Econlib

The Library

Other Sites

 Front Page arrow Titles (by Subject) arrow CCCCLI: EXPERIMENTS, OBSERVATIONS, AND FACTS, TENDING TO SUPPORT THE OPINION OF THE UTILITY OF LONG, POINTED RODS, FOR SECURING BUILDINGS FROM DAMAGE BY STROKES OF LIGHTNING. - The Works of Benjamin Franklin, Vol. V Letters and Misc. Writings 1768-1772

Return to Title Page for The Works of Benjamin Franklin, Vol. V Letters and Misc. Writings 1768-1772

Search this Title:

Also in the Library:

Subject Area: Political Theory

CCCCLI: EXPERIMENTS, OBSERVATIONS, AND FACTS, TENDING TO SUPPORT THE OPINION OF THE UTILITY OF LONG, POINTED RODS, FOR SECURING BUILDINGS FROM DAMAGE BY STROKES OF LIGHTNING. - Benjamin Franklin, The Works of Benjamin Franklin, Vol. V Letters and Misc. Writings 1768-1772 [1904]

Edition used:

The Works of Benjamin Franklin, including the Private as well as the Official and Scientific Correspondence, together with the Unmutilated and Correct Version of the Autobiography, compiled and edited by John Bigelow (New York: G.P. Putnam’s Sons, 1904). The Federal Edition in 12 volumes. Vol. V (Letters and Misc. Writings 1768-1772).

About Liberty Fund:

Liberty Fund, Inc. is a private, educational foundation established to encourage the study of the ideal of a society of free and responsible individuals.


CCCCLI

EXPERIMENTS, OBSERVATIONS, AND FACTS, TENDING TO SUPPORT THE OPINION OF THE UTILITY OF LONG, POINTED RODS, FOR SECURING BUILDINGS FROM DAMAGE BY STROKES OF LIGHTNING.

read at the committee appointed to consider the erection of conductors to secure the magazines at purfleet, august 27, 1772

Experiment I

The prime conductor of an electric machine, A, B, (see Plate I.) being supported about ten inches and a half above the table by a wax stand, and under it erected a pointed wire, seven inches and a half high and one fifth of an inch thick, and tapering to a sharp point, communicating with the table; when the point (being uppermost) is covered by the end of a finger, the conductor may be full charged, and the electrometer1 will rise to the height indicating a full charge; but the moment the point is uncovered, the ball of the electrometer drops, showing the prime conductor to be instantly discharged and nearly emptied of its electricity. Turn the wire its blunt end upward (which represents an unpointed bar), and no such effect follows, the electrometer remaining at its usual height when the prime conductor is charged.

Observation

What quantity of lightning a high, pointed rod, well communicating with the earth, may be expected to discharge from the clouds silently in a short time, is yet unknown; but I reason from a particular fact to think it may at some times be very great. In Philadelphia I had such a rod fixed to the top of my chimney, and extending about nine feet above it. From the foot of this rod, a wire (the thickness of a goose-quill) came through a covered glass tube in the roof, and down through the well of the staircase; the lower end connected with the iron spear of a pump. On the staircase opposite to my chamber door, the wire was divided; the ends separated about six inches, a little bell on each end; and between the bells a little brass ball, suspended by a silk thread, to play between and strike the bells when clouds passed with electricity in them. After having frequently drawn sparks and charged bottles from the bell of the upper wire, I was one night awakened by loud cracks on the staircase. Starting up and opening the door, I perceived that the brass ball, instead of vibrating as usual between the bells, was repelled and kept at a distance from both; while the fire passed, sometimes in very large, quick cracks from bell to bell, and sometimes in a continued, dense, white stream, seemingly as large as my finger, whereby the whole staircase was enlightened as with sunshine, so that one might see to pick up a pin.1 And from the apparent quantity thus discharged, I cannot but conceive that a number1 of such conductors must considerably lessen that of any approaching cloud, before it comes so near as to deliver its contents in a general stroke; an effect not to be expected from bars unpointed, if the above experiment with the blunt end of the wire is deemed pertinent to the case.

Experiment II

The pointed wire under the prime conductor continuing of the same height, pinch it between the thumb and finger near the top, so as just to conceal the point; then turning the globe, the electrometer will rise and mark the full charge. Slip the fingers down, so as to discover about half an inch of the wire then another half inch, and then another; at every one of these motions discovering more and more of the pointed wire; you will see the electrometer fall quick and proportionably, stopping when you stop. If you slip down the whole distance at once, the ball falls instantly down to the stem.

Observation

From this experiment it seems that a greater effect in drawing off the lightning from the clouds may be 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 away, 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 inmediately 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 the 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 wire 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.

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 down 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 owe our first knowledge of the nature and operations of lightning to observations on such small experiments; and that, on carefully comparing the most accurate accounts of former facts, and the exactest relations of those that have occurred since, the effects have surprisingly agreed with the theory; it is humbly conceived that in natural philosophy, in this branch of it at least, the suggestion has not so much weight; and that the farther new experiments, now adduced in recommendation of long, sharp-pointed rods, may have some claim to credit and consideration.

It has been urged, too, that, though points may have considerable effects on a small prime conductor at small distances, yet, on great clouds and at great distances, nothing is to be expected from them. To this it is answered, that in those small experiments it is evident the points act a greater than the striking distance; and, in the large way, their service is only expected where there it such nearness of the cloud as to endanger a stroke; and there, it cannot be doubted, the points must have some effect. And, if the quantity discharged by a single pointed rod may be so considerable as I have shown it, the quantity discharged by a number will be proportionally greater.

But this part of the theory does not depend alone on small experiments. Since the practice of erecting pointed rods in America (now near twenty years), five of them have been struck by lightning, namely: Mr. Raven’s and Mr. Maine’s in South Carolina, Mr. Tucker’s in Virginia, Mr. West’s and Mr. Moulder’s in Philadelphia. Possibly there may have been more, that have not come to my knowledge. But, in every one of these, the lightning did not fall upon the body of the house, but precisely on the several points of the rods; and, though the conductors were sometimes not sufficiently 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 it 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 occasional 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 acquainted with is scarce to be expected.1

B. F.

August 27, 1772.

[1 ]Mr. Henley’s.

[1 ]M. de Romas saw still greater quantities of lightning brought down by the wire of his kite. He had “explosions from it, the noise of which greatly resembled that of thunder, and were heard (from without) into the heart of the city, notwithstanding the various noises there. The fire seen at the instant of the explosion had the shape of a spindle, eight inches long and five lines in diameter. Yet, from the time of the explosion to the end of the experiment, no lightning was seen above, nor any thunder heard. At another time the streams of fire issuing from it were observed to be an inch thick, and ten feet long.” See Dr. Priestley’s History of Electricity, pp. 134-136 first edition.

[1 ]Twelve were proposed on and near the magazines at Purfleet.

[1 ]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, 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.