Chapter VI: is devoted to the Aurora Borealis, which plays about the (1)

magnetic pole, and is an electrical phenomena of the upper strata of the atmosphere; and Chapter VII. is an attempt to explain the auroral light as “probably produced by the collision of the subsidiary atoms when they are in the act of electro-apposition.”

The pamphlet is said to be a condensed account of the discoveries of the author in matters connected with atmospheric electricity—discoveries which were described in papers handed to the Royal Society, but which that Society did not read. The Royal Society were wise.

LE COUP DE FOUDRE DE L’ILE DU RHIN PRES DE STRASBOURG. PAR M. F.
HUGUENY. 4to. Paris. 1869.

(_Abstracted by G. J. Symons, F.R.S._)

A very full account of an accident by ball lightning. The facts are set out as clearly as possible, the authority is given for every statement, and most carefully engraved plans and engravings are given of all the necessary details. It does not bear upon the question of lightning conductors except in that it shows that a discharge of globular lightning traversed a horizontal distance of 919 yards, passed in front, but below the top, of a building which had three good conductors upon it and struck a chestnut tree, which was by no means the highest tree in the locality.

DIRECTIONS FOR CONSTRUCTING LIGHTNING RODS. _From_ “_Essays on
Meteorology_,” _by_ PROFESSOR JOSEPH HENRY.

(Smithsonian Miscellaneous Collections. 8vo., 1871.)

(_Abstracted by A. J. Frost_).

1. The rod should consist of round iron, of not less than ¾ of an inch in diameter. A larger size is preferable to a smaller one (ordinary gas pipe may be employed). Other forms of rod, such as flat or twisted, will conduct the lightning, and in most cases answer sufficiently well. They tend, however, to give off lateral sparks from the sharp edges at the moment of the passage of the electricity through them, which might, in some cases, set fire to very combustible materials.

2. It should be throughout its whole length in perfect metallic continuity, either by screwing the parts firmly together or by welding.

3. The rod should be covered with a coating of black paint.

4. It should be terminated above with a single point, the cone of which should be encased with platinum not less than 1/20 inch in thickness.

5. The shorter and more direct the rod is in its course to the earth the better; acute angles should be avoided.

6. It should be fastened to the house by iron eyes, which may be insulated by cylinders of glass; this, however, is not absolutely necessary.

7. The connection to the earth should be as perfect as possible—in cities nothing is better for this purpose than to unite it to the gas or water pipes. When a connection cannot be formed in this way the rod should terminate in a well containing water, or if this is not practicable it should terminate in a plate of iron, or some other metal buried in moist ground. It should, before it descends to the earth, be bent, so as to pass off nearly at right angles to the side of the house, and be buried in a trench, surrounded with powdered charcoal.

8. The rod should, in preference, be placed on the west side of the house, and on chimnies where a current of heated air ascends during the summer season.

9. A single rod may be placed on small houses, and its elevation should be at least half of the distance to which its protection is expected to extend.

10. Metallic roofs should be united with the lightning rods.

11. As a general rule, large masses of metal within the building, particularly those which have perpendicular elevation, ought to be connected with the rod.

ON LIGHTNING AND LIGHTNING CONDUCTORS. BY W. H. PREECE, Mem. Inst. C.E.

(Journal of the Society of Telegraph Engineers, 27 November, 1872.)

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

The author refers to the Escurial having been on fire seven times—four of them certainly from lightning; yet no lightning conductor is fixed even now.

The average deaths from lightning in England are eighteen per annum; in France, ninety-five.

From January 1, to July 31, 1872, 9·26 per cent. of instruments, of different forms, used in the telegraph offices, were injured by lightning.

Electricity is force, not matter, and _Current_ is a well-defined term which implies a transference of electricity from one place to another.

Thunderstorms differ only in degree from the phenomena which cause the ordinary snapping sparks from the machine.

In any case there must be two conducting masses in opposite electrical states, separated by a non-conductor or dielectric.

The light is the effect of the discharge, and is simply incandescent matter. It indicates the path of the discharge and nothing more.

Death by lightning is painless.

_Potential_ is that function of electricity which determines its motion from one point to another.

The path of electrical discharge is prepared beforehand by induction.

The particles of air, &c., are in a state of “tottering equilibrium.” A moving ship, a man on horseback may destroy this, and we have a discharge with all the effects of light, heat, and mechanical energy.

It is very doubtful whether thunder-clouds are themselves the sources of electricity, producing thunder and lightning; they are more probably, mere accumulators as the coatings of a Leyden jar.

Clouds have been known to be absent during a discharge.

Moreover, the charge of a Leyden jar exists not in the coatings but in the dielectric separating them.

So the discharge exists in the air and not in the clouds.

Sheet lightning is a mere reflection of forked.

Evidence proves that some such phenomena as ball or globular lightning exists, and an explanation of it has been given by C. Varley.

Discharge is invariably through the line of least resistance. It may be through metals, bricks, trees, animals, and not always in a single track; it is often divided into two, three, or even four lines.

Thus, an electrical discharge in air, is simply a discharge between two electrified conductors, of such different potentials as to break the resistance of the dielectric separating them.

There is nothing hidden, mysterious, or unknown in it.

A ship is a prominent object; generally a conductor, and reduces the line of resistance between the sea (inner coating) and the cloud (exterior coating of the condenser) determining discharge.

Trees, buildings (except tall spires, &c.) are less prominent.

The effects of lightning experienced on telegraph wires, poles, and instruments by direct discharge are less numerous than those by induction, and seldom destructive.

There were only two cases in the past season where line wires (No. 8 iron, diameter 0·170 inch) were absolutely fused.

Accumulation of a charge upon a cloud converts it into a powerful inducing body.

It induces in the wire an opposite electrical state. Discharge takes place. The cloud suddenly loses its coercive power. The wire recovers its neutral condition, and produces a powerful current in opposite direction.

Wires are affected although buried two feet underground. Unprotected poles are often destroyed. In one case, twenty successive poles were so.

Instruments have had their cases burst out, wood-work has been burnt, and the wires of electro-magnets, &c., been fused.

Clouds are not perfect conductors, so do not part with all their discharge at once. There may be several successive discharges.

_Protection._—Sir. W. S. Harris’s system approved.

_Houses._—Unnecessary expense is often incurred in protecting them.

A warm flue, terminating in a metal grate, is a dangerous conductor, as it ends in the room and not in earth: hence so many accidents indoors. A lightning conductor should expose a prominent metallic point, and offer a path of little or no resistance thence to earth.

Hitherto expensive plates or ropes have been used for this. But the author thinks galvanized iron wire ¼ inch diameter amply sufficient for any dwelling house.

Telegraphic poles, protected by lightning conductors of No. 8 wire (½ the above size), have never been injured.

In one case, fifteen per cent. of unprotected poles have been struck.

But no case of damage has occurred for many years since the poles were earth-wired. The cross-arms are often damaged as far as the earth wires, never below.

The author can conceive no case in which ½ inch standard galvanized iron wire is not ample.

The conductor should be solid and continuous from the gilded or platinum point to the ground.

Joints should be well soldered. Chains and linked rods should not be used.

Earth connections should be formed with iron gas- or water-mains, or be several feet in coke, or in a well.

Each conductor should make a separate earth.

All masses of metal in the line of probable discharge are to be connected with conductor.

Conductors should be examined periodically, they should not be insulated, nor be near soft metal gas-pipes, nor bent in acute angles.

The area of protection appears to be that of a cone, whose radius is equal to the height of the conductor.

One conductor is enough for small houses, but each stack of chimneys should have one in connection with the main conductor.

Lead roofs and iron pipes are easily made into protectors for buildings.

Details given for protecting telegraph apparatus.

The telegraph companies abandoned the use of protectors. The Post Office re-introduced them with good results. The Indian telegraph apparatus is protected, and accidents scarcely ever occur.

_Prevention._—Points prevent the accumulation of charges. But with very tall conductors—as to spires—a current results constantly in one direction, producing electrolytic action and destruction of conductor, as proved by one at Llandaff Cathedral.

So earth should be made with large masses of metal, as gas- or water-mains.

Galvanised iron fastenings should not be used to secure copper conductors to buildings, as galvanic action would be set up.

_Appendix._—Letters quoted from Mr. Latimer Clark and Dr. Faraday as to damage to underground wires from lightning.

The _Discussion_ on Mr. Preece’s paper was conducted by Prof. Abel, Capt D. Galton, Mr. G. J. Symons, who referred to Dr. Franklin’s suggestion as to cold fusion, Prof. Ayrton, who entered at length into the system of prevention used with the Indian telegraphs, Sir W. Thomson, and Mr. Latimer Clark.

Mr. Preece replied, more especially alluding to the phenomena of fire balls.

LIGHTNING RODS AND HOW TO CONSTRUCT THEM.——BY JOHN PHIN, C.E. New York.
1873.

(_Abstracted by W. H. Preece, C.E._)

The author is not an electrician nor a patentee, but the Editor of an engineering paper called the _Technologist_. The book is written chiefly to counteract the machinations of a great nuisance in the United States, called the “lightning rod man.” The Author thinks a good rod as important as a fire insurance policy. Every case of injury that he has examined was due to defective rods, or to the absence of them. The lightning rod is an American invention. He mentions several cases of marked immunity from accident due to proper conductors, notably St. Paul’s and the Monument, London; the Cathedral, Geneva; and St. Mark’s, Venice.

The lightning rod should form the path of least resistance, and it may be of iron or of copper. If of iron he prefers a flat bar 1 inch by ¼ inch, weighing 13 ounces per foot, or No. 00 copper, weighing 6½ ounces per foot. He also advocates copper rope.

He thoroughly believes in the conduction through the mass of the metal, and quotes (p. 12) several experiments in support of that view.

He believes in a good earth and in connecting all waterspouts, eaves, gutters, and metal work generally with the earth and with the conductor; he thinks one good rod enough, and sees no reason why lightning rods should not be painted, indeed, thinks it better to do so, for they become less unsightly; he has no faith in points, nor in gilding, or platinising; he recommends instead cast iron caps to chimneys; he discards insulation as absurd, and suggests that rods may be tacked, or stapled, or strapped to buildings, although he prefers staples; recommends strongly that wet earth should be reached, and that as large a metal surface as possible should be exposed to the ground and embedded in coke; he does not like any connections with the gas pipes.

He suggests that iron conductors may be welded or have merely butt joints, but recommends solder with copper, after being bound with fine wire.

He adduces the fact that Mr. Brooks, of Philadelphia, measured the resistance of three rods attached to three buildings that had been damaged, and found the average to be above the resistance of one hundred miles of telegraph wire.

TRAITÉ DES PARATONNERES, &c. PAR A. CALLAUD. Paris. 1874. Royal 8vo.

(_Abstracted by Latimer Clark, C.E._)

This work consists of 171 pages. It commences with a short history of the subject, which occupies the first chapter. The remaining nineteen chapters treat successively of the collecting points and their mode of action; the conducting rods and the methods of attachment to different classes of building, and their connection with the earth, with concluding observations.

The second chapter treats of the height of conductors and the area protected, in which he follows the usual rules, and recommends lofty rods, their office being not only to safeguard the building, but to withdraw electricity silently from the air and thus prevent strokes of lightning or diminish their violence.

In Chapter III., after citing the opinions of many other writers, he strongly advocates protectors furnished with sharp points of platina, or some inoxydisable metal, securely screwed and soldered on to copper rods, and condemns points of iron or copper. Throughout the work he treats cost as a secondary consideration and considers it false economy to spare any expense necessary to ensure the thorough perfection of the whole system.

In Chapters IV., V., and VI. he gives drawings of connections and of various forms of weathercocks.

In Chapter VII. he recommends multiple points, especially in mountainous countries and where storms are prevalent. He also points out that many buildings are naturally protected by the metal roofs and ornaments belonging to them. So long as these are connected with the ground, he prefers that the projecting rod should be of round iron of considerable length and in one piece, and the conducting cable should wind round it as a collar, and be strongly attached to it by set screws and soldering. He does not advise that all the masses of metal within a building should be connected with a conductor, especially if they are in proximity with human beings, but with a well-made conductor he considers it safer to leave them isolated. (Chapter IX.)

For the conductor he recommends Gay Lussac’s construction, viz., a rod of iron about ⅝ inch square, carried by iron supports, or a twisted cable of iron wires having a diameter of ⅝ inch to ¾ inch, well tarred or galvanised, 6 or 8 feet from the soil these are securely united to an iron bar ⅝ inch to 1 inch diameter. If of copper they may be smaller. Has seen rods of copper of ⅜ inch effectually protect churches, but regards this as a minimum size for a length of 80 feet and ¾ inch as a maximum. The single wires of the cords may have a diameter of 1 millimetre; the joints are made by splicing the strands together and soldering them. (He recommends conductors of straw in some cases for country use. Chapter X.)

The conductor is led along the ground in a channel of half drain tiles, surrounded with coke and terminates in a copper grapnel embedded in a basket of coke. (Chapter XIII.)

Chapters XIV., XV., and XVI. gives details of the construction of lightning conductors for tall chimneys, powder magazines, and ships.

In Chapter XVIII. he gives numerous examples of the utility of conductors, and in Chapter XIX. he gives a _resumé_ of his instructions, again insisting on the perfect continuity of the connections and the perfection of all the parts; these instructions are also embodied in a note read before the Academie des Sciences, in 1862, a copy of which is given at page 167 of M. Callaud’s work.

BLITZABLEITER-ANLAGEN. PROF. C. ZENGER’S SYMMETRISCHE BLITZABLEITER. C.
Korte and Co., Prague.

(_Abstracted by G. J. Symons, F.R.S._)

This is really a trade circular, but it gives, in a compact form, the considerations which have induced Prof. Zenger to propose his new system, and a description of the mode in which it is carried out. In the first place it may be well to reprint from the _Meteorological Magazine_, Vol. VIII. (1873), page 155, the report of the paper read by Prof. Zenger at the British Association Meeting.

PROF. ZENGER, ON THE ACTION OF SYMMETRICAL CONDUCTORS AND LIGHTNING
CONDUCTORS.

Professor Zenger read a paper, on this subject, illustrating it with
the well-known experiment in physics of placing two insulated
hemispheres of brass plate in contact with another insulated sphere
of brass. If the former were charged with electricity and removed
from the inner brass sphere, there was found no trace of electricity
on its surface. The electricity was shown to be accumulated on the
surface of the outer spherical conductor, with equal tension in
every point of the surface. Professor Zenger showed that if the
outer hemispheres were replaced by two circular wires, no action
whatever in the inner conductor was found. He said it was easy to
see that this simple experiment might prove useful in regard to the
construction of electric apparatus and of lightning conductors to
protect buildings, and even whole cities, from the destructive
action of atmospheric lightning. He had, therefore, endeavoured to
ascertain the effects if any other form of a symmetrically-arranged
conductor were used, instead of a circular form. In the first
instance, he had tried the parabolic wires joined to the
electroscope; next, a rectangular wire with five different openings.
If placed exactly in the middle of the rectangular wire, no action
was observed; if placed eccentrically, however, there was small but
increasing action; and if he placed a needle or another
sharp-pointed instrument between the protecting wire and the
electroscope, he still better observed the different action produced
by placing the electroscope in an eccentrical position. He therefore
thought that it was possible by symmetrical wires placed on
buildings, or over whole cities, so to procure an entire protection
from atmospherical electricity. If the electric clouds should even
enter between the objects protected and the protecting wires, their
activity would be greatly diminished, for the wires would become
immediately charged, and nearly all the electricity accumulated on
their surface without any danger to the protected buildings.

Mr. Glaisher, who had taken the chair in the temporary absence of
the president, said their thanks were due to Professor Zenger for
his communication upon a subject so important. What they wanted to
know was the distance at which buildings were protected by a
lightning conductor, and Professor Zenger’s assertion that the
sections of a globe were as effective as the whole globe itself,
would be an important addition to scientific knowledge if proved to
be so.

Professor Clerk-Maxwell, who said he had paid some attention to the
subject of shielding bodies from electrical action by means of the
wire, feared that the form that Professor Zenger had given them
would be rather difficult to work out mathematically.

Professor Zenger said that the correspondent of the _Engineer_
newspaper had just informed him that the instrument hut of the
Atlantic Telegraph Company at Valencia was protected by wires on the
principle he had just mentioned, and the plan of protecting the hut
had been devised by Mr. Cromwell Varley.

We now pass on to Messrs. Korte’s paper, which refers entirely to the application of this symmetrical principle to buildings. They begin by claiming that Prof. Zenger’s system is the only one based upon scientific investigations and practical experiments, and that although far better than the primitive arrangements generally adopted it costs no more. They urge that the conductors should be symmetrically arranged, and yet they say that they should lead to the side of the house most exposed to the weather. They recommend that the upper terminal should be a long oval of gilt brass, something like a blunt spear-head, and that, in ordinary cases, a single copper rod of 0·20 inch diameter (_not_ a rope of that size) will be sufficient; it is to be taken through porcelain insulators, and the earth terminal is to be a copper plate nearly ¼ of an inch thick, buried from 6 to 9 feet deep in coke.

PROTECTION OF LIFE AND PROPERTY FROM LIGHTNING. By W. MCGREGOR. Bedford,
1875. 8vo. 43 pages.

(_Abstracted by Latimer Clark, C.E._)

Mr. McGregor does not give any new facts in connection with lightning, but discusses the theory and action of conductors, and quotes numerous opinions from other writers, with practical suggestions and precautions to be observed in fixing conductors.

Among the principal opinions adduced are the following:—

1. Professor Jenkin’s statement, that if a conductor be armed with a point, the electricity passes into the air rapidly in times of excitement by induction, and so equalises the tension of the surrounding atmosphere as to mitigate, or, in some cases, to prevent the discharge of lightning.

2. De la Rive’s observation that a slight break of continuity in a conductor is filled by a succession of brilliant sparks during a storm, though there be no lightning; that blunt points or balls are equally effective when struck, but are more usually accompanied by explosion than by continuous discharge.

3. The opinions of De la Rive, Dr. Mann, and Preece, that a conductor practically protects a conical space—of which the radius is about double the height—and that the conductor should therefore extend to some height above the building.

4. Ganot’s opinions that a conductor should terminate in a point or points, have sufficient sectional area, be thoroughly connected with the earth, and be connected with lateral metallic surfaces of large extent if it passes near them; either iron or copper may be used, and existing rain and water pipes, &c., may be utilised; but the joints should be made carefully and tested. Chimneys with soot act as dangerous conductors, and should therefore be protected.

The author does not give any precise directions as to the best form or size of conductors.

LYNILDENS FARLIGHED I NORGE. BY H. MOHN, Kristiania. 1875.

(_Abstracted by C. Terkelsen._)

The author, having been specially commissioned to enquire into and investigate the danger of lightning in Norway, found that lighthouses, telegraph stations, and other much exposed buildings, which were provided with conductors, did not by far suffer so much as churches, which in the most cases were unprotected.

Out of about 100 churches reported to have been struck by lightning, only three were provided with lightning conductors: on the first, Kongsberg, the conductor was in good order, and the church was comparatively uninjured; the second church, Fossnes, built of wood, had a conductor, but made of zinc wire, which melted, and of course left the church unprotected; on the third, Brónó (struck 17th October, 1872), the wire had rusted, where it joins the earth, and the church was destroyed.

The author gives a full description of the different cases.

Of 100 churches struck by lightning, fifty-six were totally destroyed, and had to be rebuilt; twenty-four of that number were churches built of stone, twenty-nine of wood; the building material of the remaining three is unknown. It would thus appear that stone buildings are almost as much exposed to be damaged by lightning as wooden ones. Of the above-named churches only one can be said to have been saved by a lightning conductor, viz., Kongsberg. In 1820 the lightning struck the church, set fire to a great part of the wood-work, and did other damage. The tower was then covered with sheet iron. In 1852 the lightning struck the tower again, which, however, then was provided with a conductor consisting of two thin copper plates, 2½ inches wide, fastened on the north and south side of the tower, and both beginning with the iron rod, on which the vane is fastened; but this rod did not end in a point, but in a gilt cross. The conductors were carried down the brickwork of the church to the field, and across the market place, and ended in an old water-butt. When the concussion took place one of the lightning conductors was disabled; but no material injury was done to the tower. In 1872, July 16th, the lightning struck a farmhouse about 700 feet from the above-mentioned church; the farmhouse being about thirty feet, and the tower about 150 feet high.

The construction of a lightning conductor ought to be as follows: It consists of the following three chief parts. (1) The receiver; (2) the conductor; (3) the earth connection. The receiver consists of a copper point 8 inches long and ¾ inch thick; which is screwed into an iron rod, 1½ to 2 inches thick. The screw must fit well and the flats of the copper and iron fittings must be well connected and afterwards soldered round the joint to prevent water and air from rusting the iron. There are various ways of fastening the receiver to the building, but the engineer is generally guided by circumstances. The conductor may be made of iron or copper in the shape of rods or wire twisted like rope. If made of iron rods they should be round and ⅝ to ¾ inch thick; if iron wire-rope is used the thickness must be equal to a rod of ¾ inch; if made of copper the rod must be at least ¼ inch thick, or if made of copper wire-rope ⅜ inch. In both cases the conductor is put in metallic connection with the receiver, and then guided into earth.

The earth connection is merely a continuation of the conductor and must be buried as deep as possible in the earth, and reach the water, if it is to be found.

The end which reaches the water may be constructed in various ways, according to circumstances, but it is of the greatest importance that the earth conductor never gets dry. If there is great difficulty in getting at the water, the earth conductor may be constructed in the following manner. It is made of copper, and has joined to it as many branches as are thought necessary. Each branch has rivetted or soldered to it a copper plate 1 or 2 feet square; they are carried as far away from the building as possible, and buried deep into the earth. Besides this there must be laid an extra conductor, perfectly metallically connected with the chief conductor just under the surface of the earth, alongside of it, out from the building, with as many branches, and as long, as possible. This conductor becomes efficient, as soon as the surface of the earth gets wet through rain, which generally falls during a thunderstorm.

LECTURE DELIVERED BEFORE THE SOCIETY OF ARTS, _28th April, 1875_. By R.
J. MANN, M.D.

(_Abstracted by E. E. Dymond, F.M.S._)

Draws attention in the first place to certain established principles.

Different powers of various substances for conducting electricity.

Electrical induction.

In dull fine weather the surface of the earth negative, the surrounding air commonly positive, the surface of the sea positive.

How a thunder storm begins, gradually approaching cloud, lightning between it and earth. According to Delisle and Petit, a lightning stroke _may_ extend 9 or 10 miles, but for ordinary circumstances the striking distance varies between 650 and 6,500 feet. The lightning stroke follows the line of least resistance, and invariably falls upon the most prominent conducting substance, and passes through substances affording an easy way and offering small resistance without disturbing their molecular condition; shatters bad conductors; heats, sometimes melts, good but insufficient ones.

Describes the various forms of lightning—flash, diffused, sheet, and ball.

A continuous rod of good conducting metal must be carried from the top of the building to the ground. Describes varying carrying capacities of iron, zinc, or copper; recommends from his experience in South Africa, 42–strand rope of 1/16th inch galvanised iron wire.

The disintegrating energy is mainly expended on the extremities of the conductor.

In Natal he used to enclose the top of the rope in a tube of stout zinc, finished at the top by a gilded ball of wood, and he opened the strands of the wire above it into a brush. The French electricians strongly recommend a cluster of points.

The earth contact must be good and damp. The French system of Callaud described.

Gay Lussac recommended that all large metallic masses should be brought into connection with the conductor, and the conductor not insulated from the building. M. Callaud, on the contrary, adopts insulating supports for the conductor, and condemns the connecting of metals in the building.

The metals used in the construction of the buildings may be utilised as conductors; rain pipes, metal ventilating pipes, but not soft metal gas pipes.

ON THE PROTECTION OF BUILDINGS FROM LIGHTNING. BY PROFESSOR J. CLERK
MAXWELL, F.R.S.

(Reprinted from the _Report of the British Association for the
Advancement of Science_, 1876.)

Most of those who have given directions for the construction of
lightning conductors have paid great attention to the upper and
lower extremities of the conductor. They recommend that the upper
extremity of the conductor should extend somewhat above the highest
part of the building to be protected, and that it should terminate
in a sharp point, and that the lower extremity should be carried as
far as possible into the conducting strata of the ground, so as to
“make” what telegraph engineers call “a good earth.”

The electrical effect of such an arrangement is to _tap_, as it
were, the gathering charge, by facilitating a quiet discharge
between the atmospheric accumulation and the earth. The erection of
the conductor will cause a somewhat greater number of discharges to
occur at the place than would have occurred if it had not been
erected, but each of these discharges will be smaller than those
which would have occurred without the conductor. It is probable,
also, that fewer discharges will occur in the region surrounding the
conductor. It appears to me that these arrangements are calculated
rather for the benefit of the surrounding country, and for the
relief of clouds labouring under an accumulation of electricity,
than for the protection of the building on which the conductor is
erected.

What we really wish is to prevent the possibility of an electric
discharge taking place within a certain region, say, the inside of a
gunpowder manufactory.

If this is clearly laid down as our object, the method of securing
it is equally clear.

An electric discharge cannot occur between two bodies unless the
difference of their potentials is sufficiently great compared with
the distance between them. If, therefore, we can keep the potentials
of all bodies within a certain region equal or nearly equal, no
discharge will take place between them. We may secure this by
connecting all these bodies by means of good conductors, such as
copper-wire ropes; but it is not necessary to do so; for it may be
shown by experiment that if every part of the surface surrounding a
certain region is at the same potential, every point within that
region must be at the same potential, provided no charged body is
placed within the region.

It would therefore be sufficient to surround our powder-mill with a
conducting material (to sheathe its roofs, walls, and ground-floor
with thick sheet-copper), and then no electrical effect could occur
within it on account of any thunderstorm outside.

There would be no need of any earth-connection. We might even place
a layer of asphalt between the copper floor and the ground, so as to
insulate the building. If the mill were then struck with lightning,
it would remain charged for some time, and a person standing on the
ground outside and touching the wall might receive a shock; but no
electrical effect would be perceived inside, even on the most
delicate electrometer. The potential of every thing inside, with
respect to the earth, would be suddenly raised or lowered, as the
case might be; but electric potential is not a physical condition,
but only a mathematical conception, so that no physical effect could
be perceived.

It is therefore not necessary to connect large masses of metal, such
as engines, tanks, &c., to the walls, if they are entirely within
the building.

If, however, any conductor, such as a telegraph wire or a metallic
supply-pipe for water or gas, comes into the building from without,
the potential of this conductor may be different from that of the
building, unless it is connected with the conducting shell of the
building. Hence the water or gas supply-pipes, if any enter the
building, must be connected to the system of lightning-conductors;
and since to connect a telegraph-wire with the conductor would
render the telegraph useless, no telegraph from without should be
allowed to enter a powder-mill, though there may be electric-bells
and other telegraph apparatus entirely within the building.

I have supposed the powder-mill to be entirely sheathed in thick
sheet-copper. This, however, is by no means necessary in order to
prevent any sensible electric effect taking place within it,
supposing it struck by lightning. It is quite sufficient to enclose
the building with a network of good conducting substance. For
instance, if a copper wire, say No. 4, B.W.G. (0·238 inch in
diameter), were carried round the foundation of a house, up each of
the corners and gables, and along the ridges, this would probably be
a sufficient protection for an ordinary building against any
thunderstorm in this climate. The copper wire may be built into the
wall to prevent theft, but it should be connected to any outside
metal, such as lead or zinc on the roof, and to metal rain-water
pipes.

In the case of a powder-mill, it might be advisable to make the
network closer by carrying one or two additional wires over the roof
and down the walls to the wire at the foundation. If there are
water- or gas-pipes which enter the building from without, these
must be connected with the system of conducting-wires; but if there
are no such metallic connections with distant points, it is not
necessary to take any pains to facilitate the escape of the
electricity into the earth.

It is desirable, however, to provide for the safety not only of the
building itself, but of the system of conductors which protects it.
The only parts of this system which are in any danger are the points
where the electricity enters and leaves it. If, therefore, the
system terminates above in a tall rod with a sharp point, and
downwards in an “earth wire,” the external discharge will be almost
certain to occur at the ends of these electrodes, and the only
possible damage will be the loss of a few particles from their
extremities; but even if the rod and wire were destroyed altogether,
the building would still be safe.

ON BOILER AND FACTORY CHIMNEYS AND LIGHTNING CONDUCTORS. BY R. WILSON.
1877.

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

The author refers to the wide-spread disbelief in the efficiency of conductors, the common opinion being that metallic bodies, especially when pointed, attract lightning, and are therefore dangerous. This is quite erroneous.

“On an electrified cloud passing over a pointed conductor, the opposite and induced electricity of the earth is discharged from the point of the conductor, and the cloud and air are often thereby neutralized without producing lightning at all. But when a discharge does take place, the conductor offers a line of comparatively small resistance.”

The author further says that, “if electrified clouds be driven to the erection in such masses that the opposite electricity does not stream away from the point of the conductor in sufficient quantities to prevent a spark from passing, the spark or flash will pass from cloud to conductor in preference to any neighbouring point.”

He refers to the safety of conductors, as shown by Sir W. S. Harris’s reports.

When injury to buildings has occurred where lightning rods are fixed, they have been “ignorantly and wrongly applied,” or joints have rusted, the rods been broken, or earth contact has become imperfect.

He refers to Harris and Faraday as to sectional area of conductor. Considers a rope to be better than a rod, as it is less liable to be fractured and to have badly formed joints.

The upper extremity should project into the air as high as the diameter of the chimney top.

The rod should not be inside a chimney, as gases are liable to injure it.

The conductor should communicate with all metal in the chimney.

Insulation is not required.

All contact between copper and iron should be avoided on account of galvanic action.

Earth contact should be tested every year. Anderson’s galvanometer approved of for this.

NOUVEAU PARATONNERRE ACCEPTÉ PAR L’ACADÉMIE DES SCIENCES. PAR JARRIANT.
8vo. Paris. 1877.

(_Abstracted by G. J. Symons, F.R.S._)

This pamphlet is really a letter by M. Francisque Michel respecting some new patterns of lightning conductors made by M. Jarriant, and submitted to the Académie des Sciences by M. le Comte du Moncel. The author states that there have been many theories as to the advantage of conductors rising to great heights above buildings, and that, on the other hand, some persons have urged that buildings should bristle all over with points, and thus prevent any disruptive discharge. He thinks that, owing to the translation of the storm-cloud by the wind, these short points will not always have time to act, and says that the only rational plan is to place a conductor high above the house it is intended to protect, and so constructed that it, and it alone, offers a path of scarcely appreciable resistance to the electric discharge. He says that in Germany they put a metal sphere on the top of the conductors, but in France, both the Academy and the Commission of the City of Paris have advised that they should terminate in a point.

M. Francisque Michel says that formerly a conductor was supposed to protect all objects within a cone whose base had a radius of twice the height of the conductor; but that he and M. Félix Lucas had investigated the question geometrically, and have arrived at the conclusion that the radius cannot exceed 1·75 of the height. Hence, in many buildings, it became necessary either to increase the number of the conductors or to make them more lofty, both alternatives leading to increased expense. M. Jarriant’s design, which consists of galvanized angle iron bolted together, enables the increased elevation to be obtained at a price twenty per cent. below that of the old patterns. The angle irons themselves offer much surface, their angles are useful for discharging electricity, and they carry at the top the copper terminal recommended by the Académie.

A PRACTICAL TREATISE ON LIGHTNING CONDUCTORS. BY HENRY W. SPANG.
Philadelphia. 1877.

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

“The identity of electricity, manifested by friction, with that contained in the atmosphere, was not fully verified until Franklin’s experiment with his kite in June, 1752.”

“In restoring the equilibrium between the opposite electricities of high potential, the discharge will pass by the shortest path, even though a poor conductor, in preference to a longer path through a good conductor.”

The electricity of the earth is usually negative—of the atmosphere, usually positive.

He quotes experiments at Kew to this effect.

The friction of solid and liquid particles against the earth, and against each other in the air, produced by the wind, is a source of atmospheric electricity.

The height of the lower part of the thunder-clouds above the sea in the United States averages about 2,500 feet.

Dense thunder-clouds are good conductors, and are electrified to a certain extent by the induction of the electricity contained in the surface-earth. As electricity accumulates in the thunder-clouds it acts by induction on the surface-earth, and causes a corresponding increase of potential in the earth and the objects thereon.

He alludes to the vitreous tubes (fulgurites), 5 feet to 75 feet deep, as being formed by electricity passing to the subterranean water-bed through sand or other dry earth.

A highly positively electrified cloud within 3,000 feet of a building causes the latter to be intensely negatively electrified by induction.

So also the earth beneath the building and the upper portion of the subterranean water bed.

Whatever offers the _least_ resistance to the stroke will be its chosen path and it will never leave a very good line of conductors, which is in a short path between two opposite electricities, for an inferior one.

151 persons are killed by lightning annually in the United States, France, England, and Switzerland.

He quotes Sir W. S. Harris’s system for the Navy as preventive.

There is no absolute safety anywhere out of doors. It can only be found inside a structure having good conductors, with good earth connections.

Conductors cannot prevent disruptive discharge. They simply furnish a good path for lightning which passes over them without doing any damage.

_Protective Area._—A committee appointed in 1875 by the Prefect of the Seine reports as protected, a circular space whose radius is equal to 1·45 [Should be 1·75, see page (67). Ed.] of height of conductor. But this is not always to be relied upon.

It is necessary that a conductor extend along the ridge, gable ends, and eaves of a house, and above each chimney.

Lightning is electricity of very high potential, and the difference of conductivity between the resistance of copper and iron to a lightning discharge is small and practically amounts to nothing.

Iron rod conductors not to be less than 7/16 inch diameter. No case is recorded where such a rod, properly connected with the earth, has been fused or greatly heated by lightning.

Paint or an ordinary amount of rust does not affect conductivity.

A conductor of large surface exercises a much greater protective action than the same quantity of metal in the form of a wire or solid rod.

Not because electricity in motion resides on the surface, but that the expansive action of a discharge may have a wider scope _through_ the metal.

So iron rain water-pipes are good conductors, and should be connected with metal spouting, conductor on ridge, &c.

Cable conductors bend easily and can be made in one length, so often answer better than bars.

If earth connection is good, rusty joints are of little consequence.

Conductors are not to be insulated.

Iron pipes for gas, water, heating, &c., also iron columns extending from basement to near the roof are to be connected with conductor and earth terminals.

The pipes on each side of gas meter are to be connected by iron bands.

Air terminals are to rise about 4 feet above each chimney or other elevated projection.

High steeples to have horizontal conductors round them at every 20 feet in height connected with vertical conductors.

One terminal in the centre of a building not over 25 feet long or wide is sufficient, or one at each end of the ridge. One to each 20 feet of a large building, with one at each end and to each chimney, &c.

When the horizontal portion of a lightning conductor, or path along the roof of a building from ridge to eaves (_sic_) exceeds 50 feet in length, the path becomes rather indirect for a lightning discharge, which is then apt to select a shorter route through the building.

The upper part of terminal need not be gilt.

Points are practically of no use.

Chimneys are very likely to be struck, owing to the heated air rising from them.

Provide against this by metal caps.

There is danger also, owing to the vapour rising from them, from barns stored with new hay or grain, stables, schools, churches, &c., containing many people, flocks of sheep, &c.

Earth terminals must be in moist ground.

The author quotes Prof. F. Jenkin as to the difference of conductivity between well moistened and perfectly dry earth (as porcelain, &c.) in electricity of low potential, as 1,000,000,000,000 to 1.

Gas and water mains usually 4 feet or so deep in dry earth, therefore not good conductors.

Examples quoted of injury to their joints by lightning, which passed from conductors to the mains.

Suggests, as earth terminal, an iron pipe, 10 feet long, 2 inches diameter, open at each end, perforated at sides, put in vertically, and having the water from pipes for rain and waste led into it.

To be 8 feet from foundation.

Gives engravings of numerous forms proposed for conductors, most of them being defective, and none show improvement on Franklin’s round rod.

Copper rods held by iron staples, and connected with iron earth terminals, are bad, owing to galvanic action.

Copper wires in cable conductors become brittle, and snap when vibrated by the wind; sometimes, also, they are eaten away by electrolytic action.

He gives a drawing of a house protected as suggested by him, viz., by metal rain water-pipes connected with the metal gutters and ridge; also with his improved earth terminal by a good iron bar conductor.

Gas, water, and other pipes are to be connected together, and with conductor.

These often give better path for lightning than the conductors.

But dangerous if without proper earth terminal.

He disagrees with Prof. C. Maxwell’s theory as to disconnecting the metal covering, &c., of buildings from the earth.

Lightning conductors detached from buildings do not afford absolute protection.

Lightning has great affinity for gas-holders, so one of the nearest guide columns should be connected by a metallic conductor with the pipe leading to street main, and also with a vertical earth terminal.

When a telegraph line is altogether metallic, well insulated upon poles, &c., and not metallically connected with the earth, the electricity of a storm-cloud will not exert so strong an inductive influence upon it as upon a line whose ends terminate in the earth.

Line wire is often melted, poles and apparatus shattered, and employés sometimes killed.

As a remedy, a galvanised iron wire is now fastened to every fourth pole by iron staples, from 4 inches above the top of the pole to a coil about 10 feet long of iron wire beneath its lower end.

UEBER BLITZABLEITER UND BLITZSCHLÄGE IN GEBÄUDE WELCHE MIT
BLITZABLEITERN VERSEHEN WAREN. VON G. KARSTEN. Kiel. 8vo. 1877.

(_Abstracted by R. Van der Broek._)

In this pamphlet Dr. Karsten gives an account of two cases in which buildings that were provided with lightning conductors were damaged by lightning. The author states that the statistics for the year 1873 show that in Schleswig-Holstein twenty-six per cent. of all the cases of fire were caused by lightning; 1/130th part of these cases occurred in the towns and the remainder in the country.

Do lightning conductors guarantee absolute protection? The author answers this question as follows: There is no absolute certainty in empirical matters; each new case may direct our attention to circumstances that had been overlooked. If lightning conductors cannot be said to ensure perfect safety, they certainly afford a very high degree of protection.

The flash of lightning which struck the church at Garding, on the 18th of May, 1877, fractured the conductor in fifteen places and pierced the wall of the steeple in two places. The inefficiency of the conductor resulted from the carelessness with which it was fixed; the line was laid down the north side of the steeple and fastened with twenty-five wall eyes; these wall eyes were hammered too deep into the wall, thus damaging the line and forming a short and sharp bend in each case, besides also unduly straining the wire. The damage to the steeple was the consequence of a neglected secondary circuit. There are an excessively large number of tie-rods in the steeple; the heads of these rods are not connected together, neither are they, except in one case, in close proximity to any of the larger masses of metal that are about the building. The conductor passed close to one of those heads; the south side of the steeple, where the opposite head is, becoming wet through the rain, a secondary circuit was formed, and a return shock followed; the damage to the steeple was trifling.

The rod was provided with a conical point rather blunt but surmounted by a short platinum point. The copper line-wire was of good material—not of a uniform thickness, but at the weakest places not weighing less than 240 grammes per lineal metre (8 oz. per yard or rather less than ¼ inch diameter if solid). The earth-plate was sunk into a well 10 metres deep, and tested faultless after the discharge.

ÉTUDE SUR LES PARATONNERRES LEUR CONSTRUCTION LEUR INSTALLATION. PAR
JARRIANT. 8vo. Paris. 1878.

(_Abstracted by G. J. Symons, F.R.S._)

This pamphlet opens with two pages devoted to the consideration of Michaëlis’s work published in 1783, “De l’effet des pointes placées sur le Templè de Salomon;” then it becomes more practical, refers to the Academy of Bordeaux propounding in 1750 the question as to the identity of lightning and electricity, and to Franklin’s letter in the same year to Collinson, giving his reasons for believing in the analogy; states that the experiments suggested by him were repeated by Buffon and Dalibar in March, 1752, and subsequently repeated at Marly before Louis XV. Then the writer refers to the erection of the first conductor in France, to the popular displeasure which it excited, and to the long legal process before the proprietor was allowed to keep it in position.

The author thinks that in many cases it is better to slightly increase the number of conductors than to make them of excessive length, because the latter course causes them to fatigue and jar the roof timbers by their vibration with the wind.

Respecting platinum points he speaks strongly and to the following effect:—“I have already mentioned that Franklin’s first conductor was melted. Since then, the upper terminals of conductors have been made of platinum, because it is the least fusible, the least oxidizable of all metals, and a very suitable one for making into points. Moreover, the sharper a point the greater its preventive action, and hence I condemn every conductor without a platinum point. Although some manufacturers employ simple copper cones, which may certainly last some time without deterioration, believing in the desirability of the points being always in perfect order, I reject their system entirely.”

Few persons are used to making platinum points, it is a Parisian speciality, those which the author prefers, form a cone of about 10 degrees at the opening of the point and are about 1½ inches long, then screwed and soldered into a mass of copper forming a nut on the conical copper rod, which is 1 foot or 1 foot 6 inches long. The platinum point thus mounted can only give rise to a galvanic action so extremely feeble as not in the least to affect the durability of the apparatus. Some persons for the sake of cheapness suppress this platinum point, but they are wrong, the saving is slight and the result defective. The author objects to conductors made of bar iron because the joints are always defective, and if the section be too small they may be so heated as to set fire to the charcoal in which the lower extremity is buried.(!) However, the author prefers a rope, but he does not say whether of iron or copper, and he puts a strand of hemp in the middle so as to make it more pliable.

“Arrived at the ground the conductor ought not to be in immediate contact with the earth, for the damp would slowly destroy it; we avoid this (?) by making it pass through a trough filled with coke. Experience has shown that iron thus buried in coke undergoes no change even during thirty years.... Broken coke is better than charcoal because of the great quantity of water which it absorbs.”

The author then says that after passing through this trough the conductor must be continued into a well, or into very moist earth, and should end with a discharger like a fork with many prongs.

He recommends that all the iron be galvanized.

Although the concluding paragraph, coming from a manufacturer, sounds rather like self-recommendation, it undoubtedly contains important truths. M. Jarriant says:—

“I cannot too strongly advise that in erecting conductors those specialists should be employed, whose studies and constant practice enable them to ensure perfect work. It is necessary also that every workman should remember that in placing a lightning conductor he holds in his hands the lives of men, that he should feel conscientiously interested in the perfection of his work, and, finally, that he should feel that it is a mission which he fulfils, and not a mere matter of trade at which he works.”

REPORT ON THE LIGHTNING CONDUCTORS OF THE SMALL ARMS AMMUNITION FACTORY
AT DUM DUM, CALCUTTA. BY W. P. JOHNSTON. Government Telegraph Press.
1878. 4to.

(_Abstracted by W. H. Preece, C.E._)