277 



LIGHTNING CONDUCTOR. 



LIGHTNING CONDUCTOR. 



278 



The amount of misapprehension on this subject would form a long 

 history if properly brought together. Soon after the first employment 

 of lightning conductors in Europe, an opinion prevailed that wheu 

 their extremities were pointed they created a danger which did not 

 exist before, and that they attracted lightnings which would, without 

 the rods, have discharged themselves at a distance ; and in order to 

 diminish the risk, it was proposed to crown the summits of the rods 

 with knobs or balls of metal. This notion was entertained by the 

 Abbe* Nollet, in France ; but in the Royal Society of London the 

 question concerning the relative, efficacies of pointed and knobln 1 

 conductors was agitated with great vehemence, chiefly through the 

 obstinacy of Mr. Wilson, one of the Fellows, who, in 1778, made him- 

 self the head of a party in support of the latter kind of conductors, in 

 opposition to the president. [PKINGLE, JOHN, in BIOG. Div.] It is 

 remarkable that both George III. of England aud Frederick of Prussia 

 placed themselves on the side of the opponents oi Franklin ; the first 

 king giving the preference to balls over points ; and the other, while 

 he consented to have conductors raised on his barracks and powder- 

 magazines, prohibited the erection of them on his palace of Sans Souci. 

 It is now admitted that the prejudice against pointed lightning con- 

 ductors was entirely without foundation : those conductors have been 

 found not only to protect buildings when struck, but also to diminish 

 the number of shocks which in a given time they have experienced. 

 An experiment which was made by Beccaria, in 1753, might have 

 shown the superiority of pointed conductors over those with balls ; for 

 that distinguished electrician set up on the roof of the church of San 

 Giovanni, at Turin, a metallic-rod bent near the top and terminating 

 in a point : the upper part was capable of being turned round by 

 means of a silk line, so that the point could be directed upwards or 

 downwards at pleasure, and the lower part of the rod terminated upon 

 substances which were imperfect conductors of electricity. On direct- 

 ing the point towards the sky when a thunder-cloud passed over the 

 church, electrical sparks issued in abundance from the foot of the rod ; 

 but when the point of the rod was reversed so that the bend was 

 upwards, few or none were obtained. The conducting-rod set up by 

 Professor Richmann, at St. Petersburg, may be said to have been the 

 cause of his death (in 1753) ; but the house would most probably have 

 been struck if there had been no conductor. The immediate cause of 

 the melancholy accident was that the rod led into the apartment, and 

 the unfortunate professor was standing too near its lower extremity. 

 (' Phil. Trans.,' vol. xlviii.) 



There has also been much discussion concerning the distance to 

 which the protecting influence of the rod extends; the radius of pro- 

 , as it is called : the French electricians have even gone so far as 

 to deSne it. According to them, this influence is limited by the 

 circumference of a circle described about the rod with a radius equal 

 to double its height above the top of the building. It will be seen 

 presently that there is no ground for this assumption. 



The conditions under which a lightning conductor protects a building 

 not being understood, faults of construction have frequently occurred, 

 and buildings and ships armed with inefficient conductors have more 

 than once been struck. Thus in 1777 the magazine at Purfleet was 

 struck by lightning : the shock took place on an iron cramp which 

 united two stones of the cornice, at the distance of 2i feet from the 

 lightning conductor, measured horizontally ; and the upper extremity 

 of the rod was 11 feet above the top of the roof on which it was 

 erected. Again, in 1781, the workhouse at Heckingham in Norfolk, 

 though provided with eight rods, was struck by lightning at a spot 

 which was distant 65 feet measured horizontally from the nearest rod, 

 while the pointed summit of the Litter was 22 feet above the level of 

 the part struck ; and many other instances might be cited. So, also, 

 the most elevated objects are not always those which are struck by 

 lightning; for in 1829, the sails of a mill at Toothill, in Essex, being 

 at rest, the electric fluid left untouched the arm which was in a 

 vertical position, and fell on a knob of iron at the middle of one of 

 those which wag inclined to the horizon in an angle of 45 ; and it has 

 frequently happened that buildings containing in their walls conducting 

 substances have been struck much below their summits by a lateral 

 action of the fluid ; bars or plates of metl in- the side walls having 

 served to assist its progress to the earth better than the materials on 

 the roof. This happened to the cathedral at Lausanne in 1783. 



Now what is the action of a lightning conductor ? We have seen in 

 the previous article that the general conditions of a thunder-storm 

 consist in the charging and discharging of a stratum of air situated 

 between a mass of clouds and the earth beneath, which are in opposite 

 electrical states. Now, it will be evident that any artificial elevation 

 on the earth's surface, such as a church on land or a ship at sea, is, with 

 respect to a thunder-storm, a more point in the lower terminating 

 plane of the great electrical disturbance. The electrical forces cannot 

 be supposed to operate exclusively between such an elevation and a 

 charged cloud. The building or the ship is a mass of matter acci- 

 dentally placed in a position favourable for the union of the two 

 opposite electricities in a given direction. Electrical discharge always 

 takes place on those bodies which tend to assist its progress. It' a 

 church or a ship happen to form part of the line of least resistance, the 

 lightning will pass down that line, and if any damage is done, it is 

 because the structure is made up of good and bad conductors ; and the 

 remedy is, to connect all the good conductors into one uniform system 



of conduction, so that the lightning, wherever it may happen to strike, 

 may find an easy passage to the earth or the sea. 



Hence, then, it is not in general sufficient to erect a rod above the 

 highest point of the building and lead it in a vertical line down to the 

 ground ; but it is necessary in an extensive building to connect all the 

 masses of metal, roof-coverings, water-pipes, clamps, &c., into one 

 consistent conducting system ; and, more than this, if several points of 

 the building project into the air, each point should have its appropriate 

 lightning-rod. Thus, in a three-masted ship, it is not sufficient to 

 furnish the main-mast with a conductor ; for cases are on record of 

 the lightning striking and damaging one of the other masts, which was 

 unprotected, and taking no notice of the protected main-mast. We 

 must, therefore, dismiss from our minds all notions of the radius of 

 protection. A lightning conductor only protects the part to which it 

 is applied, aud those parts with which it is in metallic connection ; and 

 even this protection will fail unless the rod be of sufficient capacity to 

 carry the heaviest discharge of lightning that ever occurs. The metal 

 must, moreover, be a good conductor even among metals ; it must be 

 uninterrupted by any breaks or solutions of continuity, and terminate 

 in the earth where the soil is sufficiently damp to serve as a conductor ; 

 for if the ground be very dry it may serve as an insulator, and render 

 the conductor comparatively useless. We must also get rid of the 

 notion that the conductor attracts the lightning to the building or the 

 ship. It is a line of least resistance ; the lightning passes down it 

 because it can do so more easily by that route than by any other ; the 

 conductor is, in fact, passive, and has no more voice in the matter than 

 the rain-shoot in discharging the water that pours down it. The more 

 perfect, then, the conductor, the greater is the protection ; and this 

 perfection consists in sufficient capacity, good conducting power, and 

 proper distribution over all the parts of the building. Hence chains, 

 and in general wire ropes, should be avoided, as the form interferes 

 with the conducting power ; and there are instances of chains and wire 

 ropes being knocked to pieces by the lightning, and the fragments 

 falling on the deck in red-hot rain, and setting the ship on fire. 



The practical mind of Franklin first suggested the idea of providing 

 buildings with a continued line of conduction for transmitting 

 the electricity to the earth. The first lightning conductor in 

 England was erected in 1762 by Dr. Watson, at Payneshill. 

 In Roman Catholic countries there was some difficulty in intro- 

 ducing the " heretical conductors," as they were called, until 

 the mob was assured that the gilt points placed at the top to 

 prevent corrosion had been blessed by the pope. The first 

 conductor for ships was in the form of chains, consisting of 

 long links of copper rod about a quarter of an inch in diameter, 

 united by small eyes, turned in each extremity ; the chain 

 was tied to a rope, and so attached to the mast-head, whence 

 it proceeded over the side of the ship into the sea. Such a 

 conductor would doubtless contribute somewhat to the pre- 

 servation of a ship, supposing it were in its place during a 

 thunder-storm, aud that the lightning struck the mast to which 

 it was applied. The first condition was almost as difficult as 

 the second, for the wisdom of the Admiralty had led to the 

 packing-up of each conductor in a box, to be taken out and 

 used as occasion required. As might be expected, there are 

 cases where a storm has overtaken aud damaged a ship before 

 the conductor could be unpacked ; and there are other cases in 

 which the lightning has struck and shivered tb,e fore-mast, to 

 which no conductor was applied, while it avoided the protected 

 main-mast ; and there is a third set of cases in which the man 

 sent up with a wire rope or chain conductor has been knocked 

 down by the lightning in the act of applying it. 



In the year 1821, Sir W. Snow Harris submitted to the 

 Lords of the Admiralty a plan for protecting ships from the 

 ravages of lightning. It consisted in the application of capacious 

 conductors, so permanently fixed as to render them an integral part of the 

 vessel, and independent of the crew under all circumstances. According 

 to this plan, the masts are converted into lightning conductors by incor- 

 porating with them a double set of copper plates, so as to produce an 

 elastic metallic line along then- surface, capable of resisting any strain 

 which the spars themselves could support, and connecting these plates 

 with bands of copper leading through the side under the deck-beams, 

 and with the large bolts leading through the keels and keelson, aud 

 including by other connections all the principal metallic masses 

 employed in the construction of the hull. The accompanying figure 

 shows the line of conduction on the masts, from the vane-spindle to 

 the stem ; aud these conductors are connected with the hull aud with 

 the sea in the following manner : Under the step is placed a double 

 copper conductor, immediately over the heads of a number of the long 

 bolts which pass through the keelson and the keel, so that when the 

 step of the mast is in its place there is a direct metallic communication 

 between the copper bands on the step and the copper outside the ship. 

 Thus there is an extensive capacious conducting-line from the vane- 

 spindle to the sea ; and in order to maintain a more* complete con- 

 nection between the different masts at the junction of the caps, a stout 

 copper hinge or tumbler is employed ; it is fixed on the cap, and falls 

 against the conductor without being able to turn far back, so that it 

 has just motion enough to follow the yielding or change of the spar. 

 In the figure, A represents the copper plate fixed on the cap, N M an 



