a different and more easy Manner, which any one may try, as follows.

Make a small Cross of two light strips of Cedar, the Arms so long as to reach to the four Corners of a large thin Silk Handkerchief when extended; tie the Corners of the Handkerchief to the Extremities of the Cross, so you have the Body of a Kite; which being properly accommodated with a Tail, Loop and String, will rise in the Air, like those made of Paper; but this being of Silk is fitter to bear the Wet and Wind of a Thunder Gust without tearing. To the Top of the upright Stick of the Cross is to be fixed a very sharp pointed Wire, rising a Foot or more above the Wood. To the End of the Twine, next to the Hand, is to be tied a silk Ribbon, and where the Twine and the silk join, a Key may be fastened. This Kite is to be raised when a Thunder Gust appears to be coming on, and the Person who holds the String must stand within a Door, or Window, or under some Cover, so that the Silk Ribbon may not be wet; and Care must be taken that the Silk Ribbon may not touch the Frame of the Door or Window. As soon as any of the Thunder Clouds come over the Kite, the pointed Wire will draw the Electric Fire from them, and the Kite, with all the Twine will be electrified, and the loose Filaments of the Twine will stand out every Way, and be attracted by an approaching Finger. And when the Rain has wet the Kite and Twine, so that it can conduct the Electric Fire freely, you will find it stream out plentifully from the Key on the Approach of your Knuckle. At this Key the Phial may be charg'd; and from Electric Fire thus obtain'd, Spirits may be kindled, and all the other Electric Experiments be perform'd, which are usually done by the Help of a rubbed Glass Globe or Tube; and thereby the Sameness of the Electric Matter with that of Lightning compleatly demonstrated.

!!!Note from Stephen Tubbs: Don't try to repeat this kite experiment! It is very dangerous and it was a miracle that Franklin survived!!!


by Benjamin Franklin, Paris, Sept. 1767

Experiments made in electricity first gave philosophers a suspicion that the matter of lightning was the same with the electric matter. Experiments afterwards made on lightning obtained from the clouds by pointed rods, received into bottles, and subjected to every trial, have since proved this suspicion to be perfectly well founded; and that whatever properties we find in electricity, are also the properties of lightning.

This matter of lightning, or of electricity, is an extream subtile fluid, penetrating other bodies, and subsisting in them, equally diffused.

When by an operation of art or nature, there happens to be a greater proportion of this fluid in one body than in another, the body which has the most, will communicate to that which has least, till the proportion becomes equal; provided the distance between them be not too great; or, if it is too great, till there be proper conductors to convey it from one to the other.

If the communication be through the air without any conductor, a bright light is seen between the bodies, and a sound is heard. In our small experiments we call this light and sound the electric spark and snap; but in the great operations of nature, the light is what we call lightning, and the sound (produced at the same time, tho' generally arriving later at our ears that the light does to our eyes) is, with its echoes, called thunder.

If the communication of this fluid is by a conductor, it may be without either light or sound, the subtle fluid passing in the substance of the conductor.

If the conductor be good and of sufficient bigness, the fluid passes through it without hurting it. If otherwise, it is damaged or destroyed.

All metals and water, are good conductors.--Other bodies may become conductors by having some quantity of water in them, as wood, and other materials used in building, but not having much water in them, they are not good conductors, and therefore are often damaged in the operation.

Glass, wax, silk, wool, hair, feathers, and even wood, perfectly dry are non-conductors: that is, they resist instead of facilitating the passage of this subtle fluid.

When this fluid has an opportunity of passing through two conductors, one good, and sufficient, as of metal, the other not so good, it passes in the best, and will follow it in any direction.

The distance at which a body charged with this fluid will discharge itself suddenly, striking through the air into another body that is not charged, or not so highly charg'd, is different according to the quantity of the fluid, the dimensions and form of the bodies themselves, and the state of the air between them.--This distance, whatever it happens to be between any two bodies, is called their striking distance, as till they come within that distance of each other, no stroke will be made.

The clouds have often more of this fluid in proportion than the earth; in which case as soon as they come near enough (that is, with the striking distance) or meet with a conductor, the fluid quits them and strikes into the earth. A cloud fully charged with this fluid, if so high as to be beyond the striking distance from the earth, passes quietly with making noise or giving light; unless it meets with other clouds that have less.

Tall trees, and lofty buildings, as the towers and spires of churches, become sometimes conductors between the clouds and the earth; but not being good ones, that is, not conveying the fluid freely, they are often damaged.

Buildings that have their roofs covered with lead, or other metal, and spouts of metal continued from the roof into the ground to carry off the water, are never hurt by lightning, as whenever it falls on such a building, it passes in the metals and not in the walls.

When other buildings happen to be within the striking distance from such clouds, the fluid passes in the walls whether of wood, brick or stone, quitting the walls only when it can find better conductors near them, as metal rods, bolts, and hinges of windows or doors, gilding on wainscot, or frames of pictures; the silvering on the backs of looking-glasses; the wires for bells; and the bodies of animals, as containing watry fluids. And in passing thro' the house it follows the direction of these conductors, taking as many in it's way as can assist it in its passage, whether in a strait or crooked line, leaping from one to the other, if not far distant from each other, only rending the wall in the spaces where these partial good conductors are too distant from each other.

An iron rod being placed on the outside of a building, from the highest part continued down into the moist earth, in any direction strait or crooked, following the form of the roof or other parts of the building, will receive the lightning at its upper end, attracting it so as to prevent its striking any other part; and, affording it a good conveyance into the earth, will prevent its damaging any part of the building.

A small quantity of metal is found able to conduct a great quantity of this fluid. A wire no bigger than a goose quill, has been known to conduct (with safety to the building as far as the wire was continued) a quantity of lightning that did prodigious damage both above and below it; and probably larger rods are not necessary, tho' it is common in America, to make them of half and inch, some of three quarters, or an inch diameter.

The rod may be fastened to the wall, chimney, etc. with staples of iron.--The lightning will not leave the rod (a good conductor) to pass into the wall (a bad conductor), through those staples.--It would rather, if any were in the wall, pass out of it into the rod to get more readily by that conductor into the earth.

If the building be very large and extensive, two or more rods may be placed at different parts, for greater security.

Small ragged parts of clouds suspended in the air between the great body of clouds and the earth (like leaf gold in electrical experiments), often serve as partial conductors for the lightning, which proceeds from one of them to another, and by their help comes within the striking distance to the earth or a building. It therefore strikes through those conductors a building that would otherwise be out of the striking distance.

Long sharp points communicating with the earth, and presented to such parts of clouds, drawing silently from them the fluid they are charged with, they are then attracted to the cloud, and may leave the distance so great as to be beyond the reach of striking.

It is therefore that we elevate the upper end of the rod six or eight feet above the highest part of the building, tapering it gradually to a fine sharp point, which is gilt to prevent its rusting.

Thus the pointed rod either prevents a stroke from the cloud, or, if a stroke is made, conducts it to the earth with safety to the building.

The lower end of the rod should enter the earth so deep as to come at the moist part, perhaps two or three feet; and if bent when under the surface so as to go in a horizontal line six or eight feet from the wall, and then bent again downwards three or four feet, it will prevent damage to any of the stones of the foundation.

A person apprehensive of danger coming from lightning, happening during the time of thunder to be in a house not so secured, will do well to avoid sitting near the chimney, near a looking glass, or any gilt pictures or wainscot; the safest place is in the middle of the room, (so it be not under a metal lustre suspended by a chain) sitting on one chair and laying the feet up in another. It is still safer to bring two or three mattrasses or beds into the middle of the room, and folding them up double, place the chair upon them; for they not being so good conductors as the wall, the lightning will not chuse an interrupted course through the air of the room and the bedding, when it can go thro' a continued better conductor the wall. But where it can be had, a hamock or swinging bed, suspended by silk cords equally distant from the walls on every side, and from the cieling and floor above and below, affords the safest situation a person can have in any room whatever; and what indeed may be deemed quite free from danger of any stroke by lightning.

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