Abbildungen der Seite
PDF
EPUB

expected from long, pointed rods, than from short ones; I mean from such as show the greatest length above the building they are fixed on.

EXPERIMENT III.

Instead of pinching the point between the thumb and finger, as in the last experiment, keep the thumb and finger each at near an inch distance from it, but at the same height, the point between them. In this situation, though the point is fairly exposed to the prime conductor, it has little or no effect; the electrometer rises to the height of a full charge. But the moment the fingers are taken aicay, the ball falls quick to the stem.

OBSERVATION.

To explain this, it is supposed, that one reason of the sudden effect produced by a long, naked, pointed wire is, that (by the repulsive power of the positive charge in the prime conductor) the natural quantity of electricity contained in the pointed wire is driven down into the earth, and the point of the wire made strongly negative; whence it attracts the electricity of the prime conductor more strongly than bodies in their natural state would do; the small quantity of common matter in the point not being able by its attractive force to retain its natural quantity of the electric fluid, against the force of that repulsion. But the finger and thumb, being substantial and blunt bodies, though as near the prime conductor, hold up better their own natural quantity against the force of that repulsion; and so, continuing nearly in the natural state, they jointly operate on the electric fluid in the point, opposing its descent, and aiding the point to retain it; contrary to the repelling power of the prime conductor, which would drive it down. And this may also serve to explain the different powers of the point in the preceding experiment, on the slipping down the finger and thumb to different distances.

Hence is collected, that a pointed rod, erected between two tall chimneys, and very little higher, (an instance of which I have seen,) cannot have so good an effect, as if it had been erected on one of the chimneys, its whole length above it.

EXPERIMENT IV.

If, instead of a long, pointed wire, a large, solid body (to represent a building without a point) be brought under and as near the prime conductor, when charged; the ball of the electrometer will fall a little; and, on taking away the large body, will rise again.

OBSERVATION.

Its rising again shows that the prime conductor lost little or none of its electric charge, as it had done through the point; the falling of the ball while the large body was under the conductor therefore shows, that a quantity of its atmosphere was drawn from the end where the electrometer is placed, to the part immediately over the large body, and there accumulated ready to strike into it with its whole undiminished force, as soon as within the striking distance; and, were the prime conductor movable like a cloud, it would approach the body by attraction till within that distance. The swift motion of clouds, as driven by the winds, probably prevents this happening so often as otherwise it might do; for, though parts of the cloud may stoop towards a building as they pass, in consequence of such attraction, yet they are carried forward beyond the striking distance before they could by their descending come within it.

EXPERIMENT V.

Attach a small, light lock of cotton to the under side of the prime conductor, so that it may hang down towards the pointed wire mentioned in the first experiment. Cover the point with your finger, and, the globe being turned, the cotton will extend itself, stretching down towards the finger, as at a; but, on uncovering the point, it instantly flies up to the prime conductor, as at b, and continues there as long as the point is uncovered. The moment you cover it again, the cotton flies down again, extending itself towards the finger; and the same happens in degree, if (instead of the finger) you use, uncovered, the blunt end of the wire uppermost.

OBSERVATION.

To explain this, it is supposed that the cotton, by its connexion with the prime conductor, receives from it a quantity of its electricity; which occasions its being attracted by the finger that remains still in nearly its natural state. But, when a point is opposed to the cotton, its electricity is thereby taken from it, faster than it can at a distance be supplied with a fresh quantity from the conductor. Therefore being reduced nearer to the natural state, it is attracted up to the electrified prime conductor; rather than down, as before, to the finger.

Supposing farther, that the prime conductor represents a cloud charged with the electric fluid; the cotton, a ragged fragment of cloud (of which the underside of great thunder-clouds are seen to have many), the finger, a chimney or highest part of a building. We then may conceive, that, when such a cloud passes over a building, some one of its ragged, under-hanging fragments may be drawn down by the chimney, or other high part of the edifice; creating thereby a more easy communication between it and the great cloud. But a long, pointed rod, being presented to this fragment, may occasion its receding, like the cotton, up to the great cloud; and thereby increase, instead of lessening the distance, so as often to make it greater than the striking distance. Turning the blunt end of a mire uppermost (which represents the unpointed bar), it appears that the same good effect is not from that to be expected. A long, pointed rod, it is therefore imagined, may prevent some strokes; as well as conduct others that fall upon it, when a great body of cloud comes on so heavily that the above repelling operation on fragments cannot take place.

EXPERIMENT VI.

Opposite the side of the prime conductor place separately, isolated by wax stems, Mr. Canton's two boxes with pith balls suspended by fine linen threads. On each box lay a wire, six inches long and one fifth of an inch thick, tapering to a sharp point; but so laid, as that four inches of the pointed end of one wire, and an equal length of the blunt end of the other, may project beyond the ends of the boxes; and both at eighteen inches distance from the prime conductor. Then charging the prime conductor by a turn or two of the globe, the balls of each pair will separate; those of the box, whence the point projects most, considerably; the others less. Touch the prime conductor, and those of the box with the blunt point will collapse, and join; those connected with the point will at the same time approach each other, till within about an inch, and there remain.

Vol. v. 56

OBSERVATION.

This seems a proof, that, though the small, sharpened part of the wire must have had a less natural quantity in it before the operation, than the thick, blunt part, yet a greater quantity was driven doicn from it to the balls. Thence it is again inferred, that the pointed rod is rendered more negative; and, farther, that if a stroke must fall from the cloud over a building, furnished with such a rod, it is more likely to be drawn to that pointed rod than to a blunt one; as being more strongly negative, and of course its attraction stronger. And it seems more eligible, that the lightning should fall on the point of the conductor (provided to convey it into the earth) than on any other part of the building, thence to proceed to such conductor. Which end is also more likely to be obtained by the length and loftiness of the rod; as protecting more extensively the building under it.

It has been objected, that erecting pointed rods upon edifices is to invite and draw the lightning into them; and therefore dangerous. Were such rods to be erected on buildings, without continuing the communication quite down into the moist earth, this objection might then have weight; but, when such complete conductors are made, the lightning is invited, not into the building, but into the earth, the situation it aims at, and which it always seizes every help to obtain, even from broken, partial metalline conductors.

It has also been suggested, that, from such electric experiments, nothing certain can be concluded as to the great operations of nature; since it is often seen, that experiments, which have succeeded in small, in large have failed. It is true, that in mechanics this has sometimes happened. But, when it is considered, that we

« ZurückWeiter »