Imágenes de página
[ocr errors]
[merged small][merged small][merged small][merged small][ocr errors][graphic][ocr errors]
[merged small][ocr errors][merged small][merged small][graphic][merged small][graphic][ocr errors][merged small]




from the clouds may be expected from long conductor therefore shows, that a quantity of pointed rods, than from short ones; I mean its atmosphere was drawn from the end where from such as show the greatest length, above the electrometer is placed to the

part immethe building they are fixed on.

diately over the large body, and there accumulated ready to strike into it with its whole

undiminished force, as soon as within the Instead of pinching the point between the striking distance; and, were the prime conthumb and finger, as in the last experiment, ductor moveable like a cloud, it would apkeep the thumb and finger each at near an proach the body by attraction till within that inch distance from it, but at the same. height, distance. The swift motion of clouds, as the point between them. In this situation, driven by the winds, probably prevents this though the point is fairly exposed to the prime happening so often as otherwise it might do: conductor, it has little or no effect; the elec- for, though parts of the cloud may stoop totrometer rises to the height of a full charge. wards a building as they pass, in consequence But the moment the fingers are taken away, of such attraction, yet they are carried forthe ball falls quick to the stem.

ward beyond the striking distance, before they

could by their descending come within it. To explain this, it is supposed, that one reason of the sudden effe

produced by a Attach a small light lock of cotton to the long naked pointed wire is, that (by the re- underside of the prime conductor, so that it pulsive power of the positive charge in the may hang down towards the pointed wire prime conductor) the natural quantity of elec- mentioned in the first experiment. Cover the tricity contained in the pointed wire is driven point with your finger, and the globe being down into the earth, and the point of the wire turned, the cotton will extend itself, stretchmade strongly negative; whence it attracts ing down towards the finger, as at a; but on the electricity of the prime conductor more uncovering the point, it instantly flies up to strongly thar bodies in their natural state the prime conductor, as at b, and continues would do; the small quantity of common there as long as the point is uncovered. The matter in the point, not being able by its al moment you cover it again, the cotton flies tractive force to retain its natural quantity down again, extending itself towards the of the electric fluid, against the force of that finger; and the same happens in degree, if repulsion.—But the finger and thumb being instead of the finger) you use, uncovered, the substantial and blunt bodies, though as near blunt end of the wire uppermost. the prime conductor, hold up better their own natural quantity against the force of that repulsion; and so, continuing nearly in their To explain this, it is supposed that the cotnatural state, they jointly operate on the elec- ton, by its connexion with the prime conducttric fluid in the point, opposing its descent and or, receives from it a quantity of its electriaiding the point to retain it; contrary to the city; which occasions its being attracted by repelling power of the prime conductor, which the finger that remains still in nearly its nawould drive it down. And this may also serve tural state. But when a point is opposed to to explain the different powers of the point in the cotton, its electricity is thereby taken the preceding experiment, on the slipping from it, faster than it can at a distance be down the finger and thumb to different dis- supplied with a fresh quantity from the contances.

ductor. Therefore being reduced nearer to Hence is collected, that a pointed rod erect- the natural state, it is attracted up to the ed between two tall chimnies, and very little electrified prime conductor; rather than higher (an instance of which I have seen) can-down, as before, to the finger. not have so good an effect, as if it had been Supposing farther that the prime conductor erected on one of the chimnies, its whole represents a cloud charged with the electric length above it.

fluid ; the cotton, a ragged fragment of cloud

(of which the underside of great thunderIf, instead of a long pointed wire, a large chimney or highest part of a building. We

clouds are seen to have many) the finger, a solid body (to represent a building without a then may conceive that when such a cloud point) be brought under and as near the prime conductor, when charged; the ball of the passes over a building, some one of its ragged electrometer will fall a little; and on taking by the chimney or other high part of the edi

under-hanging fragments may be drawn down away the large body, will rise again.

fice; creating thereby a more easy commu

nication between it and the great cloud.-But Ils rising again shows that the prime con- a long pointed rod being presented to this ductor lost little or none of its electric charge, fragment, may occasion its receding, like the as it had done through the point: the falling cotton, up to the great cloud; and thereby of the ball while the large body was under the increase, instead of lessening the distance, so

VOL II....2 S




« AnteriorContinuar »