In telegraphy, as well as in the question of lightning rods, attention
has been but incidentally paid to the improvement of ground
conductors, and this point has not been the object of that careful
study that has been bestowed upon the establishment of aerial lines.
It is only recently that the interest created by lightning rods has
given rise to new forms of conductors differing from those formerly
used. The publications of the Prussian Academy of Sciences of from
1876 to 1880 contain some information of special importance in regard
to this. It is stated therein that the effect of ground conductors may
be notably increased by the division of the earth plates and the use
of metallic rods, without necessitating a greater output of material.
These facts, however, have not as yet been put to profit in practice
for the reason, perhaps, that the considerations, which have remained
general, have not at once permitted of obtaining forms what could be
employed with perfect knowledge of the results. This is what led Mr.
Ulbricht, of Dresden, to make calculations for a few forms of
conductors, and to test their approximate values. The results of these
researches are printed in the _Elektrotechnischen Zeitschrift_ for
1883 (p. 18).
[Illustration]
The equations found show, in the first place, that there exist three
means of obtaining a considerable effect, as regards the ground
conductor, with a slight expenditure of material: The cylindrical
electrode may be drawn out into the form of a bar or wire; the plate
may be rendered narrow, and elongated in the form of a ribbon; and,
besides, the annular plate may be enlarged in lessening the metallic
surface.
Finally, a short, open cylinder with a vertical axis may be formed by
curving a narrow plate or ribbon. It is not necessary to see the
formula to recognize the fact that this cylinder must behave like a
ribbon and a flat ring. The radius increasing, and the surface
remaining constant, the resistance of the earth here likewise
approaches zero.
As the resistance of the earth is inversely proportional to the
diameter of the plates, the zero resistance can also be reached by
dividing a plate _ad infinitum_. As the parts of the plate may be
brought quite close to each other without perceptibly interfering with
the action, a _network_ has finally been reached by a division carried
very far, yet limited, and by connecting the parts with one another by
conducting cylinders.
If we seek to determine what forms of ground conductors are efficient
and economical under given conditions, we shall have to begin by
informing ourselves as to the choice of material to be used for the
electrode, and shall then have to ascertain whether putting it in the
ground will or will not necessitate much outlay. The most suitable
material is copper, which may be used with advantage, in that it lasts
pretty well underground, and that the facility which it may be worked
permits of easily giving it more appropriate forms than those that can
be obtained with cast iron, which is of itself less costly.
If the burying in the ground requires little or no labor, as when
there exist ponds, rivers, and wells, or subterranean strata of water
near the surface of the earth, elongated forms of conductors will be
employed, such as the solid or hollow cylinder, the wire, the ribbon,
the narrow ring, and the network. Plates approaching a square or
circular shape are not advantageous. But if the ground has to be dug
deeply in order to sink the conductor, the form of the electrode must
be more condensed, and selected in such a way that the necessary
action may be obtained with a minimum output of copper and labor. For
great depths, and when the ground will permit of boring, an elongated
and narrow cylinder will be used. Such a system, however, can only be
employed when the cylinder is surrounded by spring water, since,
without that, an intimate contact with earth that is only moist,
cannot be obtained with certainty. In earth that is only moist and for
moderate depths, preference may be given to an electrode laid down
flat. The digging necessary in this case is onerous, it is true, but
it permits of very accurately determining the state of the earth
beneath and of obtaining a very perfect adherence of the electrode
therewith. Two forms, the annular ribbon or the flat ring and the
network, present themselves, according to calculations, as a
substitute for copper plates, which are so expensive; and these forms
are satisfactory on condition that the labor of digging be not notably
increased. These forms should always have a diameter a little greater
than that of the plate. The flat ring and the network, however, offer
one weak point, which they possess in common with the plate, and that
is, their dimensions cannot be easily adapted to the nature of the
ground met with without a notable increase in the expense. Now, if the
ground should offer a conductivity less than what was anticipated, and
it were desired to increase the plate, say by one-third, it would be
impossible to do so as a consequence of the closed form.
One important advantage is realized in this respect by combining the
ring and the network in the form of a reticulated ring having a
diameter of from 1 to 1½ meters. On cutting this ring at a given place
and according to a certain radius we obtain the reticulated ribbon
shown in the accompanying figure. The thickness of the wires is 2.5
mm., and their weight is 0.475 kilo. per meter. L, L, and L are the
points at which the conducting cable is soldered. A reticulated ribbon
of copper can be made in advance of any length whatever, and,
according to local exigencies, it may be easily curved and given the
form of a flat or cylindrical ring of varying width. Even though the
ribbon has already been cut for a ring of given diameter, it may be
still further enlarged by drawing it out and leaving a bit of the ring
open, so as to thus obtain a nearly corresponding diminution in the
resistance. Such a resistance may be still further diminished by
rendering the ring higher, that is to say, by employing an annular
cylindrical form.
After assuring himself, by experiments on a small scale, that
calculation and observation gave concordant results for the flat ring,
the author made an experiment on a larger scale with the annular
network. For practical reasons he employed for this purpose a copper
wire 2.5 mm. in diameter, which may be expected to last as long as one
of iron plate 2 mm. in thickness. Calculation showed that in a ribbon
160 mm. wide, meshes 40 mm. in breadth were advantageous and favorable
as regards rigidity. A reticulated ribbon like this, 4 meters in
length, was made and formed into a flat ring having an external
diameter of 1.42 m. and an internal one of 1.10 m. The resistance of
this ring was found to be W = 0.3485 (1/_k_), and that of a plate one
meter square, W0 = 0.368 (1/_k_).
As the conductivity of the earth is very variable, and as we cannot
have an absolute guarantee that the ramming will be uniform, it seemed
proper to make the measurements of the resistance by fixing the plate
and the ring in succession to the lower surface of a small raft, in
such a way that the contact with the water should correspond as well
as possible to the suppositions made for the calculation. As a second
ground conductor, a system of water pipes was used, and, after this, a
lightning rod conductor, etc.
Repeated and varied experiments gave, for the calculation of the
values of the resistances, equations so concordant that the following
results may be considered very approximate.
The square plate had a resistance of 35.5 Siemens units, and the
reticulated ring one of 32.5. From the first figure we deduce k =
1/91.12, that is to say, the specific conductivity of river-water is
1:91120000. Calculation, then, gives as the resistance of the earth in
Siemens units:
Calculated. Observed.
Square plate. 33.5 33.5
Annular ring. 31.76 32.5
These figures prove the accuracy of the calculations that had been
made in an approximate way.
The experiments were performed upon the Elba, above Dresden. Other
experiments still had reference to the influence of immersion. In
order to diminish polarization, only instantaneous currents from the
measuring pile were employed. It was to be supposed that the current
of water through which the bubbles of gas were removed from the
electrodes would not have permitted of a notable resistance of
polarization. Later measurements, made upon a ribbon buried, like the
plates, in the earth, gave likewise most favorable results.
As a result of these experiments, the State railways of Saxony have,
in such cases as were practicable, introduced the annular network of
copper. There are some manufacturers, too, who seem desirous of
adopting this system, although it has hardly emerged from the period
of experiment. The pecuniary advantages that will result from an
application of it ought, it would seem, to dispel a large proportion
of the criticisms directed against the erection of lightning rods,
from the standpoint of expense, and contribute to extend an
arrangement which may be considered as a very happy one.
If we compare the square plate with the equivalent annular network,
constructed as above indicated, and which should possess, according to
the author an external diameter of 1.26 m. and of 3.45 m., we find
that:
The square plate, 1 mm. thick weighs 8.9 kilos.
" 2 " " " 17.8 "
The annular network " 1.64 "
The cost of reticulated ribbon per meter amounts to about 4.4 francs,
supposing it to be arranged as shown in the cut.
As term of comparison, we may admit that the following forms are
nearly the equivalent of a horizontal, unburied plate one meter
square.
Length. Diameter.
Vertical cylinder buried 1.40 m. 0.13 m.
" " " 1.80 m. 0.06 m.
Vertical bar " 2.60 m. 0.013 m.
Horizontal bar " 5.20 m. 0.013 m.
Horizontal flat ring 1.32 m. in external diameter, and 1.08 m.
internal.
Horizontal network 1.01 m. square, and having meshes of the same size
as those of the reticulated ribbon.
Horizontal reticulated ribbon 3 m. in length and of the structure
described.
Horizontal annular ring 1.26 m. in external diameter, 0.94 m.
internal.
In conclusion, let us meet an objection that might be made to the
accuracy of the hypotheses that serve as a base to the preceding
calculations, in cases where ground plates for lightning rods and not
for telegraphs are concerned. Between the two ground plates of a
telegraph line there is generally a distance such that the curves of
the current undergo no deviation in the vicinity of one of the
electrodes (the only part important for integrations) through the
influence of the other. But it might be admitted that such would prove
the case with a lightning rod in a storm, at the time of the passage
of the fluid into the earth. The ground plate here is one of the
electrodes, and the other is replaced by the surface of the earth
strongly charged to a great distance under the storm clouds. If we
suppose (what may be admitted in a good lightning rod) that there no
longer occurs any spark from the point downward, the curves of the
current, in starting perpendicularly from the ground plate, would be
obliged to leave their rectilinear trajectory and strike the surface
of the earth at right angles. When the electricity flows through a
plane surface into an infinite body, it is only when such surface
presents a very great development that the respective potentials
decrease very slowly in the vicinity of the said surface. No notable
modification occurs, then, in the curves of equal potential, in the
vicinity of the ground plate through the action of this extended
charge, nor consequently any modification in the curves of the
current; but the electricity which spreads has but a short distance to
travel in order to overcome the most important resistances.
The calculations of resistances given above have, then, the same value
for discharges of atmospheric electricity.--_Bull. du Musee de
l'Industrie._
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