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Page 30
Without any desire to seek an application for an experiment that is
simply curious, we yet believe that there is here a phenomenon of a
nature to be studied by physicists. Discoveries in telephony and
microphony have certainly opened up to science, as regards both theory
and practice, new horizons that still promise other surprises for the
future. But to return to the observatory: The success obtained by the
exhibition of the French Society of Physics shows that these reunions
respond to a genuine need--that of instructing in and popularizing
science. While warmly congratulating the organizers of these meetings,
we may express a wish that the good example set by the Society of
Physics may be followed by other societies. We are convinced in
advance that an equal success awaits them.--_La Nature._
* * * * *
ON THE ARRANGEMENT OF GROUND CONDUCTORS.
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.
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