APPENDIX K. NOTES respecting LIGHTNING CONDUCTORS, collected in PARIS in May, 1881, by Messrs. Preece and Symons.

The information which we obtained may perhaps be most conveniently grouped under the names, arranged in alphabetical order, of the authorities whose opinions or whose practice we quote. These gentlemen are M. AndrouËt, who, under the direction of M. Alphand, the City Engineer, has charge of all the lightning conductors attached to the municipal buildings of Paris, M. Borrel, of 47, rue des Petits Champs, who has been making lightning conductors nearly all his life, M. le Comte du Moncel, who is well known as perhaps the highest authority in France upon the practical application of electricity, and lastly M. Jarriant who is manufacturer to the municipality, and also, we believe, to the War Department, besides having a large connection among architects and engineers.

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M. AndrouËt accompanied us in a thorough examination of the conductors as they are now fixed upon the south gallery of the Louvre, temporarily occupied as the Hotel de Ville de Paris. They were stated to be only temporarily fixed, because the offices of the PrÉfet of the Seine will be removed to the new Hotel de Ville as soon as it is rebuilt, but they were said nevertheless to be in almost all respects conformable to the instructions issued by the municipality. The tiges were iron rods, 10 m. (33 feet) high, with rather blunt terminals of gilded copper; they were 35 m. (115 ft.) apart. All were united by a horizontal copper rope, ½ inch diameter (used instead of iron bars 0·8 in. square, because of the temporary nature of the work), which was led along the roof through iron holdfasts or crutches, which were carefully soldered to the metal roof. All joints in the rope were spliced and heavily soldered. For Æsthetic reasons the main conductor is carried down inside the building, through various closets, &c., and finally, after a rather circuitous course, it finds its earth terminal in a plate of copper 1 m. (3 ft. 3 in.) square, immersed in the Seine. Although the roof is well covered with metal no separate connections with earth are made. M. AndrouËt tests the conductivity from every tige in the spring of each year, using a very portable apparatus, consisting of two LeclanchÉ’s cells and a trembling bell.

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M. Borrel showed us various specimens of conductors, of earth terminals, and also his portable testing apparatus. He also gave us a copy of the Instruction sur les Paratonnerres, which he issues, and from which we make a few extracts, especially as in several respects M. Borrel’s views are expressed with unusual clearness, and although in holding some of them he stands alone:

“A lightning conductor is a preventive agent destined to convey to moist earth, or preferably to water, the electricity contained in a cloud. When strong earth tension is produced by the passage of an oppositely electrified cloud, the beneficial action of the conductor is indicated by the luminous brush discharge from the top of the conductor.

“It is generally considered that a conductor protects a cone of revolution, having for its base the height of the point above the roof multiplied by 1·75, and for its summit the point. If, therefore, the point be 6 m. (20 ft.) above the roof, it will protect a base 10½ m. (35 ft.) radius. M. Borrel supplies round upper terminals of galvanised wrought iron about 10 m. (33 ft. high), and tapering from a diameter of 4 inches at the base to ¾ inch at the top.

“Having found that long exposure to the weather destroys iron wire ropes, and even copper ones, if made of many small wires, he has adopted where ropes are necessary, four or five rods nearly 0·20 in. diameter, so slightly twisted as not to strain the metal. By this means the numerous interstices of the ordinary ropes are avoided, and much greater durability is insured.

“Where iron bars are used he employs galvanised wrought iron in square bars, the sides ranging from 0·63 in. to 0·90 in.

“To allow for variations of length produced by changes of temperature, he always inserts, in long roof conductors, a compensator, which is merely a loop of copper tape.

“M. Borrel says that it is especially upon the earth connection that the efficacy of a conductor largely depends; there must be a metallic mass, with a large surface, and he describes his pattern of ‘perd fluide.’ It is composed of two sheets of galvanised wrought iron 3 ft. long, 6½ in. wide, and ½ an inch thick, hacked into sharp points in order to facilitate the discharge of the electricity. He alludes to Callaud’s basket of coke, but says that its efficiency has not been absolutely demonstrated. M. Borrel insists upon the perd fluide being immersed in the water of a well, and one preferably not less than 2 ft. in diameter. He strongly objects to insulators, and says that he always makes metallic connection between the gutters, rain-water pipes, &c. and his conductors. From the surface of the earth to 6 ft. above it, he encloses his conductor in a wooden case in order that no one may touch it during a storm.”

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We had a long conversation with M. le Comte du Moncel, of whose remarks the following is a prÉcis:—

He objects to square iron bars because their angles have a tendency to facilitate lateral discharge.

He objects to conductors being painted, because he believes that the surface of a conductor acts electro-statically. He knows that the brass wire rope occasionally used for lighthouses is often destroyed, but thinks that the theory enunciated in the Report of the AcadÉmie des Sciences, 18th December, 1854 (see Appendix F., p. 62), can hardly be maintained, and believes it to be more probable that the rope was in a very bad state of oxidation.

Thinks that conductors should possess both sectional area and surface. Does not attach much importance to extremely sharp points, but thinks that the suggestion of one stout central one to receive a disruptive discharge, surrounded by three or four needles to facilitate silent discharge, would be good.

The following statement was quoted from the Report of 20th May, 1875 (see Appendix F., page 68), that, “if a 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.” Count du Moncel said that the paragraph referred chiefly to buildings on large solid rocks, but that obviously there is every degree of quality in the earth contact which can be obtained; and that although it is easy to decide at the two extremes, it is difficult to say how bad the earth must be in order to render the erection of a conductor inadvisable.

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M. Jarriant, who is the manufacturer employed upon the Municipal buildings of Paris (and author of two pamphlets, of which abstracts are given in Appendix F, pages (111) and (115)), accompanied us through his works, and afforded us all the information which we could desire.

He showed us a large collection of platinum points of various patterns, ranging in cost from 12s. to 60s. each; he also showed us some which had been employed by other makers, which were merely hollow sheaths of platinum filled in with soft metal in order to reduce the cost.

He had also a large variety of upper terminals, including the patterns used by the City of Paris, by the War Department for its military establishments, and by civil engineers and architects.

We saw specimens of the ropes, rods, &c., usually supplied. The iron ropes were galvanized and ¾ in. diameter. The copper ropes were made of six twisted strands of copper wire enclosing a central core of hemp, the total diameter being ½ an inch. The iron bars were square galvanized wrought iron 0·80 in. square, in lengths of 16½ feet, rabbetted at the ends with two holes for bolts. To make a joint a strip of foil is laid between the two faces, the bolts are screwed up, and then the whole joint is very heavily soldered.

Among various works in progress, we saw a highly decorated wrought iron cross for the roof of a church, which cross would become the summit of the conductor, its top and the extremity of each arm being furnished with a short copper terminal tipped with a platinum point.

We were much struck by the fact that in France, where so much attention has been given to lightning protection, there should be so much diversity of practice. The Municipality adopt one system, the State another, the War Department a third, and each individual manufacturer has, as in England, his hobby.

We desire to record our thanks to Mr. J. Aylmer, C.E., for making the various arrangements, by which we were able to see so much in the comparatively short time at our disposal, and also for accompanying us throughout.

P.S.—A very convenient form of a rough testing apparatus has been made, by the Silvertown Co., for one of the writers; it consists of one LeclanchÉ cell, a trembling bell, a key, and a pair of terminals to attach insulated wires to the top and bottom of the lightning rod, all fixed in a neat portable mahogany box, and with its aid any one can readily examine the conductivity of his lightning rod.