Chapter VI: is devoted to the Aurora Borealis, which plays about the (3)

If instead of a hill or building we erect a solid rod of metal, G H, then the field will be distorted as shown in fig. 2. Now it is quite evident that whatever be the relative distance of the cloud and earth, or whatever be the motion of the cloud, there must be a space _d d´_ along which the lines of force must be longer than _c c´_ or H D; and hence there must be a circle described around G as a centre which is less subject to disruptive discharge than the space outside the circle; and hence this area may be said to be protected by the rod G H. The same reasoning applies to each equipotential plane; and as each circle diminishes in radius as we ascend, it follows that the rod virtually protects a cone of space whose height is the rod, and whose base is the circle described by the radius G _c_. It is important to find out what this radius is.

Fig. 3.
]

Let us assume that a thunder-cloud is approaching the rod A B (fig. 3) from above, and that it has reached a point D´ where the distance D´ B is equal to the perpendicular height D´ C´. It is evident that if the potential at D´ be increased until the striking-distance be attained, the line of discharge will be along D´ C´ or D´ B, and that the length A C´ is under protection. Now the nearer the point D´ is to D the shorter will be the length A C´ under protection; but the minimum length will be A C, since the cloud would never descend lower than the perpendicular distance D C.

Supposing, however, that the cloud had actually descended to D when the discharge took place. Then the latter would strike to the nearest point; and any point within the circumference of the portion of the circle B C (whose radius is D B) would be at a less distance from D than either the point B or the point C.

“_Hence a lightning-rod protects a conic space whose height is the length of the rod, whose base is a circle having its radius equal to the height of the rod, and whose side is the quadrant of a circle whose radius is equal to the height of the rod._”

Upon this rule the author makes the following concluding remarks:

“I have carefully examined every record of accident that I could examine, and I have not yet found one case where damage was inflicted inside this cone when the building was properly protected. There are many cases where the pinnacles of the same turret of a church have been struck where one has had a rod attached to it; but it is clear that the other pinnacles were outside the cone; and therefore, for protection, each pinnacle should have had its own rod. It is evident also that every prominent point of a building should have its rod, and that the higher the rod the greater is the space protected.”

SHORT ACCOUNT OF THE STRIKING BY LIGHTNING OF THE RAILWAY TERMINUS AT ANTWERP, ON THE 10TH OF JULY, 1865. BY M. MELSENS, Member of the Royal Academy of Belgium.

(_Abstracted by R. Van der Broek._)

On the date mentioned, between three and four o’clock in the afternoon, a violent storm burst over Antwerp, during which the lightning struck the Railway Terminus, without, however, occasioning any other damage than the perforation of a single hole in one of the glass squares of the roof.

The author states that the effect of the discharge on this square of glass, which was about 4^{mm} (0·2in.) thick, was remarkable; it appeared as if it had been traversed by a projectile from below, the perforation, viewed from above, being broken and chipped, whilst viewed from below it showed a clean edge. The sinuosities caused by the chipping on the upper surface had rounded edges, and the glass appeared to have been subjected to incipient fusion. Not a single fragment of glass was found on the glass squares or in the gutters of the roof.

The author arrives at the following conclusions: The square of glass was pierced in the same manner as any square of similar nature and dimensions, placed in identical circumstances, would be, were it traversed by a spherical projectile fired at a low velocity from a firearm. The fracture resembled one that would be produced by a missile thrown from below, that is to say, from the earth to the sky.

The form of the opening indicated that the earth was positively electrified.

The author notices that, according to M. F. Duprez, negative electricity generally shows itself in abnormal conditions of the atmosphere, during storms, rains, &c., and when the wind blows from the western quarters between N. and S. Now, on the day in question, it rained and the wind blew from the west.

The author publicly thanks M. Ruhmkorff for his skilful and disinterested co-operation in proving the correctness of his (the author’s) view of the distribution of the electricity at the Antwerp discharge. M. Ruhmkorff has, at request, pierced squares of ordinary glass about 1^{mm} (0·04in.) thick by the discharge of his great induction apparatus charged by a powerful Leyden battery.

ON LIGHTNING PROTECTORS WITH POINTS, CONDUCTORS, AND MULTIPLE EARTH CONNECTIONS, A DETAILED DESCRIPTION OF THE LIGHTNING PROTECTOR ERECTED ON THE TOWN HALL OF BRUSSELS IN 1865, WITH AN ACCOUNT OF THE PRINCIPLES ADOPTED IN THE CONSTRUCTION, BY M. MELSENS, MEMBER OF THE ROYAL ACADEMY OF SCIENCES OF BELGIUM.

(_Abstracted by R. Van der Broek._)

As the author states in his preliminary observations that it is impossible to give a complete condensed description of the Lightning Protector, which he erected on the Town Hall at Brussels, we will merely draw attention to a number of facts, regarding the system followed, some of them, we believe, of a novel description.

M. Daniel Colladon, the author states, has observed that as a rule lightning does not strike a single part or prominent point of the objects that are struck or destroyed by it; and that, in the majority of cases, it does not strike in the form of a single spark, but in the form of a sheet with one or more principal centres of intensity. The correctness of this observation, the author considers fully borne out by the ravages which the electric discharge committed on the Town Hall at Brussels, on the 10th September, 1863. He gives an elaborate description of the effects of the flash on the building. It is interesting to note that the ravages principally took place at the side exposed to the west north-west wind, which was blowing at the time the building was struck.

In the ensuing winter the Municipal Council of Brussels took into consideration the necessity of protecting the Town Hall against a similar disaster, and the author was requested to superintend the erection of lightning protectors on the building.

The characteristics of the author’s system, as exemplified by the lightning protectors erected on the Brussels Town Hall, may be briefly summarised as follows:—

1. The points are very numerous—of three kinds; some long, sharp, and
gilded, others of middling length, made of iron; and finally some
small and very sharp, consisting of copper.

2. The points are replaced by _aigrettes_ (brushes of points diverging
from a common base).

3. The conductor is not insulated.

4. The connections are simple and unchangeable, the joints are each
embedded in a mass of zinc.

5. The surface exposed to the air is considerable.

6. The conductor consists of thin, and numerous wires, which are very
flexible, so as easily to be led round all the corners of the
buildings.

7. The conductor is made of galvanised iron.

8. The earth connections are multiple: firstly, a well within which a
large surface of metal is plunged; and, secondly, two enormous
networks of metal pipes, offering an immense contact surface with
the earth. One of these networks is in direct communication with
all the reservoirs and all the water sources of the environs of
Brussels and also in indirect communication with two rivers and
two canals.

The author has arrived at the conclusion that the height of the rod is a secondary question, as the radius of protection has not been determined by irrefutable proofs, and as that length is, in comparison with the distance and the extent of the thunder-clouds, so small a factor that it may safely be neglected. The author states that he has been greatly gratified to meet with the same opinion in a paper which Mr. W. H. Preece published in Vol. I., No. 3, page 366, of the Journal of the Society of Telegraph Engineers for 1872: “When we consider the distance of the cloud and the area of its surface, the height of a building vanishes in the general figure.”

The author points out that M. Perrot has endeavoured to demonstrate by experiment that the neutralizing area of a lightning protector surmounted by a crown of sharp points is far more extensive than that of an ordinary protector. M Perrott further thought, and MM. Babinet and Gavarret shared his opinion, that it is sufficient to shelter the ordinary protector from discharges of lightning by arming it with numerous, long, sharp, and well conducting divergent points. M. Gavarret after having repeated Mr. Perrott’s experiments, found the results so conclusive that he wrote to the author in the beginning of 1865: “It is at the present time no longer permitted to erect lightning protectors with single points.”

The metal of which the points are made must be a very good conductor. With regard to their conductivity, the metals follow each other in the following order: copper, silver, iron, platinum. No metals are used but those which resist fusion. The author rejected platinum and silver: the former because it fuses very readily by the electric discharge; and the latter, because it has, in his opinion, no advantage over copper.

The conductor, although galvanized, received several coats of paint; but the points (_aigrettes_) of course remained metallic. With regard to the general principle of connecting the protector with any masses of metal which may be about the building, the author has ever since 1865 endeavoured to demonstrate, that it is not sufficient, as might at first sight be supposed, to form that connection at one single point; there must be at least two points of contact, so as always to ensure a closed metallic circuit.

The contact with the water presents a surface of about ten square metres (12 sq. yds.), bringing both surfaces of the cylinder into account.

With regard to the earth connection, the author quotes M. Perrot, who remarks that with the ordinary protector the surface immerged offers a resistance at least 10,000 times greater than the conductor itself; it is therefore necessary to increase the surface of the earth-plate as much as possible.

In order to retard as much as possible the oxidation of the cylinder, the author introduced two hectolitres (6 bushels) of lime into the well, thus rendering the water alkaline.

DE L’APPLICATION DU RHE-ÉLECTROMÈTRE AUX PARATONNERRES DES TÉLÉGRAPHES.
PAR M. MELSENS.

(_Abstracted by R. Van der Broek._)

In this pamphlet the author describes in § 1 an apparatus to show the presence of atmospheric electricity in telegraph wires.

In §§ 2 and 4 he explains how the apparatus is joined up in the Belgian telegraph offices.

§ 3 contains a résumé of observations made at the government telegraph offices between June, 1875, and March, 1876.

The author states in this paragraph that, on the 19th of June, 1875, the Rheo-Electrometer at the office at Louvain, showed a deflection of 85° East, although there was not the slighest appearance of atmospheric electricity. The fact was, that at the time a thunder storm was raging at Beverloo, distant from Louvain about 40 kilometres (25 miles).

TROISIÈME NOTE SUR LES PARATONNERRES. PAR M. MELSENS.

(_Abstracted by R. Van der Broek._)

On the 3rd of July, 1874, the church of Ste. Croix, at Ixelles, was struck by lightning. The building was provided with a lightning protector, which was constructed as follows: The point consisted of a platinum cone of about 30° (the form officially adopted in France in 1855), all the supports of the protector were soldered with zinc. This was attached to the steeple, and rose to 53 metres or 174 feet above the pavement. It consisted of an iron rod 18 mm. (0·71 in.) in diameter (M. E. Sacré’s system). The conductor passed from the principal roof along the roofs, descending to a point near a pump, behind the vestry, where the well (W) was situate. There is an abundance of water in the well, which is about 7 m. (23 ft.) deep. The conductor terminated in the well, by a cast-iron plate 0·65 m. (2 ft. 1 in.) by 0·50 m. (1ft. 8 in.), thus presenting a surface of 0·654 ⬜ m. (7 sq. ft.). A little in front of the transept there is a supplementary rod B 5·25 m. (17 ft. 3 in.) high, 11 m. (36 ft.) distant from the point (c in diagram) which was struck; and 22 m. (72 ft.) distant from that point there was a second rod D, whose height was 9 m. (29½ ft.) above the top of the roof.

The damage to the church was trifling, but the author contends that the fact of the church having been struck at all, proves that a building armed with a protector constructed on the usual principle is not completely protected.

A. Principal conductor on steeple.
B. D. Two supplementary receiving rods.
C. Stone cross at end of transept, which was struck,
W. Well in which conductor made earth connection.

QUATRIÈME NOTE SUR LES PARATONNERRES. PAR M. MELSENS.

(_Abstracted by R. Van der Broek._)

This treats § 1 of observations on the distribution of the spark of electric batteries and machines over numerous metallic conductors of different sections, lengths, and nature, and on the passage of electricity of tension in bad conductors.

§ 2. Effects of soldered joints on the conductivity and the resistance of conductors. Interrupted lightning protectors.

§ 3. The distribution of sparks from Holtz’s machine and Ruhmkorff’s coil over two conductors outwardly identical, but one of iron and the other of copper. Comparative resistance to fusion and rupture for iron and copper conductors. Identical damage produced by discharges in several homogeneous and solid conductors.