Electrical Review and Western Electrician
July 6, 1912
The Disturbing Influence of Solar Radiation on the Wireless Transmission of Energy
by Nikola Tesla
When
Heinrich Hertz announced the results of his famous experiments in
confirmation of the Maxwellian electromagnetic theory of light, the
scientific mind at once leaped to the conclusion that the newly
discovered dark rays might be used as a means for transmitting
intelligible messages through space. It was an obvious inference, for
heliography, or signaling by beams of light, was a well recognized
wireless art. There was no departure in principle, but the actual
demonstration of a cherished scientific idea surrounded the novel
suggestion with a nimbus of originality and atmosphere of potent
achievement. I also caught the fire of enthusiasm but was not long
deceived in regard to the practical possibilities of this method of
conveying intelligence.
Granted even that all difficulties were successfully overcome, the
field of application was manifestly circumscribed. Heliographic
signals had been flashed to a distance of 200 miles, but to produce
Hertzian rays of such penetrating power as those of light appeared
next to impossible, the frequencies obtainable through electrical
discharges being necessarily of a much lower order. The rectilinear
propagation would limit the action on the receiver to the extent of
the horizon and entail interference of obstacles in a straight line
joining the stations. The transmission would be subject to the
caprices of the air and, chief of all drawbacks, the intensity of
disturbances of this character would rapidly diminish with
distance.
But a few tests with apparatus, far ahead of the art of that time,
satisfied me that the solution lay in a different direction, and after
a careful study of the problem I evolved a new plan which was fully
described in my addresses before the Franklin Institute and National
Electric Light Association in February and March, 1893. It
was an extension of the transmission through a single wire without
return, the practicability of which I had already demonstrated. If my
ideas were rational, distance was of no consequence and energy could
be conveyed from one to any point of the globe, and in any desired
amount. The task was begun under the inspiration of these great
possibilities.
While scientific investigation had laid bare all the essential facts
relating to Hertz-wave telegraphy, little knowledge was available
bearing on the system proposed by me. The very first requirement, of
course, was the production of powerful electrical vibrations. To
impart these to the earth in an efficient manner, to construct proper
receiving apparatus, and develop other technical details could be
confidently undertaken. But the all important question was, how would
the planet be affected by the oscillations impressed upon it?
Would
not the capacity of the terrestrial system, composed of the earth and
its conducting envelope, be too great? As to this, the theoretical
prospect was for a long time discouraging. I found that currents of
high frequency and potential, such as had to be necessarily employed
for the purpose, passed freely through air moderately rarefied.
Judging from these experiences, the dielectric stratum separating the
two conducting spherical surfaces could be scarcely more than 20
kilometers thick and, consequently, the capacity would be over 220,000
microfarads, altogether too great to permit economic transmission of
power to distances of commercial importance. Another observation was
that these currents cause considerable loss of energy in the air
around the wire. That such waste might also occur in the earth's
atmosphere was but a logical inference.
A number of
years passed in efforts to improve the apparatus and to study the
electrical phenomena produced. Finally my labors were rewarded and the
truth was positively established; the globe did not act like a
conductor of immense capacity and the loss of energy, due to
absorption in the air, was insignificant. The exact mode of
propagation of the currents from the source and the laws governing the
electrical movement had still to be ascertained. Until this was
accomplished the new art could not be placed on the plane of
scientific engineering. One could bridge the greatest distance by
sheer force, there being virtually no limit to the intensity of the
vibrations developed by such a transmitter, but the installment of
economic plants and the predetermination of the effects, as required
in most practical applications, would be impossible.
Such was the state of things in 1899 when I discovered a new
difficulty which I had never thought of before. It was an obstacle
which could not be overcome by any improvement devised by man and of
such nature as to fill me with apprehension that transmission of power
without wires might never be quite practicable. I think it useful, in
the present phase of development, to acquaint the profession with my
investigations.
It is a well know fact that the action on a wireless receiver is
appreciably weaker during the day than at night and this is attributed
to the effect of sunlight on the elevated aerials, an explanation
naturally suggested through an early observation of Heinrich Hertz.
Another theory, ingenious but rather fine-spun, is that some of the
energy of the waves is absorbed by ions or electrons, freed in
sunlight and caused to move in the direction of propagation. The
Electrical Review and Western Electrician of June 1, 1912,
contains a report of a test, during the recent solar eclipse, between
the station of the Royal Dock Yard in Copenhagen and the Blaavandshuk
station on the coast of Jutland, in which it was demonstrated that the
signals in that region became more distinct and reliable when the
sunlight was partially cut off by the moon. The object of this
communication is to show that in all the instances reported the
weakening of the impulses was due to an entirely different
cause.
It is indispensable to first dispel a few errors under which
electricians have labored for years, owing to the tremendous momentum
imparted to the scientific mind through the work of Hertz which has
hampered independent thought and experiment. To facilitate
understanding, attention is called to the annexed diagrams in which
Fig. 1 and Fig. 2 represent, respectively, the well known arrangements
of circuits in the Hertz-wave system and my own. In the former the
transmitting and receiving conductors are separated from the ground
through spark gaps, choking coils, and high resistances. This is
necessary, as a ground connection greatly reduces the intensity of the
radiation by cutting off half of the oscillator and also by increasing
the length of the waves from 40 to 100 percent, according to the
distribution of capacity and inductance. In the system devised by me a
connection to earth, either directly or through a condenser is
essential. The receiver, in the first case, is affected only by rays
transmitted through the air, conduction being excluded; in the latter
instance there is no appreciable radiation and the receiver is
energized through the earth while an equivalent electrical
displacement occurs in the atmosphere.
figure 1
Another evidence is afforded by quantitative measurement which proves that the energy received does not diminish with the square of the distance, as it should, since the Hertzian radiation propagates in a hemisphere. One more experiment in support of this view may be suggested. When transmission through the ground is prevented or impeded, as by severing the connection or otherwise, the receiver fails to respond, at least when the distance is considerable. The plain fact is that the Hertz waves emitted from the aerial are just as much of a loss of power as the short radiations of heat due to frictional waste in the wire.
It has been contended that radiation and conduction might both
be utilized in actuating the receiver, but this view is untenable in
the light of my discovery of the wonderful law governing the movement
of electricity through the globe, which may be conveniently expressed
by the statement that the projection of the wave-lengths (measured
along the surface) on the earth's diameter or axis of symmetry of
movement are all equal. Since the surfaces of the zones so defined are
the same the law can also be expressed by stating that the current
sweeps in equal times over equal terrestrial areas. (See among others "Handbook
of Wireless Telegraph," by James Erskine-Murray.) Thus the
velocity propagation through the superficial layers is variable,
dependent on the distance from the transmitter, the mean value being
n/2 times the velocity of light, while the ideal flow along the axis
of propagation takes place with a speed of approximately 300,000
kilometers per second.
To illustrate, the current from a transmitter situated at the Atlantic
Coast will traverse that ocean—a distance
of 4,800 kilometers—in less than 0.006 second with an average speed
of 800,000 kilometers. If the signaling were done by Hertz waves the
time required would be 0.016 second.
Bearing, then, in mind, that the receiver is operated simply by
currents conducted along the earth as through a wire, energy radiated
playing no part, it will be at once evident that the weakening of the
impulses could not be due to any changes in the air, making it turbid
or conductive, but should be traced to an effect interfering with the
transmission of the current through the superficial layers of the
globe. The solar radiations are the primary cause, that is true, not
those of light, but of heat. The loss of energy, I have found, is due
to the evaporation of the water on that side of the earth which is
turned toward the sun, the conducting particles carrying off more or
less of the electrical charges imparted to the ground.
This subject
has been investigated by me for a number of years and on some future
occasion I propose to dwell on it more extensively. At present it may
be sufficient, for the guidance of experts, to state that the waste of
energy is proportional to the product of the square of the electric
density induced by the transmitter at the earth's surface and the
frequency of the currents. Expressed in this manner it may not appear
of very great practical significance. But remembering that the surface
density increases with the frequency it may also be stated that the
loss is proportional to the cube of the frequency. With
waves 300 meters in length economic transmission of energy is out of
the question, the loss being too great. When using wave-lengths of
6,000 meters it is still noticeable though not a serious drawback.
With wave-lengths of 12,000 meters it becomes quite insignificant and
on this fortunate fact rests the future of wireless transmission of
energy.
To assist investigation of this interesting and important subject,
Fig. 3 has been added, showing the earth in the position of summer
solstice with the transmitter just emerging from the shadow.
Observation will bring out the fact that the weakening is not
noticeable until the aerials have reached a position, with reference
to the sun, in which the evaporation of the water is distinctly more
rapid. The maximum will not be exactly when the angle of incidence of
the suns rays is greatest, but some time after. It is noteworthy that
the experimenters who watched the effect of the recent eclipse, above
referred to, have observed the delay.
Fig. 3 Illustrating Disturbing Effect of the Sun on Wireless
Transmission.