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Page 31
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 1� 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:
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