« ZurückWeiter »
of an upright bar, an inch and a half in diameter, fastened to the wall by leaden straps, and extending' ten feet above the ridge of the building, tapering from the ridge upwards to a sharp point; the upper twelve inches to be copper; the iron to be painted.
We mention lead for the underground part of the conductor, as less liable to rust in water and moist places, in the form of a pipe, as giving greater stiffness for the substance; and iron for the part above ground, as stronger and less likely to be cut away. The pieces of which the bar may be composed should be screwed strongly into each other by a close joint, with a thin plate of lead between the shoulders, to make the joining or continuation of metal more perfect. Each rod, in passing above the ridge, should be strongly and closely connected by iron or lead, or both, with the leaden coping of the roof, whereby a communication of metal will be made between the two bars of each building, for a more free and easy conducting of the lightning into the earth.
We also advise, in consideration of the great length of the buildings, that two wells, of the same depth with the others, should be dug within twelve feet of the doors of the two outside magazines; that is to say, one of them on the north side of the north building, the other on the south side of the south building; from the bottoms of which wells, similar conductors should be carried up to the eaves, there joining well with a plate of lead extending on the roof up to the leaden coping of the ridge, the said plate of lead being of equal substance with that of the coping.
We are further of opinion, that it will be right to form a communication of lead from the top of the chimney of the proof-house to the lead on its ridge, and thence to the lead on the ridge of the corridor, and
Vol. v. 55 KK
thence to the iron conductor of the adjacent end of the magazine; and also to fix a conductor from the bottom of the weathercock spindle of the clock-house, down on the outside of that building into the moist earth.
As to the board-house, we think it already well furnished with conductors by the several leaden communications above mentioned, from the point of the roof down into the water; and that, by its height and proximity, it may be some security to the buildings below it; we therefore propose no other conductor for that building, and only advise erecting a pointed rod on the summit, similar to those before described, and communicating with those conductors.
To these directions we would add a caution, that, in all future alterations or repairs of the buildings, special care be taken that the metalline communications are not cut off or removed.
It remains that we express our acknowledgments to Sir Charles Frederick, Surveyor-general of the Ordnance, for the obliging attention with which he entertained and accommodated us on the day of our inquiry.
With very great respect we are, Gentlemen,
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.
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 of the electrometer drops, showing the prime conductor 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.
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 opportunity, for the benefit of the public."
* Mr. Henley's.
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 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 f 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.
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.
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.
t Twelve were proposed on and near the magazines at Purfleet .
VOL. V. KK*