Abstract
We know in theory that the overall resistance of the earth is small
compared with that of a metal such as copper. To obtain a pair of
low-resistance ground terminals it is necessary that the cross-sectional
resistance of the surface of the earth that is presented to the two
ground rods be greater than that of the metallic conductor the
terrestrial connection is intended to replace. In that case, the
resistance offered by the earth will be equal to or greater than that of
the metal conductor. Earth can be made a conductor of as low a
resistance as desired simply by connecting the ends of the Tesla coil
transmitter and receiver ground wires to ground terminals that present a
sufficient surface area of contact. It is evident that we can diminish
the resistance offered by the ground terminal to any extent we like.
Indeed this resistance can be so reduced as to make it quite insensible
when compared to that offered by a copper wire.
"It seems many persons have formed designs for telegraphs. I, too,
formed mine, and prepared a specification of it five years ago, and that
included the plan of making one wire only serve for the returning wire
for all the rest, as in Alexander's telegraph; but even that might, I
think, be dispensed with where a good discharging train, as gas or water
pipes, at each end of the telegraph could be obtained." *
Here we have a suggestion of the use of the earth in precisely the same
way that it is employed to-day. In the period anterior to Volta's
famous discovery, when static electricity alone could be employed for
telegraphs, a single wire only was frequently employed, as we have
shown, water or the earth being employed for the return; but the true
function of the earth as part of the circuit was not then
understood—indeed, the dual character of electricity was not yet fully
comprehended — and Steinheil's was the first intelligent and practical
utilisation of the earth in circuit. According to De la Rive, the
discovery was made in this way:—
"Gauss having suggested the idea that the two rails of a railway might
be employed as conductors for the electric telegraph, Steinheil, in
1838, tried the experiment on the railroad from Nuremburg to Fiirth, but
was unable to obtain an insulation of the rails sufficiently perfect for
the current to reach from one station to the other. The great
conductibility with which he remarked that the earth was endowed, caused
him to presume that it would be possible to employ it instead of the
return wire. The trials that he made in order to prove the accuracy of
this conclusion were followed by complete success; and he then
introduced into electric telegraphy one of its greatest improvements."
Having so far simplified telegraphy by rendering unnecessary a complete
metallic circuit, Steinheil was led to speculate upon the possibility of
a still further reduction in the use of wire—whether, in fact,
telegraphing could not be carried on without wires. Having referred to
his discovery already mentioned, he proceeds thus :—" The inquiry into
the laws of dispersion according to which the ground, whose mass is
unlimited, is acted upon by the passage of the galvanic current,
appeared to be a subject replete with interest. The galvanic excitation
cannot be confined to the portions of the earth situated between the two
ends of the wire; on the contrary, it cannot but extend itself
indefinitely, and it, therefore, only depends on the law that obtains in
this excitation of the ground and the distance of the exciting
terminations of the wire, whetherit is necessary or not to have any
metallic communication at all for carrying on telegraphic intercourse."
He made experiments which proved the feasibility of influencing a magnet
through a distance of 50 feet, and there he apparently left the subject.
. . .
In July 1838, or seven months after the publication of "Corpusculum's"
letter, Steinheil made his accidental discovery in a way which we find
thus related by De la Rive:†
Gauss having suggested the idea that the two rails of a railway might be
employed as conductors for the electric telegraph, Steinheil, in 1838,
tried the experiment on the railroad from Niiremburg to Furth, but was
unable to obtain an insulation of the rails sufficiently perfect for the
current to reach from one station to the other. The great
conductibility, with which he remarked that the earth was endowed,
caused him to presume that it would be possible to employ it instead of
the return wire. The trials that he made in order to prove the accuracy
of this conclusion were followed by complete success; and he then
introduced into electric telegraphy one of its greatest improvements."
In Steinheil's own account of this discovery, he begins by pointing out
that Ampere required for his telegraphic proposal more than sixty line
wires; that Sommerring reduced the number to thirty or so; Cooke and
Wheatstone to five ; and Schilling, Gauss, and Morse to "one single wire
running to the distant station and back."
He then goes on to say:—
"One might imagine that this part of the arrangement could not be
further simplified; such, however, is by no means the case. I have found
that even the half of this length of wire may be dispensed with, and
that, with certain precautions, its place is supplied by the ground
itself. We know in theory that the conducting powers of the ground and
of water are very small compared with that of the metals, especially
copper. It seems, however, to have been previously overlooked that we
have it within our reach to make a perfectly good conductor out of
water, or any other of the so-called semi-conductors.
"All that is required is that the surface that its section presents
should be as much greater than that of the metal as its conducting power
is less. In that case the resistance offered by the semi-conductor will
equal that of the perfect conductor; and as we can make conductors of
the ground of any size we please, simply by adapting to the ends of the
wires plates presenting a sufficient surface of contact, it is evident
that we can diminish the resistance offered by the ground, or water, to
any extent we like. We can indeed so reduce this resistance as to make
it quite insensible when compared to that offered by a metallic wire, so
that not only is half the wire circuit spared, but even the resistance
that such a circuit would present is diminished by one half.
"The inquiry into the laws of dispersion according to which the ground,
whose mass is unlimited, is acted upon by the passage of the galvanic
current, appeared to be a subject replete with interest. The galvanic
excitation cannot be confined to the portions of earth situated between
the two ends of the wire; on the contrary, it cannot but extend itself
indefinitely, and it, therefore, only depends on the law that obtains in
this excitation of the ground, and the distance of the exciting
terminations of the wire, whether it is necessary or not to have any
metallic communication at all for carrying on telegraphic intercourse.
"An apparatus can, it is true, be constructed in which the inductor,
having no other metallic connection with the multiplier than the
excitation transmitted through the ground, shall produce galvanic
currents in that multiplier sufficient to cause a visible deflection of
the bar. This is a hitherto unobserved fact, and may be classed amongst
the most extraordinary phenomena that science has revealed to us. It
only holds good, however, for small distances ; and it must be left to
the future to decide whether we shall ever succeed in telegraphing at
great distances without any metallic communication at all. My
experiments prove that such a thing is possible up to distances of 50
feet. For greater distances we can only conceive it feasible by
augmenting the power of the galvanic induction, or by appropriate
multipliers constructed for the purpose, or, in conclusion, by
increasing the surface of contact presented by the ends of the
multipliers. At all events the phenomenon merits our best attention, and
its influence will not perhaps be altogether overlooked in the theoretic
views we may form with regard to galvanism itself."
* For 1837, p. 219. The full text of this interesting letter will be
found at p. 477, infra.
† Treatise on Electricity, London, 1853-58, vol. iii. p. 351
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