HÔTEL DE VILLE, BRUSSELS, AND WESTMINSTER PALACE.
The systems of lightning-conductors used for the protection of
the Hôtel de Ville and Westminster Palace seem worthy of separate
description, as showing the methods employed by Professor Melsens and
the late Sir William Snow Harris, both eminent authorities in their
respective countries. The two buildings are so entirely distinct in
their character, that it will be seen at once that very different
methods had to be employed in rendering them safe from the effects of
thunderstorms.
The Hôtel de Ville, Brussels, one of the finest Gothic structures
in the Netherlands, is fitted with an elaborate system of
lightning-conductors, erected under the superintendence of Professor
Melsens, a distinguished electrician and scientist. He has for many
years advocated the method of employing a great number of small
lightning-rods, in preference to one rod of large size, for the
protection of buildings from the effects of lightning; the main
characteristic of his system being that of covering the building with
a network of metal furnished with very many points, combined with
numerous and ample earth-contacts. This idea has been thoroughly worked
out at the Hôtel de Ville, Brussels; and probably no other building is
so completely guarded from the dangers of thunderstorms. The principal
feature of the Hôtel is a large central building, with a pinnacled
turret, from which rises a lofty spire, nearly three hundred feet high,
and adorned with four galleries, each with corner pinnacles. Upon the
top of this spire is a gilded colossal figure, seventeen feet high, of
St. Michael, holding a naked sword and standing upon a dragon. This
acts as a vane, and the point of the sword forms the highest terminal
conductor of the system. The main block of the Hôtel is ornamented with
six turrets, from each of which springs a small spire. In the rear is
a courtyard formed by buildings annexed to the front main block, and
composing the remaining three sides of this inner quadrangle.
The figure of St. Michael, all the parts of which are rivetted and
soldered together, rests on a pivot of iron, three and a half inches
in diameter, which is deeply embedded in the stone-work of the spire.
The weight of the vane produces a metallic connection with the pivot,
and the top of the platform in which the pivot is fixed is covered with
sheet-copper. Around this and in connection with the pivot are fixed
eight perpendicular galvanised iron conductors, two-fifths of an inch
in diameter, and provided with five points each. A flash of lightning
striking the statue would thus reach the pivot and then be divided
between the eight conductors. Just below the platform are placed, at
an angle of 45 degrees, eight large points six and a half feet long.
These are fastened to an iron band which encircles the spire, and are
connected with the eight conductors by means of a mass of zinc. Thus
the pivot of the statue, and consequently the statue itself, the eight
conductors, the eight large points, and the forty small points on the
conductors, constitute a protection which dominates the edifice, and
represents a circular space of about five and a half yards in diameter;
that is, between the extremities of the large points which project from
under the platform. In this manner a flash of lightning is instantly
distributed and conveyed by the conductors to the ground. It may be
mentioned here that a thin copper wire, insulated by three coatings, is
fixed on the north side of the iron band in which the large points are
fastened; the other end of this wire is left free, and can be utilised
as a conductor for a rheometer or any other electric machine which it
might be thought proper to use permanently for the registration of
lightning striking the conductors.
The eight conductors have each an unbroken continuity of about 310
feet; and they collectively show a continuous section of nearly
one inch--almost half as much again as the limit of safety given
in the ‘Instruction’ of the Paris Academy. Although, in Professor
Melsen’s opinion, rods of somewhat less diameter would have been
amply sufficient for security, he chose the largest size which could
be easily bent to the varying contours of the building, and also
as allowing for the expansion and contraction caused by changes of
temperature. If conductors of only one quarter of an inch diameter
had been used they would, it is true, have shown a total section
just above the limit of the ‘Instruction;’ but, since Coulomb has
demonstrated that tensional electricity is more particularly carried on
the surface of bodies, M. Melsens thinks it is necessary to consider
the action that this surface might exercise in the easy transmission
of electricity. Some old German writers on this subject went so far
as to assert that the conductivity was proportioned to this surface.
They therefore recommended flat bands or hollow tubes in place of rods.
Although exact figures cannot be given as to the effect due to the area
of the surface, M. Melsens considers that it is unquestionable that
the relation of the section to the surface has a marked and definite,
although at present unknown, result. In the case of the Hôtel de Ville,
Brussels, he thinks the eight conductors possess a signal advantage
over one conductor, even though it had a larger section--say one inch.
Experience will doubtless teach how to determine more precisely the
extent of this surface-action.
The eight conductors descend the length of the octagon of the spire
until they reach the first gallery; going round this they pass over
the balustrade, and then converge towards each other; are carried
over a prominence in the roof; and as they pass along gather up
other conductors of similar size from the ridges and parapets of the
buildings which form the quadrangle. Projecting vertically from these
horizontal lengths of the conductors are a large number of points and
aigrettes. The summits of the lower tower are also furnished with a
great many points. These eight main conductors are then taken down the
wall of the building into the courtyard, and at about three feet from
the ground are carried into a box constructed of galvanised iron, and
in it are connected into one solid mass by zinc, which has been poured
molten into the box. Almost throughout their length the conductors are
left loose, so as to remove all complication arising from dilatation;
the play of this dilatation being rendered easy on account of the small
section of the conductors, which bend readily.
In accounts of lightning striking buildings which have been provided
with lightning-conductors, it is almost invariably found that these
conductors are incomplete, and have generally been fixed by persons
ignorant of the scientific questions involved. When the facts in
such cases are carefully examined it is found, as a rule, that the
defect is in the connection with the water underground, or in the
bad conductivity of the earth in which the conductors terminate.
In establishing a perfect communication with the earth, M. Melsens
considers it is necessary, not only to place the conductors in contact
with water, but also to see that the contact extends over a large
surface. The Paris Academy ‘Instruction’ recommends this precaution,
but in a very vague and too succinct a manner. To the above rule may
be added another condition, namely, that the earth-connection should
be large in proportion as the site of the building is redundant in
metal products in direct or indirect contact with the ground, the
subsoil, or the damp earth of the foundations, and sometimes even with
water itself. With regard to the metal contained in the materials of
buildings, it is not sufficient to establish a connection at one point
only, as is generally supposed. On the contrary, it is important that
all the metal-work should be connected with the conductor at least at
two points, in order to realise closed metallic circuits, and thus
offer an entry and exit, or a free metallic course, for the current of
electricity. The foregoing statements have been placed here chiefly
because the principles they convey have been so rigidly, and at the
same time successfully, carried out by Professor Melsens at the Hôtel
de Ville, Brussels.
[Illustration: Fig. 3.]
To return to the eight conductors and the earth-connections provided
for them. It has been shown that these conductors, after descending
the wall of the building, reach a point about three feet from the
ground, where they are embedded in a rectangular box of galvanised
iron, which is eight inches long, three inches broad, and three and a
half inches high. In the bottom of the box are three holes, through
which pass three series of eight conductors, each series being of the
same diameter as those which descend from above; the conductivity
being thus increased threefold. All of these are formed into one mass
by the zinc, which has been poured into the box in a molten state,
so that they constitute with the eight rods from above, one integral
conducting system. In the illustration which is here given the box
is represented by B, and the eight main conductors coming down from
the building by C C. The three series of rods numbered 1, 2, 3 show
the triplicated conductors issuing from the box. The first series is
placed in communication with the water by means of an iron pipe, which
carries it underground to a well. Here the rods are inserted in a large
tube six and a half feet long and nearly two feet in diameter (see
engraving). This tube is let down almost four feet below the level
of the earth, and sustained by two chains hung on two iron holdfasts
fixed in the side. The conductors C C are fastened to this tube in the
following manner:--A small length of straight iron cylinder is placed
outside the flange of the tube; and the ends of the conductors being
arranged between the cylinder and the flange, the space _a a_ is filled
with molten zinc; thus rendering the substance of the iron tube and
that of the conductors metallically continuous. The well into which the
tube is sunk furnishes perpetually a contact of eleven square yards
between the water and the iron of the tube. Into the space _a a_ is
also introduced a large number of small galvanised iron wires to act
as auxiliary conductors; these are terminated by being brought to a
point and soldered to the mass of zinc. In order to prevent as far as
possible the formation of rust, a large quantity of lime is thrown into
the well, in order to make the water alkaline. The second series of
conductors, painted with coal-tar, is placed in a covered metal gutter
and carried some distance to a gas-main in a spot where the earth is
moist. The conductors are fixed by means of a large copper plate, which
is soldered to the gas-pipe or main. On the copper plate are fastened
sixteen large-headed brass screws, to which the conductors are secured.
This arrangement is enclosed in brickwork, the wires being painted with
coal-tar; and a quantity of boiling tar is poured on the copper plate,
over which is laid a cloth, thus preserving the whole from oxidation.
The third series of conductors is carried in a gutter, similar to that
which contains the second series, to a water-pipe in the Place de
l’Hôtel de Ville, and the wires are fixed to it in the same way.
[Illustration: Fig. 4.]
It may be added that the whole of the conductors above-ground--with the
exception of the points--are painted with oil.
Although it is correct that the coke generally placed around the
earth-connection of conductors aids by its good conductivity to bring
them in contact with a large surface of earth, Professor Melsens has
preferred to employ tar, which, it is true, is insulating, but helps
materially to preserve the conductors. It is estimated that the entire
contact between the earth and the underground surface of iron is about
300,000 square yards.
Professor Melsens thinks it is worthy of note that, although copper
is a better conductor of electricity than iron, it has less molecular
strength. Where thin iron wire would simply be beaded--without losing
its conductivity--by an exceptionally strong charge of electricity,
copper wire of the same thickness would by a similar charge be
dissipated to a black powder. Professor Melsens has verified this
in some very interesting experiments. The large use of iron in his
system of conductors on the Hôtel de Ville, Brussels, was rendered
imperative by reason of the enormous cost of sufficient copper for such
an extensive system. But Professor Melsen’s experiments, nevertheless,
give some support to the selection of iron for large and complete works
of this kind.
Sir William Snow Harris, in his arrangements for the protection of
the Palace of Westminster from lightning, has endeavoured to perfect
the general conductivity of the whole mass of the building, and so
make it assume the same relation to the electric discharge as if it
were a complete mass of metal. Westminster Palace differs in one
important respect from the Brussels Hôtel de Ville--the general level
of the roofs is covered with iron coated with zinc, and in many places
directly connected with the earth by cast-iron water-pipes. The roofing
thus constitutes, although imperfectly, and only to a limited extent,
a protection of itself. Sir William Snow Harris had, therefore,
chiefly to provide for those portions of the building which are above
the general level of the roofs, and, by the use of ample conductors
of copper, to make up for the comparatively low conductivity of the
roofing and the iron pipes which connect it with the earth.
From the terminal which forms the highest point of the large central
tower is brought a copper tube of two inches diameter and one-eighth
of an inch in thickness, the joints of which are secured by solid
screw-plugs and coupling-pieces. This tube is carried down in the
south-west angle of the tower and fastened to the masonry by metallic
staples. At the junction of the tower with the roofs the tubing is, or
at any rate was, thoroughly connected with the metal of the roof, and
then continued to the earth in as straight a course as practicable,
and there terminates in two projecting branches made of solid copper
rod. By carrying this copper tubing direct to the earth, instead of
terminating it in the metal-work of the roof, the electrical discharge
obtains a conducting medium of the same power throughout, in place of
having to leave a high power for one of lower conductivity.
The Victoria and Clock Towers, which are each 300 feet high, are both
fitted with a copper band, five inches wide and a quarter of an inch
thick. These run down the walls, and are connected with the metal of
the roof, and also with the metallic rail of the staircase within each
tower. The ornamental turrets and pinnacles of St. Stephen’s Porch
are protected by small bands of sheet-copper, two inches wide and
one-eighth of an inch thick; these are also placed in connection with
the metal of the roof.
The north and south towers of the central block, and the north and
south wing towers of the front facing the Thames, have attached to
them bands of sheet-copper running from their respective vanes to the
roofing below. The bands are connected with the metal of the roof, and
are then carried down independently to the earth, in a similar manner
to that adopted on the large central tower.
The only other prominent portion of the edifice is the ventilating
shaft of the House of Commons, where, during the sitting of Parliament,
a coke-fire is generally burning, and from which, therefore, a stream
of warm and rarefied air is constantly being emitted. The conductivity
of an ascending column of warm vapour is known to be great, and
accidents from this cause are of frequent occurrence, although very
often they are not ascribed to their true source. To obviate this
danger, the ventilating shaft is provided with a copper tube conductor,
fixed on its eastern side, and connected with the metal of the roof.
This short description of the measures adopted by Sir William Snow
Harris for the protection of Westminster Palace contains all the
salient points of the system which at that time, some twenty years
ago, was doubtless the best that could be devised. But, although
nearly 4,000_l._ was spent upon this work, from that time to this, as
far as can be ascertained, these lightning-conductors have never been
tested! It is therefore very possible, and indeed probable, that on the
occurrence of any very heavy thunderstorm they would be found wanting,
and considerable damage would ensue, the extent of which no one can
estimate.
[Illustration]