INSTRUCTIONSuponLIGHTNING RODSforPOWDER MAGAZINES,by a Committee consisting ofMM. Becquerel,Babinet,Duhamel,Fizeau,Edm Becquerel,Regnault,le MaréchalVaillant,andPouillet.

5th March, 1855.

The committee examined the points presented by Messrs. Delieul, one of platinum, made exactly as described in the report of the previous 18th December; the other, a cone similar in form, size, and external appearance, but rather less costly, being made of a cap of platinum, fixed with hard solder upon the conical end of the iron rod. It was thought that this second arrangement would not practically be inferior to the other; but it must be made by a skilful workman, who knows how to insure that the solder should take to the whole of the surfaces brought together. They see no objection to the substitution of palladium, or gold or silver of a standard of ·950 for the platinum. But all these metals are costly; few workmen know how to work in them, or at least to employ that precision, and take that minute care, which are indispensable to success. These reasons have raised again a proposition that was discussed in the former commission, which consists in making the points of copper. The copper point is 2 centimetres (·78 inch) in diameter, like the upper part of the iron rod, to which it is screwed and brazed; its length is about 20 centimetres (7·87 inches), and it terminates in a cone 3 or 4 centimetres (1·1 or 1·5 inches) high.

They see no reason why this should not be used with almost the same confidence as the preceding forms. If there is ground to fear that it may undergo changes from atmospheric influences, this is counterbalanced by certain advantages. 1st, copper is with palladium, gold, and silver, among the best conductors of heat and electricity; and the point of the cone will be much less heated than the platinum point; and 2nd, the terminal, with a copper point, is much less expensive, and can be made everywhere.

On the report being put to the vote M. Despretz could not approve the proposal to employ copper points, fearing that the deposition of carbonate or some other badly conducting matter would diminish the efficacy of the lightning rod.

14th January, 1867.

After referring to some general principles, and to the construction of lightning rods recommended in the reports of the earlier Committees: the Committee recommend, that the upper terminal including the copper point should be from 3 to 5 metres (9 feet 10 inches to 16 feet 5 inches) high; that the junction of the conductor and the upper terminal, and also the several joints of the conductor, should becovered with solder, and insist very strongly upon the necessity of communication with thenappe d’eau souterraine, which they define as “the water level in neighbouring wells which never dry up, and which retain at least 50 centimetres (19·68 inches) in depth of water in the most unfavourable seasons.”

The special arrangements to be adopted in setting up lightning rods for powder magazines are: not to fix them on the building itself but outside the surrounding walls. For each large sized magazine (27·89 metres, by 20 metres, and 11 metres high, equal to 91 feet 6 inches by 65 feet 7 inches, and 36 feet high) there should be three conductors—two near the ends of the long side of the enclosing wall most exposed to storms, and the third in the middle of the opposite side. The upper terminals should be only 5 metres (16 feet 5 inches) high, and should be raised on a pier, a mast, or other support 15 metres (49 feet 2 inches) high, down which the conductor should be led to the ground. There should be a circuit which the Committee callcircuit de ceinturecarried entirely round the enclosing wall to which each conductor should be joined, and a conductor should be carried from the most convenient point of this circuit to the underground water. For middle sized magazines two terminals and supports, and for small magazines one terminal and support will suffice; but in all cases there should be acircuit de ceinture. This need not be deep below the surface, nor covered over; it may even be in an open gutter, but a conductor must be led from it to the underground water, even if in order to do this it is necessary to carry the conductor several hundred metres or several kilometres. It need not, however, be made of bars and carried all the way in a trench, but it may be made of six wires 6 or 7 millimetres (about ·25 inches) in diameter, and carried on posts like telegraph wires, except that they need not be insulated.

20th May, 1875.

The Committee find that platinum tips are useless, and recommend instead that the point of the terminal should be made of pure copper, 50 centimetres (19·7 inches) long, and terminating in a cone, forming an angle of 30°. This should be scarfed, pinned, and soldered to the end of the terminal. The terminal should be of wrought iron in one length, and where possible galvanized; but on no account painted. The connection with the conductor should be by a piece fitted and bolted; and, lastly, the whole joint should be well covered with solder.

The Committee consider that on an ordinary building a terminal will effectively protect a cone, having the point for its apex, and a base whose radius is 1·75 of its height. But in practice the terminalsmay be much farther apart, if there is acircuit des faites. This is defined as a metallic conductor, which extends without break over the ridges of all the buildings which it is intended to protect, and which is joined by metallic contact to all the upper terminals and to the conductor, and consequently to the underground water which alone forms the common reservoir. All pieces of metal of any considerable size should be connected with the conductor.

If the conductor is made of iron bars, they should be galvanized if possible, and the joints should be fitted, bolted, and finally covered with solder. If the bars cannot be galvanized, they should be well painted. The Committee recommend the employment, especially in thecircuit des faites, of an arrangement for compensating for the lengthening and shortening of the bars by the variations of temperature. This is made by inserting in the circuit a curved band of copper which will yield to the movement of the rods. If the conductor is made of galvanized iron wire rope, each wire should be 2·5 or 3 millimetres (·09 to ·11 inch) in diameter, and there should be such a number of them that the sum of their sectional areas shall be equal to one-fifth more than that of a bar of iron 20 millimetres (·78 inch) square. The rope should be all in one piece, and the joints with the terminal and earth connection should be covered with solder.

The supports should not be insulated, and there should be as few as possible of them. At the underground end of the conductor should be fixed a large sheet or hollow cylinder of metal, and this should be always, even in the greatest droughts, plunged at least 1 metre (3 feet 3 inches) into the subterranean water. If from any cause this water cannot be reached, the conductor may be joined to one of the main water-pipes of the city; but if the conductor cannot be led either to the subterranean water or to a main water-pipe, no lightning-rod should be erected. It would do more harm than good.

In the case of buildings of any importance, two or more conductors leading to the subterranean water should be employed. It should be so arranged that the underground part and the earth connection may be easily inspected and cleaned from rust, and the whole should be inspected and cleaned at least once a year, at the end of the autumn. The Committee is of opinion that it would be better to put all the lightning-rod work in the hands of special workmen, under the control of an agent appointed by the administration, and not to trust it to the blacksmiths and locksmiths usually employed. The Committee lastly recommend that they should be permanently appointed, and meet every year after the inspection, to report and decide upon the steps to be taken to remedy any defects that may be discovered.

This report gives a detailed description of the state of the lightning rods attached to the public buildings of Paris.

In most cases the upper terminals were of great length, some of them as much as 9 metres (nearly 30 feet) in height; the conductorswere in almost all cases of iron, either in bars or wire rope; the earth connections were of various kinds and extent.

The report frequently states that the points were blunted; that the upper terminals and the conducting rods were deeply rusted; that especially at the joints the conductors were seriously deficient; and that the underground portion was greatly deteriorated by rust.

A description is given of an accident from lightning to the church of St. Sulpice; but this building had no lightning rod. In the case of the church of St. Clotilde there are five upper terminals, two on the two spires, the remaining three along the ridge of the main roof. The building was amply protected as far as its length was concerned, but the transept was not so thoroughly protected. The five terminals were joined to a conductor which went round the building, and was connected with the ground. A second conductor led from one of the terminals to the ground, where it terminated in a second pit. The conductors were made of iron rods 18 millimetres (·71 inch) in diameter, joined by collars and pinned and the whole covered with paint. They terminated in distributors plunged in the underground water in walled pits. They were supported by insulated collars. The building has an iron roof. The church had been struck by lightning at least four times since the lightning rods had been erected. The first time, twelve years ago, the lightning struck the rod placed on the transept, and carried away the platinum tip of the copper point. Since then the rod has received another discharge, and the copper point is bent to the S.W. In January, 1872 or 1873, the lightning struck the western tower, and shattered one of the stones above one of the windows of the staircase.

“One of the platinum tips is gone, and many are blunted. The conductivity of the conductor is very bad, and the joints are very much damaged: hence the accident to the tower. The greater number of the glass insulators are broken, or gone altogether.”

In the case of the church of St. Eloi, which had one terminal on the spire, one conductor, formed of iron wire rope 2 centimetres (·78 inch) in diameter, joined at 3 metres (9 feet 10 inches) above the ground to an iron rod 25 millimetres (·97 inch) in diameter, which entered the ground and ended without branches in a pit filled with charcoal. The soil was dry and calcareous. The conductor was made up of many lengths of rope, old pieces apparently having been used; the joints were in bad condition, and needed soldering. The underground part was deeply rusted.

“In September, 1874, lightning struck the spire, twisted the conductor, broke the terminal, threw down the part above the cross, and made great cracks in the apse.”

During the building of the Mairie of the 20th Arrondissement, the lightning struck a fir-pole in the scaffolding. It did not do any damage, being carried away by the chain attached to the pole, from which it took all the rust, and being thence conducted by some pieces of iron roof framing lying on the ground.

There are several other accounts of accidents, but they are mostly represented by the foregoing examples.

INSTRUCTIONSas to the application ofLIGHTNING CONDUCTORSfor protection ofPOWDER MAGAZINES, ETC.

Issued with Army Circulars, dated May 1st, 1875.

(Abstracted by Prof. T. Hayter Lewis, F.S.A.)

1. The principles adopted by Sir W. S. Harris, as shown in the Appendices A and B, to this paper, still held to be sound.

2. The terminating plane of action of lightning is sometimes beneath the surface of earth, which, if moist, forms good medium for diffusion of electricity.

3. Dry soil is to be regarded as non-conducting matter.

4. Therefore conductor to be taken into soil permanently damp.

5–6. Underground magazines are usually in dry soil, and should

therefore be fitted with conductors as in the case of similar magazines above ground.

8–9. Casemated batteries of modern construction, with magazines in basement should have conductors on the parapet or terreplein from end to end of battery, attached to vertical conductor into earth. Flagstaff should have conductor. In large works there should be several points 5 feet above top of building. Iron verandahs and railings are good conductors when with good earth connections.

10. Iron buildings are good conductors. But if covered with asphalte, concrete, &c., rods or points must be provided projecting above asphalte, &c., and with good earth connections. Iron shields should be connected with conductors.

11. Copper is recommended as best conductor; it is not liable to corrosion, and very durable.

12. But if exposed to injury, or likely to be stolen or corroded, copper may be replaced by iron, provision being made for its smaller conductivity—viz., ⅕th that of copper.

13. Copper rods to be ½ inch diameter; copper tubes to be ⅝ × ⅛ inch thick; copper bands to be 1½ × ⅛ inch thick.

14. If the conductor be of iron, solid rods to be 1 inch diameter; solid bands to be 2 inches wide × ⅜ inch.

15–16. The fusing temperature of copper is 1994° Faht.; whereas that of iron is 2786° Faht. So far there is a marked advantage over copper. But it rusts easily, and then the electrical resistance is immensely increased. Roughly speaking, an equal conducting power may be obtained either in iron or copper for the same cost, the number of iron conductors being greater in proportion to the less cost, and the more conductors being the better.

17–19. Expansion and contraction are to be carefully provided against;e.g.by suitable bends at intervals in long lines of horizontal conductors and by bearing collars, allowing of slip in vertical lines.

20. Soldered or welded joints are desirable, but not absolutely necessary.

21. Gives engravings of connections recommended by Sir W. S. Harris, where soldered joints cannot be used, and which fulfil the conditions specified in sections 17–19.

22. Soldered or welded joints to be used where discharge is possible with unsoldered joints, and likely to ignite dust or inflammable substances near.

23. Iron may be connected by similar joints as for copper, or by screw joints as for gas pipes. No white lead to be used, it being a bad conductor.

24. Iron flat bands may be connected by rivets or screws, working in slots, to provide for expansion, each surface in contact being at least six times the sectional area of band.

25. Copper bands to be similarly connected. Joints between different metals may be soldered, screwed, or rivetted, the extent of surface in contact being regulated by the dimensions of the metal of the least conducting power. Access of moisture to surfaces in contact must be prevented, on account of local galvanic action and decomposition.

26. No precise limit can be fixed to protecting power of conductors. In England the limit is usually assumed as being the radius of the height from ground. It may be sufficiently correct for practical purposes, but cannot always be relied upon.

27. Conductors do not attract lightning; they only diminish the resistance due to the air. Even a change in the nature of the soil -over which a cloud passes may produce a discharge.

28. One angle of a building may receive a discharge, though another angle have a conductor. So every prominent part of a building containing explosive material should have a conductor.

29. In buildings of uniform height, provide a solid rod 5 feet above it at each end, and at each 45 feet in length; if the conductor be of iron the top should be gilt.

30. Buildings not over 20 feet long to have one vertical conductor at end, and a horizontal conductor on ridge.

31. If 20 to 40 feet long to have one vertical conductor in centre, and one along ridge, as last.

32. If 40 feet long to have two vertical conductors; if 100 feet long three conductors; in both cases with conductor along ridge.

33. Similar principles to be adopted in larger and more complicated buildings.

34. Each prominent part should have a conductor. The value of three or four points to terminals is not apparent unless the points are widely separated.

35. Conductors are to be connected horizontally,e.g., by ridge or eaves, which, when of metal, should be invariably connected with conductor. All metal surfaces whatever to be also so connected.

36. Sir W. S. Harris considers the relative conductivity of the several metals as being—of lead 1, tin 2, iron 2½, zinc 4, copper 12. So lead cannot be altogether depended on.

37. Avoid long lengths of horizontal conductors without earth contacts, as the currents might leave the conductor, and pass to earth, causing danger. Avoid sharp angles.

38. Good earth connections most important. Conductors are to be led into springs or wells or earth permanently wet. Not into watertight tanks. Shingle, dry sand, or dry mould are not sufficient.Provide several earth connections in all large systems of conductors as a precaution.

39. Lead conductors into ground in trenches 18 inches deep. Not less than 30 feet of metal to be in contact with moist earth.

40. Lead a flow of water over trenches if possible,e.g., from rain water pipes.

41. Trenches in rocky or dry soil to be 30 to 120 feet long, so as to obtain all moisture possible.

42. Connections in trenches may be of old iron, forming continuous metallic surface, the trenches to be filled with cinders or coal ashes. Water pipes form excellent earth connections, but gas pipes are dangerous.

43–44. Frequently inspect conductors, especially as to joints connecting different metals and defects in iron from rust.

45–46. Galvanize iron, care being taken that the coating is good.

47. Great care to be taken in case of contact of zinc coating with other metals, especially copper.

April 8th, 1875.

FRED. E. CHAPMAN.Inspector General of Fortifications.

FRED. E. CHAPMAN.

Inspector General of Fortifications.

1. The earth’s surface and clouds are the terminating surfaces of electric actions, and buildings, &c., are only points, as it were, of earth’s surface in which the whole action vanishes.

2. Electricity when confined to substances resisting its progress, as air, glass, dry wood, stones, &c., exerts a terribly explosive power.

3. But when confined to bodies, such as metals, offering small resistance, its violent expansion or disruptive action is greatly reduced or avoided altogether, and becomes a continuous current comparatively quiescent. But if body be small, as wire, it may be heated or fused. Resistance is so small that a shock has traversed copper wire at the rate of 576,000 miles a second; resistance increases with length and diminishes with area of section of conductor.

4. So a building metallic in all its parts, or a man in armour is safe.

5. So endeavour to bring buildings into the same passive or non-resisting state as if of metal.

6. So conducting channels of copper should be systematically applied to walls, either in plates united in series one over another, not less than 3½ inches wide and 1/16th and ⅛th of an inch thick, or of stout copper pipe not less than 3/16ths of an inch thick, and 1½ to 2 inches diameter, fixed to building by braces or copper nails or clamps. Terminals to be solid metal rods, projecting above to a moderate and convenient height. Earth connections to be by one or two branches, leading out about a foot below ground—if possible into moist ground, but if dry, use old iron or other metallic chains so as to expose a large metallic surface.

7. All metals in roof, &c., of building to be connected with main conductors; any prominent chimney to have a pointed conductor taken along it to metals of roof.

8. An electrical discharge never leaves a perfect conductor to passto a very bad one, so the apprehension of lateral discharge is absurd. Furious discharges have fallen on the conductors to the masts of H.M. ships, and passed through copper bolts in bottom without injury even to persons leaning against the conductors.

9. Metallic bodies have no specific attraction for electricity more than wood or stone have; all matter is indifferent so far as regards a specific attraction. Lightning falls indiscriminately upon trees, rocks and buildings, whether with metals about them or not;e.g., at Plymouth Dockyard in May, 1841, a granite chimney, 120 feet high, without any metal in it, was struck, and yet it was within 300 feet of a clock-tower of equal height, having metal weathercock, a dome covered with metal and large conductor along it to ground. The damage ceased where the chimney passed through a massive metallic roof, having a conductor from it to the ground. Here the lightning fell on a building, which, according to the popular idea, held no “invitation” in preference to a structure whichdidhold such “invitation.”

10. If efficient conductors provide free and uninterrupted course for electrical discharge, it will follow that course without danger to general structure; ifnot, then this irresistible agency will find a course for itself and shake all imperfect conducting matter in pieces in doing so. The great object is to provide a line or lines of small resistance in given directions, less than the resistance in any other line of the building. The conductor no more attracts lightning than a gutter or water pipe attracts a flow of water.

11. It follows that a magazine if of metal would be safer than if built in the usual way. Metallic gutters and ridges, with continuous metallic communications to earth, are unobjectionable.

Note.—It is as wrong to isolate conductors from buildings by glass or resin, as it would be to place rain water pipes 10 feet from the building from which they should carry off the water.

An instance is given of an iron conductor which was placed 10 feet from a house, the latter being, notwithstanding, struck at the point nearest to the conductor, which was untouched.

12. Pointed terminations tend to break the force of lightning when it falls on them. Before explosion a large amount of discharge passes off through pointed conductors.

Pointed conductors should be solid copper rods, about ¾ inch diameter, and a foot long, united by brazing to the conducting tube. It is not necessary to gild the points, or form them of platinum. Sometimes even, this would be detrimental, as platinum has only half the conducting power of copper. The oxidations of the surface of conductor is of little moment; and in case of copper very trifling. In any case the conducting surface is better than the bad—or non-conducting air. The electric telegraph wires work well, though enclosed by gutta percha or other non-conducting matter. It is sufficient if the terminal solid rod be even roughly pointed. But even a ball, a foot diameter, would be a point as opposed to 1,000 acres of charged clouds.

Note.—Experience contradicts the idea that the conductor protects a certain area. The foremast of a ship has been struck, though the mainmast has been protected by conductor.

13. Copper linings to doors and windows of magazines, are not objectionable, but useless for keeping out lightning. They should be connected with the general system of conductors.

1. A given quantity of electricity melts the same quantity of metal, whether in a solid or hollow form. So far it is immaterial which form the conductor has. But supposing the mass of metal to be so large that the heating effect may be neglected. It is proved that the greater the surface, the less is its intensity or power at any point, the intensity approaching the second power or square of the surface inversely. It is important to give the charge free room of expansion by increasing the surface of conductor, so as to reduce the mechanical activity of shock to the least possible. Rectangular flat bars may be employed.

2. A rain water pipe communicating with main conductor, should have earth connection. All imperfect substances, as masonry, and ship masts, transmit a certain portion of electricity without explosive action. One great use of the conductor is to relieve the wood or masonry of the quantity it cannot discharge without explosion.

3. Conductors of small iron rod or wire are very objectionable. They commonly rust at the joints, and have fallen to pieces, and often been knocked to pieces by lightning. Iron may, certainly, be employed with advantage, but should be galvanized. Zinc is an even better conductor than iron; and being spread over the surface is not open to the objection of making a conductor of two metals of unequal conducting power. A good and efficient conductor might be formed of galvanized iron. It should be of wrought iron, galvanized, of 2 inches diameter, with screwed joints ofextrathickness. Copper tubing is, however, always to be preferred.

4. In dry or rocky soil, complete the conductor by leading old iron chains out from the walls in several directions, or by leading a flow of water over them. Fortunately a thunder storm is usually attended by heavy rains. The iron chains should extend 30 to 50 feet, and be a foot or 18 inches under ground. The termination in a large surface of moist earth would be preferable to that in a well, as the action is a superficial one of expansion in all directions. In thetinleaf coatings of the electrical jar, the charge is not influenced by the thickness of metal.

W. SNOW HARRIS.

(Abstracted by G. J. Symons, F.R.S.)

The Gunpowder exploded at 4.30 p.m. on August 6th, 1878, during the greatest intensity of a violent thunderstorm. The building, was brick, with brick arched roof, length 9 feet, width 5 feet, height6 feet (internal dimensions). The store had a uniform thickness of three bricks, and was furnished at the one end with an iron door, at the other end with a lightning conductor. The conductor consisted of a copper wire rope, 10 gauge copper wire, the rope being 7/16 inch thick, having four points at the top (one large one in the centre, and three smaller ones round it), it extended to about 13 feet above the top of the building, and about the same length was carried into the ground and terminated in a drain. The conductor had been erected in 1876, by Mr. John Bisby, of Leeds, and was fixed to a pole distant about 2 inches from the end of the building opposite to that in which the iron door was fixed, it was not connected with the iron door in any way. No one was near the store when the powder exploded, and it seems probable that the earth connection of the conductor was bad, that the mass of iron in the door offered at least an equally good path—and that the gunpowder was ignited by a flash passing between, the two imperfect conductors.

View and Plan of Bruntcliffe Gunpowder Store

“The only structural damage effected was produced by the impingement of bricks, which striking with great force, had in a few instances, partially penetrated or displaced brick work in the dwelling-houses and buildings, and a portion of the iron of an iron church was broken by a piece of projecteddébris. A brick was driven through a window in one of the houses at three hundred yards, and broke a bedstead. As far as I have been able to discover no other structural injury was occasioned.”

This accident appears to suggest several conclusions:—

“In the first place it appears to me to afford a striking confirmation of the principle which has been repeatedly and emphatically enunciated by Sir William Snow Harris and other authorities on the subject of lightning conductors, that in order to secure an efficient protection for a given building, all the metal of the building, and as far as possible the whole of the structure itself, should be brought into actual connection with the system of conduction; in other words, that the general conducting power of the mass of the edifice should be completed, and all attractive and prominent parts allied in one protective combination, so as to “bring the whole” (as it has been expressed by Sir William Snow Harris) “as nearly as may be into that passive or non-resisting state which it would assume, supposing the whole were a mass of metal.” In the present case, assuming the conductor itself to have been efficient, a point which there seems no sufficient reason for doubting, the system of conduction was obviously defective. Not only was the whole length of the building left unprotected, the conductor having been on a pole at one end, and carefully insulated from the building, but the iron door which was at the opposite end, was absolutely unconnected therewith, and was not itself supplied with any earth connection.”

“It appears clear, therefore, that even what may be deemedper sean efficient lightning conductor,i.e.a conductor, which considered alone, offers a path of little or no resistance even to a powerful electric current, does not afford a reliable protection to a building unless it be scientifically applied, and with due regard to those principles upon which the more eminent authorities on electrical science are agreed. To a disregard of these principles, especially in respect of the iron door being left out of the system of conduction, and unconnected therewith, I believe the present accident may be attributed.”

(Phil. Trans., 1773, p. 42, and 1778, Part I., p. 232.)

(Abstracted by Prof. W. G. Adams, F.R.S.)

Report of a Committee consisting of the Hon. Henry Cavendish, Dr. Watson, Dr. Franklin, Mr. J. Robertson, Mr. Wilson, and Mr. Delaval, appointed by the Royal Society, “to consider of a method for securing the powder magazine at Purfleet.”

A powder mill at Brescia having blown up in consequence of being struck by lightning, the Board of Ordnance applied to Mr. B. Wilson to know in what way the powder magazine could be protected. He recommended that a blunt conductor should be employed, whereas Dr. Franklin recommended a pointed conductor. The Committee met and Dr. Franklin read a paper on the subject, and the report of the Committee was in conformity with Dr. Franklin’s views.

The Committee went to Purfleet and examined the buildings. They found that the barrels of powder, when the magazines were full,lay piled on each other up to the spring of the arches; on each barrel were four copper hoops, which with vertical iron bars formed broken conductors within the building. These iron bars were ordered to be removed.

The Committee advised that ateach endof each magazine a well should be dug in or through the chalk, so deep as to have in it at least four feet of standing water. From the bottom of this water should arise a piece of leaden pipe to or near to the surface of the ground, where it should be strongly joined to the end of an upright iron bar, aninch and a halfin 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 of copper, the iron to be painted.

Lead was mentioned for the underground part as less liable to rust, 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. 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, so making metallic communication between the two bars of each building.

It was also advised that two wells be dug within twelve feet of the doors, one to the north of the north building and the other to the south of the south building, and that metallic communications be made between the water in them and the leaden coping of the roof.

The Board house stood 150 yards from the magazines, on elevated ground, and was a “lofty building with a pointed hip-roof, the copings of lead down to the gutters, from which leaden pipes descend at each end of the building into the water of wells of forty feet deep, for the purpose of conveying water forced up by engines to a cistern in the roof.”

As to the Board-house, they thought 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 building below it; they therefore proposed no other conductor for that building, and only advised erectinga pointed iron rodon the summit, similar to those before described and communicating with those conductors.

Mr. Wilson dissented from that part of the Report which recommended that each conductor should be pointed, because,he says, “by points we solicit the lightning, and may promote the mischief by drawing the charges from charged clouds, which would not discharge at all on the building if there were no points on the conductors.” By experiments made and appealed to at the Committee the difference in the effects between pointed and blunted conductors is as twelve to one. Mr. Wilson states that, “A thunder cloud, therefore, if it acted at 1200 yards distance upon a point, would require a blunted end to be brought within the distance of 100 yards, and beyond those limits would pass over it without affecting it at all.” He also says, “Thelongerthe conductors are above the building, the more danger is to beapprehended from them. I have always considered pointed conductors as being unsafe by their great readiness to collect the lightning in too powerful a manner.”

Mr. Wilson adds an account of an accident to St. Paul’s Church, and some curious reasoning on it in support of his own views. (See Phil. Trans. 1773, p. 59–61.)

On the 15th of May, 1777, the Board House at Purfleet was struck by lightning, and some of the brickwork damaged (See Phil. Tran., 1778, Pt. I., p. 232). About 6 p.m., after heavy rain through the day, a heavy cloud hung over the house for some time, and Mr. Nickson, who watched it from the house and gives the account, says he suspected that some of the conductors might find employment from it. He had not been long at the window before a violent flash of lightning and clap of thunder came together. The lightning struck one of the iron cramps that hold the coping, and made a dent in the lead of the cramp and the stone adjoining it, throwing some stone down and slightly disturbing about a cubic foot of brickwork at A. The iron cramp was situated over a plate of lead, and the ends of it, inserted in the stone, came within 7 inches of that plate, which communicated with the gutter, and served as a fillet to it; this gutter was part of the main conductor of the building. The lightning struck through the stone, &c., to the corner of the plate, fusing a very small portion of it. From this point no farther effect of the lightning could be traced. At the distance of seven feet and a-half from the place struck, a large leaden pipe went down from the gutter to a cistern of water in the yard. It is remarkable that the surface of one of the hip-rafters, four inches and a-half in diameter, covered with lead (communicating with the gutter), andreaching within twenty-eight inchesof the place struck, seems not to have been at all affected. The distance from the point of the conductor on the house to the part struck was forty-six feet.

View of Board House, at Purfleet

A fresh Committee of the Royal Society, consisting of Mr. Henly,Mr. Lane, Mr. Nairne, and Mr. Planta, recommended a channel to be made from cramp to cramp round the parapet, filled with lead, and connected in four places with the main conductor on the roof of the building.

Mr. Wilson again dissented from their report, and attributed the hanging of a heavy cloud over the house (it being calm at the time) to the presence of the pointed lightning conductor.

An account of Mr. Wilson’s elaborate series of experiments at the Pantheon on a long cylinder to illustrate the effects of pointed and rounded conductors occupies seventy pages of the Philosophical Transactions; and another Committee of the Royal Society, consisting of Sir John Pringle, Dr. Watson, Henry Cavendish, W. Henly, Bishop Horsley, T. Lane, Lord Mahon, E. Nairne, and Dr. Priestley, report in favour of having additional conductors ten feet high, with copper eighteen inches long, finely tapered and acutely pointed placed upon the magazines. They conclude that “elevated rods are preferable to low conductors terminated in rounded ends, knobs, or balls of metal,” conceiving that, the experiments and reasons, made and alleged to the contrary by Mr. Wilson, are inconclusive.

Mr. Wilson’s objections are again urged by Dr. Musgrave, but called in question by Mr. Nairne (see Phil. Trans., 1778, Pt. 2, p. 823), who makes a series of experiments to illustrate the advantage of pointed conductors.

Both Mr. Wilson’s and Mr. Nairne’s experiments agree in showing that “pointed conductors draw off the electricity from a cloud at a much greater distance than those which are blunted.” Mr. Wilson objecting that this draws the charged cloud from a greater distance; and Mr. Nairne concluding that “a charged body is exhausted of more of the fluid by a pointed than by a blunted conductor,” and so is not likely to cause so much damage since it discharges itself more gradually.

By Benjamin Franklin.Fifth edition. London, 1774.

(Abstracted by Prof. T. Hayter Lewis, F.S.A.)

The author shows that pointed bodies draw off electricity much more effectually than blunt ones.

When the land is hot, “the lower air is rarified and rises; the cooler, denser air above, descends.”

The clouds meet over the heated place, “and if some are electrified and others not, lightning and thunder succeed, and showers fall.”

“As electrified clouds pass over a country, high hills and trees, towers, spires, masts, chimneys, &c., as so many points, draw the electrical fire and the whole cloud discharges there.”

Therefore it is dangerous to take shelter under a tree. It is safer to be in the open fields, especially if the clothes are wet.

Metals are fused, possibly without heat; the lightning creating a violent repulsion of the particles of the metal it passes through.

[He afterwards admits this opinion to be erroneous.]

Describes experiments with sharp-pointed metallic bodies, and says: “May not the knowledge of this power of points be of use to mankind in preserving houses, churches, ships, &c., from the stroke of lightning, by fixing on their highest parts upright rods of iron, made sharp as a needle, and gilt, to prevent rusting; and from the feet of these rods lead iron wire down the outside of the building into the ground; or down one of the shrouds of a ship and her side till it reaches the water.”

“Would not pointed rods probably draw the electrical fire silently out of a cloud before it came nigh enough to strike, and thereby secure us from that most sudden and terrible mischief?”

He mentions the case of the topmast heads of a ship being struck, but having flames upon them like very large torches before the stroke.

He thinks that if there had been a good wire conductor from the heads to the sea there would have been no stroke or damage.

He records the experiments on the 10th of May, 1752, at Marly, of M. D’Alibard, who placed upon an electrical body a pointed bar of iron 40 feet high. In a thunder storm sparks of fire were attracted from it.

Again, at Paris, on the 18th of May, with the same result, by M. de Lor, with a bar of iron 99 feet high upon a cake of resin 3 inches thick and 2 feet square.

Similarly in London in July, 1752, by Mr. Canton.

He refers to other experiments.

He experimented in 1752 with a kite of thin silk (as being able to bear the wet), having a very sharp-pointed wire fixed to its top, above which it rose about a foot. The kite was raised by twine, the part in the hand being made of silk and kept quite dry.

The pointed wire will draw the electric fire from thunder clouds, and when the rain has wet (sic) the kite and twine, so that it conducts the electric fire freely, they will be electrified, and the electric fire will stream out plentifully on the approach of the knuckle.

“Spirits may be kindled, &c., as with a rubbed glass or tube, and thereby the sameness of the electric matter with that of the lightning be completely demonstrated.”

September, 1752. He erected “an iron rod to draw the lightning into his house in order to experiment on it.”

After many experiments, he concluded that “the clouds of a thunderstorm are most commonly in a negative state of electricity, but sometimes in a positive state.” The latter, he believed, rare.

“So that, for the most part, in thunderstrokes, it is the earth that strikes into the clouds, and not the clouds into the earth.”

In the contrary (rare) case the cloud was, “I conjecture, compressed by the driving winds or some other means, so that part of what it had absorbed was forced out, and formed an electric atmosphere round it in its denser state, so communicated positive electricity to my rod.”

“The electric fluid, moving to restore the equilibrium between the cloud and the earth, takes, in its way, all the conductors it can find (v.page 132 of Franklin’s book)—as metals, damp walls, moistwood, &c.—and will go considerably out of a direct course for the sake of the assistance of a good conductor.”

“Explosions only happen when the conductors cannot discharge it as fast as they receive it, by reason of their being incomplete, disunited, or too small, or not of the best materials for conducting.”

He supposes that a wire ¼ inch diameter will conduct the electricity of any one stroke of lightning ever known.

Iron is the best material, as least liable to fuse.

“Pointed rods erected on buildings and communicating with moist earth would either prevent a stroke, or, if not prevented, would conduct it so that the building should suffer no damage.

He gives instances of a small wire acting as conductor and saving the building, though the wire, being too small, was utterly destroyed.

His theory as to the crooked course of lightning is as follows:

“Who knows but that there may be, as the ancients thought, a region of this fire (electric) above our atmosphere, prevented by our air and its own too great distance of attraction from joining our earth. Yet some of it be low enough to attach itself to our highest clouds,” which thence become electrified, &c.

“I am still at a loss about the manner in which clouds become charged with electricity, no hypothesis I have yet formed perfectly satisfying me.”

He describes how he and others have been struck down by electric shocks without feeling pain or sustaining permanent injury.

For protecting powder magazines, erect a mast not far from it, and 15 or 20 feet above the top of it, with a thick iron rod fastened to it, reaching down till it comes to water.

“In buildings the rod may be fastened to the walls, chimneys, &c., with staples of iron. The lightning will not leave the rod (a good conductor) to pass into the wall (a bad conductor) through these staples. It would rather, if anywhere in the wall, pass out of it into the rod to get more readily into the earth.”

If the building be very extensive, two or more rods may be placed at different parts for greater security.

It is well not to sit near the chimney, or gilt objects, during a thunderstorm.

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