MM. Franklin, Le Roy, Coulomb, Delaplace and
l'Abbé Rochon made the following report.
M. the maréchal de Ségur sent two proposals to
the Academy, to equip the powder magazines in Marseilles with
lightning rods; in his letter accompanying the proposals, he
explained that the King wished the Academy to have them examined,
and to give him our advice on it. The Academy appointed us to
report on the proposals; we are going to do this with all the care
required for a subject of this importance, not only because of its
subject, but also because of its beneficial results. For nothing is
more apt to increase the number of lightning rods everywhere, than
the example which the King will give by equipping his kingdom's
powder magazines with them.
But before discussing the proposals which we
have been entrusted to examine, it will be useful to briefly review
the theory which must guide the installation of lightning rods or
conductors.
To outfit a given building in order to protect
it from lightning, using one or several conductors, there are
different circumstances to which one must pay particular
attention.
1. One must know or determine the area to be
protected, in order to decide whether it is necessary to put up one
or several rods, for this is an important point to resolve before
undertaking anything. Unfortunately, electrical experiments have
not yet taught us anything which might lead to a knowledge of the
extent of the sphere of action of a conductor's tip. But since
buildings have been equipped with them, several observations have
taughtus that some parts of the building at a distance of more than
forty-five feet from the conductor's tip were struck by
lightning. It follows quite naturally that the rods must be placed
so that their sphere of action need only protect parts situated at
a lesser distance.
2. When there are several pointed rods or
spires on a building, they must be well connected together, as well
as all the parts of the roof of the building covered in lead or
containing some other metal, such as the iron tips of weather
cocks, or of any decoration, so that the whole produces a single
body of metal elements, which, with the transmission bars, would
suitably transmit the electrical energy of a lightning bolt from
the top to the bottom of the building, no matter the direction it
came from.
3. It is no less important that these bars be
closely interconnected, so that the energy does not encounter any
resistance in going from top to bottom. For a break of continuity
in the bars always produces a larger or smaller resistance
depending on the distance between them, because the electrical
energy must jump from one bar to the other.
4. Finally, the bars must be in full contact
with moist earth, or better yet, with water, so that there is an
easy passage consistently open all along the transmission bars, to
the earth's mass, the great common reservoir of electrical
matter.
As for the height of these pointed rods, they
must be at least 12 or 15 feet or even more, if the building is
very large. It is certain that the higher they are, the larger the
radius of their sphere of activity. They should be two inches
square at the base or even more, in proportion to their height when
this exceeds 15 feet. And as for the size of the transmission bars,
it seems that a size of to inch square will be more than sufficient
to transmit the electrical energy of the most violent lightning
bolt. Certainly no example has been observed of iron bars of this
width which have been melted or altered in any way by the passage
of the bolt.
After these preliminary notions of how to
install conductors and the dimensions of their elements, we must
turn to the examination of the two proposals to equip the powder
magazines of Marseilles with lightning rods, proposals sent by M.
the maréchal de Ségur and signed by MM. Ravel, de Puy Comtal and
Pierron, the first being an officer in the Royal Artillery Corps,
and the other in the Royal Fortifications Corps.
These two proposals were created for the same
magazine, which is 31 fathoms long by 8 wide or thereabouts. In the
first proposal, three pointed rods or spires are mounted, on the
ridgepole of the magazine and along its length. To these, four more
are added, placed respectively at each corner of the building. The
second proposal also recommends three pointed rods on the rooftop,
but instead of placing them at the four corners of the building,
they are arranged in a quincunx on the two slopes of the roof, with
horizontal iron transmission bars all along it, connecting the
pointed rods to each other.
As for how the rods are fastened or clamped to
the rooftop, whose transmission bars are assembled together, and
finally how they lead to the water, the system is the same in each
proposal. In looking briefly at the drawings, the Academy will
easily understand everything we have just said about the nature of
the two proposals.
In order to decide which of the two should be
preferred, we will note that the second, with its transmission bars
laid horizontally along the length of the roof, would entail large
expenses which seem unnecessary. However, the pointed rods from
this proposal should be kept, only instead of placing them in a
quincunx pattern as proposed in the plan, they should be put in
such a way that each of them is exactly
[Fol. 93]
between the pointed rod at the midpoint of the rooftop and the
one at the edge. And instead of connecting the rods by the
transmission bars laid along the length of the roof, they should be
connected with those on the rooftop by bars which will join them in
a manner perpendicular to the length of the roof. This way, all the
pointed rods would be just as well connected, without the expense
of laying the bars horizontally all around the roof. This point
decided, we must proceed to the examination of the proposed way in
which to connect or assemble the different elements of the
lightning rods in these two proposals.
It seems that MM. Pierron and de Ravel, the
authors of these two proposals, driven by the need to secure a very
close connection between these elements, believed that no expense
should be spared to achieve this aim, but we cannot help but think
that these gentlemen pushed this scruple too far, and that the
method they propose to connect the different elements of their
conductor system would involve too much work and superfluous
expense. In fact, the screws at the ends of the transmission bars,
and the nuts in the hooks designed to accomodate the screws, would
cost a fairly considerable sum if they are to be well done and
achieve their purpose. We believe that one might proceed more
simply: consequently, here is how we think that these magazines
might be equipped, according to what has been presented to us,
combining solidity and all else which might contribute to the
economical construction of their lightning rods, in conformity with
what M. the maréchal de Ségur appears to desire, according to the
terms of his letter.
We will retain the struts for the spires in the
powder magazines proposed for protection in Marseilles and in other
ports in which they are exposed to heavy sea winds. However, other
magazines found in the Kingdom's interior might be able to do
without them, experience having shown that the pointed rods with
the dimensions given in the drawings are sufficient to resist the
efforts of the winds there; but we will make the spires higher, up
to 15 feet from their base, and directly above the place where it
will be fastened, in the freestone, we will fashion a circular
flange on each side, about two inches in diameter and inch thick.
We will make a hole in the center of it, inch in diameter; at the
end of the first transmission bar to be attached to this spire, we
will fashion another identical flange. We will do the same thing on
the other end, so that all the transmission bars will have a
latchet on each extremity. In order to connect these elements, we
will put the latchet of one of these bars against that of another
bar, putting a strip of lead between them. And by using a nut with
a headed screw, the stem of which will pass through the eye of
these flanges, they can be tightly fastened together. In this way
the bars will be well attached to each other; they will be able to
be separated easily, and this adjustment will be very easy to carry
out. In order to reinforce the transmission bars along the roof, we
will be able to establish and attach hooks in the freestone at
intervals, which will be split at the top in order to receive them.
As for the position and distribution of these pointed rods or
spires, they will be constructed as we proposed, three on the
ridgepole and two on each slope of the roof, located respectively
at the midpoint of the interval between the spire at the middle of
the building and the one at the edge. These spires on the slope
will be connected to the transmission bars placed along the roof,
as we stated above, and they will be tall enough to surpass the
ridgepole by least six feet. By this arrangement of the different
rods of the building, all the parts of the roof will be well
protected by these spires, there being no part which is not well
inside the sphere of activity discussed above. We can only applaud
the way in which it has been proposed to link the transmission bars
to water, by leading them to the sea. However, if at the other end
of the building there were soil at the surface, and the ground was
not entirely rock, the transmission bars could be made to descend
from the pointed rod at this end: this could serve to reduce the
distance that the electrical energy would have to travel to reach
the other side. The way in which the copper tips are fitted to the
spire is good, as they can be adjusted by screws; either method is
fine, but in the latter [proposal], the tip can be easily
dismantled, and this is an advantage, as it is sometimes necessary.
Some of them have been observed to be melted by the passage of the
electrical charge from a thunderbolt.
Such are the observations that we felt
necessary to make regarding these two proposals which M. the
maréchal de Ségur sent to the Academy, to equip the powder
magazines of Marseille with lightning rods; and such are the
suggestions which we believed necessary for connecting the
different elements of the system together. But before concluding
this report, we owe it to MM. Ravel de Puy Comtal and Pierron to
say that we recognize by the manner in which their proposals were
conceived, that they understand the subject matter very well, and
that they are quite capable of equipping powder magazines with
lightning rods, the task with which M. the maréchal de Ségur has
entrusted them in order to protect these magazines against the
ravages of lightning.”
Académie Royale des Sciences, minutes, tome
CIII, fol. 90 v°-95 r° (session for Saturday April 24, 1784)