« ZurückWeiter »
large and complete, was conveyed into the earth, without any material damage to the buildings. Facts then in great, as far as we have them authenticated, justify the opinion that is drawn from the experiments in small, as above related. It has also been objected, that, unless we knew the quantity that might possibly be discharged at one stroke from the clouds, we cannot be sure we have provided sufficient conductors; and therefore cannot depend on their conveying away all that may fall on their points. Indeed we have nothing to form a judgment by in this, but past facts; and we know of no instance, where a complete conductor to the moist earth has been insufficient, if half an inch in diameter. It is probable, that many strokes of lightning have been conveyed through the common leaden pipes affixed to houses to carry down the water from the roof to the ground; and there is no account of such pipes being melted and destroyed, as must sometimes have happened, if they had been insufficient. We can, then, only judge of the dimensions proper for a conductor of lightning, as we do of those proper for a conductor of rain, by past observation. And, as we think a pipe of three inches bore sufficient to carry off the rain that falls on a square of twenty feet, because we never saw such a pipe glutted by any shower; so we may judge a conductor of an inch diameter more than sufficient for any stroke of lightning that will fall on its point. It is true, that, if another deluge should happen wherein the windows of heaven are to be opened, such pipes may be unequal to the falling quantity; and, if God for our sins should think fit to rain fire upon us, as upon some cities of old, it is not expected that our conductors, of whatever size, should secure our houses against a miracle. Probably, as water drawn up into the air and there forming clouds, is disposed to fall again in rain by its natural gravity, as soon as a number of particles sufficient to make a drop can get together; so, when the clouds are (by whatever means) over or under charged with the electric fluid to a degree sufficient to attract them towards the earth, the equilibrium is restored, before the difference becomes great beyond that degree. Mr. Lane's electrometer, for limiting precisely the quantity of a shock that is to be administered in a medical view, may serve to make this more easily intelligible. The discharging knob does by a screw approach the conductor to the distance intended, but there remains fixed. Whatever power there may be in the glass globe to collect the fulminating fluid, and whatever capacity of receiving and accumulating it there may be in the bottle or glass jar, yet neither the accumulation nor the discharge ever exceeds the destined quantity. Thus, were the clouds always at a certain fixed distance from the earth, all discharges would be made when the quantity accumulated was equal to the distance. But there is a circumstance, which, by occasionally lessening the distance, lessens the discharge; to wit, the movableness of the clouds, and their being drawn nearer to the earth by attraction when electrified; so that discharges are thereby rendered more frequent and of course less violent. Hence, whatever the quantity may be in nature, and whatever the power in the clouds of collecting it, yet an accumulation and force beyond what mankind has hitherto been acquanted with is scarce to be expected. *
B. F. August 27th, 1772.
* It may be fit to mention here, that the immediate occasion of the dispute concerning the preference between pointed and blunt conductors of lightning arose as follows. A powder-mill having blown up at Brescia,
Description of a Portable Apparatus, invented by Mr.
John Canton, for the Purpose of easily demonstrating Dr. Franklin's fundamental Principles of Electricity. *
This apparatus is very simple, and takes up but little room in the pocket.
Take a rod of common wood, two feet long, an inch wide and three lines in thickness, planed by a joiner.
Divide it in four pieces, or four rods of equal length, each being six inches long.
Place two of these rods end to end on a table, and unite the contiguous ends, by pasting over them a small strip of fine linen, which, when it has become dry, will answer the purpose of a hinge, to double or fold the two pieces together at pleasure.
Unite the other two pieces in the same manner. Have two pairs of little balls, made of cork or the pith of the elder, and of nearly the size of a pea.
Attach one pair of these to the two ends of a very fine
in consequence of its being struck with lightning, the English Board of Ordnance applied to their painter, Mr. Wilson, then of some note as an electrician, for a method to prevent the like accident to their magazines at Purfleet. Mr. Wilson having advised a blunt conductor, and it being understood that Dr. Franklin's opinion, formed upon the spot, was for a pointed one, the matter was referred in 1772, to the Royal Society, and by them as usual to a committee, who, after consultation, prescribed a method conformable to Dr. Franklin's theory. But a harmless stroke of lightning having, under particular circumstances, fallen upon one of the buildings and its apparatus in May, 1777, the subject came again into violent agitation, and was again referred to the Society, and by the Society again referred to a new committee, which committee confirmed the decision of the first committee. – B. V.
* Translated from M. Dubourg's edition of Franklin's works. — EDITOR.
linen thread, about twelve inches long; then, doubling the thread in the middle, these two balls, suspended at the ends of the thread, will be in contact.
Then fasten the middle of the thread to one end of one of these pairs of rods. The threads should be very smooth, and free from any little fibres; and must be dipped once in salt water.” Proceed in the same manner with regard to the other two balls, and the other two rods.
Make little hemispherical holes in each pair of rods, to olace the balls in, when you wish to shut up the apparatus.
MODE 0 F USING IT.
To make use of this apparatus, take three large drinking-glasses, and warm and dry them well before the fire.
On one of the glasses, place one pair of the rods open, the hinge being on the upper side, and the little balls suspended at the end. Place the other pair in like manner on the second glass.
Place these two glasses near a corner of the table, in such a position that the ends of the rods from which the balls are suspended may extend beyond the table, and thus the balls may hang entirely free of the table, on either side of the same corner, while the two ends without balls are an inch apart, and the rods in a line with each other.
Rub the third glass with a silk handkerchief, of any color; a piece of black silk, however, is better than any thing else.
"Thereason why the threads should be dipped once in salt water is, that they may always continue to be good conductors; for, unless this is done, in dry weather they would sometimes become too dry to conduct freely. For this ingenious method, we are indebted to Mr. Cavendish
1. Bring the rubbed glass near one pair of balls; they will be attracted by it, and receive electricity from it; and, when you withdraw the glass, the balls will hang apart, because they will repel each other.
2. To show that this electricity is a subtile fluid, which penetrates wood, and passes readily through it from end to end, being at the same time susceptible of division and of communication ;
Without touching the wood, bring the glasses nearer each other, so that the ends of both pairs of rods will meet; and you will immediately perceive that the two separated balls will approach each other by half the distance that divides them, and that the two balls which were hanging in contact will recede from each other, to an equal distance.
3. To show that electricity does not pass into wax, although it passes into wood;
Touch the wood with a stick of sealing-wax; and you will perceive no change in the respective positions of the balls.
4. A proof that it does not enter glass is, that it is retained as long as the rods are supported on the glass.
5. But, to prove that it enters metals and animal bodies, touch the rods either with a key, or with the finger, and the electricity will instantly pass off into the ground, whence it was attracted by rubbing the glass, and each pair of balls will come together again.
6. To show that the particles of the fluid mutually repel each other, and that the natural quantity of it contained in any substance whatever can be put in motion by repulsion;
Rub the glass well, and, having separated the two pairs of rods, hold the glass over that end of one pair from which the balls are not suspended ; on the approach of the glass, you will see the balls separate and