MEANS to be adopted for ensuring PERSONAL SAFETY FROM THE EFFECTS OF LIGHTNING.(Abstracted by Prof. G. Carey Foster, F.R.S.) Works consulted:—
The danger to men and animals from the effects of lightning arises from the fact that the bodies of living animals form comparatively good conductors of electricity,—better, that is, than rain-water (probably better even than sea-water), or than trees, walls of brick or stone, hay-stacks, or in fact than almost any common objects consisting of non-metallic materials. It may be assumed that the path of a lightning-discharge striking the earth is determined by the line of least inductive resistance between the thunder-cloud and the earth. 7. The apparently capricious way in which lightning often strikes is not inconsistent with this statement. It proves, however, that the line of least inductive resistance is partly determined by atmospheric or terrestrial conditions which are not perceived by the eye. 8. Is there any evidence to show that soldiers wearing spiked helmets, or marching with fixed bayonets, are specially liable to be struck by lightning? Various ancient writers—CÆsar, Seneca, Livy, Pliny, and others—mention luminous appearances (“Fire of St. Elmo”) presented by the javelins or pikes of soldiers during thunderstorms at night. As to people indoors, we need only consider the case of those who are in buildings which are either not at all or only imperfectly protected by conductors; for, if a building is thoroughly protected, whatever is inside it is protected also. Indoors, as out of doors, we have to avoid forming part of a line of least inductive resistance. This consideration leads to such rules as the following:—Keep to the lower rooms of a house, rather than to the upper rooms; also keep as much as possible in the middle of the room you are in, but avoid being under a metal chandelier, or a lamp, or other object hung by a metal chain or wire; keep away from a stove or fire-place, especially when a fire is burning in it; keep away from large metallic objects which are not in electrical connection with the ground, especially if they are above the level of the head (as mirrors, or pictures with gilt frames, hung against the wall), or below the feet (as an iron pillar or beam supporting the floor, or an iron staircase leading to a lower storey but not continued to one above). Franklin recommends “sitting in one chair and laying the feet up in another,” or as a further precaution “to bring two or three mattresses or beds into the middle of the room, and, folding them up double, [to] place the chair upon It may be added for the comfort of the timid that Arago concludes that the danger of being struck by lightning in a town (Paris) “is less than the danger of being killed in passing along the street by the fall of a chimney, or flower-pot, or of a workman engaged upon a roof; this latter danger being [he imagines] one which occasions very little uneasiness.” Also it seems to be the universal testimony of those who have been restored after being struck by lightning that they had not been conscious of either thunder or lightning. We may accordingly conclude that all danger from a given discharge is over, not merely by the time we hear the thunder, but as soon as ever we see the flash. G. C. F. INJURY to GAS AND WATER-PIPES by LIGHTNING.The city gas company of Berlin, having expressed the fear that gas-pipes may be injured by lightning passing down a rod that is connected with the pipes, Professor Kirchhoff has published the following reply:— “As the erection of lightning-rods is older than the system of gas and water-pipes as they now exist in nearly all large cities, we find scarcely anything in early literature in regard to connecting the earth end of lightning-rods with these metallic pipes, and in modern times most manufacturers of lightning-rods, when putting them up, pay no attention to pipes in or near the building that is to be protected.” Kirchhoff is of the opinion, supported by the views of a series of professional authorities, that the frequent recent cases of injury from lightning to buildings that had been protected for years by their rods, are due to a neglect of these large masses of metal. The Nicolai Church, in Griefswald, has been frequently struck by lightning, but was protected from injury by its rods. In 1876, however, lightning struck the tower and set it on fire. A few weeks before, the church had had gas-pipes put in it. No one seems to have thought that the new masses of metal which had been brought into the church could have any effect on the course of the lightning, otherwise the lightning-rods would have been connected with the gas-pipes, or the earth connection been prolonged to proximity with the pipe. A similar circumstance occurred in the Nicolai Church in Stralsund. The lightning destroyed the rod in many places, although it received several strokes in 1856, and conducted them safely to the earth. Here, too, the cause of injury was in the neglect of the gas-pipes, which were first laid in the neighbourhood of the church in 1856, shortly before the lightning struck it. The injury done to the school-house “If it were possible,” says Kirchhoff, “to make the earth connection so large that the resistance which the electric current meets with when it leaves the metallic conducting surface of the rod to enter the moist earth, or earth water, would be zero, then it would be unnecessary to connect the rods with the gas and water-pipes. We are not able, even at immense expense, to make the earth connections so large as to compete with the conducting power of metallic gas and water-pipes, the total length of which is frequently many miles, and the surface in contact with the moist earth is thousands of square miles. Hence the electric current prefers for its discharge the extensive net of the system of pipes to that of the earth connection of the rods, and this alone is the cause of the lightning leaving its own conductor.” Regarding the fear that gas and water-pipes could be injured, the author says: “I know of no case where lightning has destroyed a gas or water-pipe which was connected with the lightning-rod, but I do know cases already in which the pipes were destroyed by lightning because they were not connected with it. In May, 1809, lightning struck the rod on Count Von Seefeld’s castle, and sprang from it to a small water-pipe, which was about 80 metres from the end of the rod, and burst it. Another case happened in Basel, July 9, 1849. In a violent shower one stroke of lightning followed the rod on a house down into the earth, then jumped from it to a city water-pipe, a metre distant, made of cast iron. It destroyed several lengths of pipe, which were packed at the joints with pitch and hemp. A third case, which was related to me by Professor Helmholtz, occurred last year in Gratz. Then, too, the lightning left the rod and sprang over to the city gas-pipes; even a gas explosion is said to have resulted. In all three cases the rods were not connected with the pipes. If they had been connected the mechanical effect of lightning on the metallic pipes would have been null in the first and third cases, and in the second the damage would have been slight. If the water-pipes in Basel had been joined with lead instead of pitch, no mechanical effect could have been produced. The mechanical effect of an electrical discharge is greatest where the electric fluid springs from one body to another. The wider this jump the more powerful is the mechanical effect. The electrical discharge of a thunder cloud upon the point of a lightning rod may melt or bend it, while the rod itself remains uninjured. If the conductor, however, is insufficient to receive and carry off the charge of electricity, it will leap from the conductor to another body. Where the lightning leaves the conductor its mechanical effect is again exerted, so that the rod is torn, melted, or bent. So, too, is that spot of the body on which it leaps. In the examples above given it was a lead pipe in the first place, a gas-pipe in the last place, to which the lightning leaped when it left the rod, and which were destroyed. Such injuries to water and gas-pipes near lightning-rods must certainly be quite frequent. It would be desirable to bring them to light, so as to COLLIERY WORKINGS STRUCK BY LIGHTNING.The Institute of Mining Engineers. A meeting of the members of the North of England Institute of Mining and Mechanical Engineers took place in the Wood Memorial Hall on Saturday, Mr. G. C. Greenwell in the chair, when the secretary read an account of an investigation which had been made into a statement that lightning had entered Tanfield Moor Colliery on the 12th of July last, and traversed the workings in several directions. Mr. Wm. Joicey kindly gave permission to examine the witnesses of the occurrence, and the workings of the colliery, so that a complete and accurate report could be drawn up of the circumstance; and on the 30th of July, Mr. C. Berkley, Mr. J. B. Simpson, Mr. W. H. Hedley, and the secretary went out to the colliery, and were met by Mr. W. Joicey, one of the owners; Mr. Pringle, the viewer; and Mr. Arkless, the resident viewer. The top of the working shaft at the colliery is 34 fathoms from the Shield Row seam. An incline bank leads northwards from the working shaft and ultimately reaches the day by a drift, and a little to the south is an up-cast shaft. The engine way leads south from the working shaft, and goes in-bye to a goaf. Between the goaf and the working shaft are two down-cast shafts. From what can be gathered the lightning passed down the working shaft and struck the flat sheets, and then divided itself into two parts, one of which went north up the incline way and probably passed out to the day by the drift, where it was supposed to have left traces of its exit in marks upon a bank near by. The other part went south along the engine way; but after passing a point where it was noticed its further course was not known. The thill of the seam is composed of soft sagger, and the roof of strong post, both of which would offer great obstruction to the absorption of the electric fluid; and the probability was that this portion of the fluid had been dissipated in the goaf, or had forced an exit by way of the down-cast shaft. The evidence taken was appended.—Joseph Kirtley, back-overman, said a light, distinct but not very bright, fell and struck the flat sheets, and split up into several lights like a lot of lighted matches. He could only see the light for a moment among the tub wheels. It ACCIDENTS by LIGHTNING at the SWAN COTTON MILL, CHADDERTON, OLDHAM. Report by J. Doherty, A.S.T.E.[On July 13th, 1880, during a thunderstorm, the large 400 light gas meter of this mill, though locked up in a cellar, and with no light near it, exploded, and the gas, which is supplied through a 4–inch main, was ignited. This was repaired, but on July 5th, 1881, during another thunderstorm, precisely the same accident occurred. At the request of Mr. Preece, F.R.S., Mr. Doherty, of H. M. Postal Telegraph Service, went and inspected the works, and forwarded the following report.—Ed.] 21st July, 1881. A very careful investigation of the Swan Mill premises has been made, with a view of arriving at some explanation of the recent injury to the gas meter, which was undoubtedly caused by lightning. The building is a large one, having for its internal supports a number of cast iron columns running from floor to basement, and on the top of the building, I am told, there are numerous iron gutters; round the various rooms are carried large iron gas pipes, and in numerous instances this gas piping is dead against the iron caps of the columns, thus the lightning may have struck any portion of the building, and the current have been conveyed, safely, by the gas piping to the large gas meter, where an imperfect joint (electrically imperfect) existed, viz., an india-rubber ring placed between the faces of the iron joint. It is to be regretted that the connecting pipes were not on the premises at the time of my visit, otherwise I could have spoken with a greater degree of certainty, but I have not the slightest doubt in my own mind as to the insulating ring between the joints being the cause of rupture. Tests were made, showing that the continuity of the present pipe is lessened by the existence of another india-rubber ring, and the oxidation of the connecting screws at another joint. I advised the Directors of the Spinning Company to connect the outlet and inlet main pipes by iron or copper wire straps. I feel convinced that if this had been done prior to 5th July the accident would not have occurred. J. Doherty. ESSAY on the effects of HEAVY DISCHARGES OF ATMOSPHERIC ELECTRICITY, as exemplified in the Storms of the Summer of 1846 * * * * and Remarks on the Use and Application of LIGHTNING CONDUCTORS. By E. Highton, Esq., C.E.(Transactions of the Society of Arts for 1846–47. London. Sm. 4to). (Abstracted by G. J. Symons, F.R.S.) The author’s primary object in studying the subject was the discovery of a method of protecting telegraphic apparatus from injury and danger. That has long been accomplished, but some remarks in the Paper seem worthy of extraction. [This seems to prove two things—(1) The fallacy of the old notion that ringing the church bells sent away thunderstorms (see also ante, p. (37);) and (2) that even very imperfect conductors, such as this conductorless steeple, carry off much electricity by the silent or brush discharge.—G. J. S.] Mr. Highton found that the leaden flashings were frequently burst up, the lead being sometimes forced up somewhat like a miniature volcano. This he attributes to the explosion of confined atmospheric air, but obviously water converted into superheated steam would yield a greater expansive force. The author quotes a case at Water Newton, Wansford, Northamptonshire, where, although the Church had tower and spire, and the whole roof was covered with lead, a tree 90 feet from the spire, and not one-third the height of the spire, was struck, but the Church was not. This the author attributes partly to the action of the leaves of the trees, and partly to there being no iron or other vertical spouting to the Church. Mr. Highton’s “Practical Rules” are literatim et verbatim:— 1st. Where a building has any quantity of vertical metallic work, it is quite necessary, for its protection against Lightning, that it should have an artificial Lightning Conductor, (unless the materials of themselves form a natural one). 2ndly. It is very desirable, that all metallic circuits, especially those in a vertical direction, should be metallically connected with the system of Lightning Conductors. 3rdly. That, in many instances, a single insulated Lightning Conductor attached to a building may become positively injurious and dangerous; as it may cause many a cloud to discharge its electric force at that point, which would otherwise have passed over, and poured its power in some other channel. 4thly. That, where Lightning Conductors are employed, they ought to be thoroughly well erected, and every course or channel that the Electric fluid has open to it carefully considered, and a division of the charge in those quarters provided against. 5thly. That a Lightning Conductor, or a system of Lightning Conductors, where properly and scientifically erected, are perfect safeguards against the effects of heavy discharges of Atmospheric Electricity. But, if improperly applied, they may become a most dangerous addition to a building. 6thly. That it is essentially necessary for the safety of the public, that all public buildings, and especially churches, should, if naturally deficient in safe and secure Lightning Conduction, have artificial Lightning Conductors erected for their protection. The above are given as a few general rules. It is difficult, however, and almost impossible, to lay down any fixed and definite rules for the erection of Lightning Conductors, to be applicable to every building; as the very form, shape, and position of the building, and the relative position of buildings in the immediate neighbourhood, so materially affect the data for the formation of those rules. In all cases, therefore, I consider it much better and safer for an Architect to call in a person of knowledge and experience in this branch of science, for directions for the proper erection of Lightning Conductors, than to trust to any printed rules whatever on the subject. THUNDERSTORMS.By Professor Tait, F.R.S. [Delivered in the City Hall, Glasgow. Nature Aug. 12th, 19th, Sept. 2nd, 9th, 1880.] (Abstracted by W. H. Preece, Esq., C.E., F.R.S.) While a few years ago no qualified physicist would have ventured an opinion as to the nature of electricity, now, thanks to Clerk-Maxwell, electric and magnetic phenomena are regarded as mere stresses and motions of the ether, and are brought within the resources of mathematical analysis. Thunderstorms are accompanied by darkness, the result of the intense shadow of peculiar thick clouds charged with electricity, whose height varies from 30 yards to 3 miles. The air is never free from electricity. Snow, sleet, hail, and “luminous rain” are frequently indications of great electrification. The atmospheric electric charge is usually positive, and is probably the result of evaporation, but clouds themselves are more generally negative. Lightning, as a source of light, is very brilliant, comparable even with the sun, but its duration is extremely short, hence its intensity is about equal to that of full moon. The motion of a flash cannot be detected; hence when people say they saw a flash going upwards or downwards, they must be mistaken. It is an optical illusion. The peculiar zigzag form, occasionally bifurcated, is that of a very large electric spark, varied by local electrification and heat. The motion of electricity is due to a difference of potential or electrical pressure. The power of a machine is measured by the utmost potential it can give to a conductor, and the time required to charge the conductor depends on its capacity. The damage which can be done by a discharge is proportional to the square of the charge, and inversely to the capacity of the receiver. Doubling a charge gives fourfold a shock. Electricity is entirely distributed on the surface of conductors. The quantity per square inch of surface is the density, and the density varies with the form of the conductor. On a very elongated body, terminating in a point, the density becomes so exceedingly great that the outward pressure of the electricity tending to escape forces a passage through the surrounding air. Proper lightning rods must be surrounded with a number of sharp points, lest one should be injured. The proper function of a lightning rod is not to parry a dangerous flash of lightning: it ought rather, by silent but continuous draining to prevent any serious accumulation of electricity in a cloud near it. Hence it must be thoroughly connected with the earth. At Pietermaritzburgh, The violent disruptive effects produced by lightning are principally due to the sudden vaporization of moisture. Heated air conducts better than cold air. Hence the killing of flocks and herds. There is little or no danger inside a thunder-cloud. Thunder-bolts (so called) are due to the vitrification of sand through which a discharge has passed. The smell that accompanies lightning is due to ozone. Sheet lightning and summer lightning are due to the lighting up of the clouds by flashes of forked lightning not directly visible to the spectator, sometimes even beneath the horizon. Thunder corresponds to the snap of the electric spark, intensified and re-echoed from clouds and surfaces. A longer zigzag flash acts successively and intermittently from portions farther and farther from the listener. Hence the crash, clap, rolling and pealing of thunder. The extreme distance that it is heard is about ten miles, although guns have been heard fifty miles. Fireball or globe lightning undoubtedly exists and is probably due to a species of natural Leyden jar, very highly charged, which no lightning rod can destroy, except, perhaps, a close net work of stout copper wires. Water is the chief agent in thunderstorms. Copious rain and hail always accompany them. Hot moist air precipitating its moisture as clouds as it ascends, cooling by expansion but warmed by the latent heat of the condensed vapour is the main spring. The condensation of aqueous vapour is accompanied by an enormous development of energy. A fall of one-tenth of an inch of rain over the whole of Britain gives heat equivalent to the work of a million millions of horses for half an hour. The mere contact of particles of aqueous vapour with those of air produces a separation of the two electricities. Aqueous vapour condenses into cloud particles, and the agglomeration of cloud particles into rain drops would enormously increase the original potential of the electrified vapour. The column of smoke and vapour discharged by an active volcano gives out flashes of lightning. Cloud caps on mountains frequently do the same. Ascending currents of air mean change of density, difference of pressure, heat condensation, and all the conditions required to produce a thunderstorm, with its effects forming “one of the most exquisite of the magnificent spectacles which nature from time to time so lavishly provides.” On the PROTECTION of BUILDINGS from LIGHTNING.By Captain J. P. Bucknill, R.E. (Abstracted by W. H. Preece, C.E., F.R.S.) In the first part of his paper the author popularly explains his own views of electricity, the causes of thunderstorms, and the purpose He advocates strange views as to the space protected by a lightning conductor, which, if true, would tend to show that there is no safety in lightning conductors at all, for according to him, the safe area rule may be upset in practice by all sorts of accidental circumstances. He has, however, not grasped the meaning of the rule. He advocates the use of iron as the best metal to use, specifying a weight of 2 lbs. per foot. He thinks wire ropes are more easily applied than rods, ribbons, or tubes, and prefers a rope 1·2 in. diam. of six strands of seven No. 11 B.W.G. wire, each round a hemp core—costing about 5d. per foot. Conductors should be specified in terms of electrical units, viz.: ·3 ohms per 1000 yards, and be continuous. Every unavoidable joint should be soldered. He has found in practice many bad joints, especially in copper conductors. At Tipner one gave 10,000 ohms, and one in the Isle of Wight 700 ohms. Each joint was apparently quite sound. He considers that lofty conductors require no additional conductivity per unit of length, and that high lightning rods are only required in exceptional situations. Several points are preferable to a single point, because the “gathering power” is increased thereby, and the chance of lightning striking other things in the immediate vicinity of the conductor is proportionately diminished; the top of the rod is less likely to be fused when struck, the stroke being divided between the various points; and also because the brush discharge is thereby facilitated. He dwells with much emphasis on the importance of the earth connection, which he regards as a joint, and advocates greater surface than is usual at present. He illustrates an excellent deep earth connection formed by a galvanised cast-iron pipe, 10 feet long and 1 foot in diameter, sunk in a well below the water level in the dryest season. He insists that both deep and shallow surface earths are required. Lastly he insists on periodical inspection, and the careful application of electrical tests. In an appendix he describes his own testing arrangements, with the results of nearly 500 tests made by him for the War Department, from which he concludes “that with the lightning conductors erected as they are at present by the War Department, electrical testing is of small value.” Nevertheless, in spite of this strong condemnation he asserts that the conductors now existing on our magazines and fortifications have never yet failed. Specification (No. 3925. September, 1880) of Samuel Vyle. LIGHTNING CONDUCTORS.(Abstracted by G. J. Symons, F.R.S.) The invention may be divided into two parts. In the first place, the inventor proposes that in lieu, for instance, of the central strand of a seven-strand copper wire rope, there shall be a central wire insulated from the others, and only connected to them at the junction with the upper terminal, while at the bottom this insulated wire is led up from the earth to some place where it is easy of access. Secondly, there is a differential galvanometer, resistance coil, and other apparatus, which being connected with the conductor and with the insulated wire, will enable the efficacy of the conductor to be read off at any time. On the PARTIAL PROTECTION of BUILDINGS.(By Prof. T. Hayter Lewis, F.S.A.) The following are suggestions whereby the ordinary materials used in building may, to some extent, be utilised as protectors against lightning:— (1) When the roofs and sides of a building are covered with galvanized sheet iron on a framework of wood, if these coverings have good earth contacts, either by themselves or through the ordinary iron rain-water pipe, the building may be considered safe. (2) Cottages and small houses have usually iron eaves gutters, slate or tile hips and ridges, cement flashings, and iron rain-water pipes. If the joints be sound, and the earth at the foot of the rain-water pipes be moist, the houses will, to a considerable extent, be protected from the level of the eaves gutters downwards. But as they will be quite unprotected about that level, a wire rope or metal tape from the top of the highest chimney to the gutters, which will very much diminish the risk, is desirable. (3) In larger buildings the gutters, rain-water pipes, hips, ridges, and flashings of the roof are often made of lead. If the pipes have good earth contacts, and conductors be fixed from the chimneys or other projections to the lead-work, the buildings will be to some extent protected. (4) When the hips and ridges of roofs are of slate, terra cotta, or other non-conducting materials, conductors along the ridges, connected with the rain-water pipes, and with points along the ridge, and to the chimneys, will be required. But all the buildings above described would be exposed to the risk of imperfect joints, bad workmanship, &c.; so that no structure can be considered as secure unless it be protected by one or more conductors of approved size and metal, and with carefully constructed connections and earth contacts. |