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 ERECTING OF CONDUCTORS TO SECURE THE MAGAZINES AT PURFLEET, AUGUST 27TH, 1772.

EXPERIMENT I.

THE prime conductor of an electric machine, A, B, (see Plate IV.) 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, and 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 electrometer will rise to the height indicating a full charge; but the moment the point is uncovered, the ball

that means, not only contributing to increase the quantity of every actual discharge, but also frequently occasioning a discharge, where it might not otherwise have happened.

"If, therefore, we invite the lightning, while we are ignorant what the quantity or the effects of it may be, we may be promoting the very mischief we mean to prevent.

“Whereas if, instead of pointed, we make use of blunted conductors, those will as effectually answer the purpose of conveying away the lightning safely, without that tendency to increase or invite it.

"My further reasons for disapproving of points, in all cases where conductors are judged necessary, are contained in a letter addressed to the Marquis of Rockingham, and published in the Philosophical Transactions, Vol. LIV. p. 247.

"There are other reasons also, which I have to offer, for rejecting points on this particular occasion, and which were mentioned at the committee. Those I shall lay before the Royal Society at another op. tortunity, for the benefit of the public."

• Mr. Henley's.

of the electrometer drops, showing the prime conductur to be instantly discharged and nearly emptied of its electricity. Turn the wire its blunt end upwards (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 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 awaked 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.* And from the apparent quantity thus discharged, I cannot but conceive that a number † 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

* 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.

Twelve were proposed on and near the magazines at Purfleet.

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 dif ferent 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.

1

If, instead of a long, pointed wire, a large, solid boay (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