To: "Gary Peterson"
Cc: "Ljubo Vujovich"
"Christopher Bach"
"Jeff Hayes"
"Jim Hardesty"
"John Wagner"
"Kelley Cole"
"Robert Uth"
"Wallace Brand"
"Huff, Lois"
"Bryden, Dan"
"Wilfred Mische"
"Liala Strotman"
"Jane Alcorn"
"Gene Genova"
"Dick Doremus"
"Chris Wesselborg"
"Carol Wiebelt"
"Betty Coveney"
"Jim Yeck"
Subject: Re: An appraisal of Tesla's wireless work.
Date: Tuesday, January 13, 2004 8:26 PM
Greetings Gary:
I hope that some of the e-mail addresses on this old list are still
valid. A retired radio man in Oregon recently asked me about Tesla's
proposed system for wireless transmission of power. As you know, there
is no easy answer to this question because there is so little clear
information about what Tesla was doing in this regard. The following is
my reply. I am passing it along to you because I thought it might be of
general interest. Perhaps someone would forward it to Margaret Cheney.
I have not attached the material referred to, but I think you can
follow the arguments without it. Most of the diagrams referred to are in
my article about Tesla in the February and May 1999 issues of the Old
Timer's Bulletin (OTB), published by the Antique Wireless Association. I
hope that no one will take offence at some of my criticism of other
people's interpretations of Tesla's wireless work, and I will not take
offence at criticism of my views.
Cheers, Henry.
Dear ....
Probably you expected a short answer to your query about Tesla’s
wireless power ideas. All I can give you is a long inconclusive one.
That is because no one is sure what Tesla had in mind, and no one is
sure that what they think he had in mind would work. So you have
uncertainty piled on uncertainty. What follows is the best that I can
do.
There are some good biographies of Tesla, but they are all weak on
the technical details of his work. You could read everything that has
been written, at least in the popular literature, without finding any
satisfactory explanation of Tesla’s scheme for wireless transmission of
power. I don’t guarantee the accuracy of my version either. The best
documented Tesla biography probably is “Tesla - Man Out Of Time”, by
Margaret Cheney, Dorset Press (1981), ISBN 0-88029-419-1. A recent one
is “Tesla - Master of Lightning”, by Margaret Cheney and Robert Uth,
Barnes and Noble (1999), ISBN 0-7607-1005-8. It has many interesting
photos. Their chapter entitled “Who Invented Radio?” is completely
erroneous, but that does not detract from the rest of the book. A very
interesting reference, if you can find it, is: “Nikola Tesla On His Work
With Alternating Currents And Their Application To Wireless Telegraphy,
Telephony, And Transmission Of Power”, by Leland Anderson, Sun
Publishing, Denver (1992), ISBN 0-9632652-0-2. This is lengthy testimony
before a legal counsel by Tesla himself about his wireless work.
There are several reasons why it is difficult to determine either
what Tesla intended to do or what he accomplished in the field of
wireless. His contributions to AC power technology in the 1880’s and
1890’s are well known because they were developed commercially by
Westinghouse. After that, starting in the 1890’s, he experimented with
high voltage, high frequency alternating current, and worked on concepts
for wireless communications and wireless transmission of useful amounts
of electric power. He worked more or less alone, and didn’t confide much
in his assistants or anyone else. He wrote articles in general and
glowing terms about what the future would bring, but they revealed
little technical information. Surprisingly, his patents also do not
throw much light on how his inventions were supposed to work. I have
enclosed copies of his most important wireless patent applications, and
you can judge for yourself. I think that the patent commissioners were
pretty indulgent with Tesla, because his patent descriptions were vague,
and he didn’t provide working models or demonstrations.
When he finally did build a large transmitting station
(Wardenclyffe) on Long Island, based on his wireless concepts, he ran
out of money before he could finish it. So no one knows yet whether the
station would have worked or whether his principles had practical value.
In spite of this absence of information or proof, there are many Tesla
fans who claim that Tesla was the real inventor of radio. This is
because most of them, especially Tesla biographers, neither understand
what radio is nor what Tesla was trying to do.
According to my interpretations, Tesla’s wireless efforts fell into
two categories: communications, and transmission of electric power.
Obviously the latter is more demanding than the former. Let us consider
wireless power transmission first. Tesla gave few details, so much of
what I say is educated guesswork. When talking about transmitting power,
he spoke of setting the whole Earth into electrical oscillation by means
of his special type of transmitter, and drawing useful electric power
from this oscillation anywhere on Earth by means of his special type of
resonant receiver.
He calculated the resonant frequencies of whole Earth electrical
oscillations, and presumably he intended to transmit power wirelessly at
these frequencies. He reasoned that since the whole Earth would be in
oscillation, electrical resistance would be negligible or of no
consequence. (Actually, he had this backwards. The resistance would have
to be negligible to get the whole Earth oscillating.) The resonant
frequencies that he calculated were a bit off because he assumed that
the electrical disturbance would travel directly through the Earth
rather than over its surface, but this error is inconsequential. The
correct frequencies are the natural resonant frequencies of the
spherical cavity between the surface of the Earth and the ionosphere.
They are called Schumann resonances, and correspond to radio waves
propagating over the surface of the Earth to the point opposite the
source, and back again to reinforce the original signal at the source.
If that round trip time is taken to be the fundamental period of the
resonant oscillation, the fundamental frequency is about 7.5 to 7.8 Hz.
Other Schumann resonance frequencies are roughly integer multiples of
this frequency. These resonances are observed as peaks in the natural
electromagnetic noise in this frequency range. Presumably Tesla wanted
to transmit wireless power at these frequencies; i.e., in the ballpark
of tens of hertz. On the other hand, he seems to have considered
frequencies in the ballpark of tens or hundreds of kilohertz for
communications, presumably because he wanted to modulate a carrier
frequency rapidly for transmission of code, speech or other information.
Although the requirements for efficient operation at frequencies
around 10 hertz are far different from the requirements at 100
kilohertz, Tesla provided only one diagram for the transmitter and
receiver in his patent applications, and did not specify the operating
frequency. In his patent applications (enclosed), he vaguely stated that
transmission would be through “the natural media”, and he emphasized
that it would be via electrical conduction, NOT via Hertzian (radio)
waves. He felt that radio was a dead end because radio waves spread out
and therefore would not provide significant power at long distances from
the transmitter, whereas electrical conduction would transmit power
around the Earth efficiently. Tesla was vague about what natural
conducting media he was going to use. Vagueness was a common ploy in
patents in those days. The more general the description of your
invention was, the more developments you could lay claim to in the
future.
Tesla sometimes talked about utilizing the conductivity of the
earth between the ground connections of his transmitter and receiver,
and sometimes talked about utilizing the conductivity of the rarefied
air in the stratosphere between the elevated “terminals” of his
transmitter and receiver. (See diagram.) If he could do both with the
same setup, he would have a closed circuit between the transmitter and
receiver. However, Tesla seems to have given up the stratospheric
conduction idea eventually because of the difficulty of getting the
terminals up to high enough altitudes and/or other problems. He seems to
have settled for the open circuit system shown in the enclosed diagram
which just utilized conduction through the earth. His patent
descriptions were so vague that this change of emphasis did not require
revising them. However, his first wireless patent application in March,
1900 (#645,576) included a long preamble about the conductivity of the
rarefied air in the stratosphere, whereas his last one in 1914
(#1,119,732) only refers to a ground connection. This was just as well,
because in my opinion Tesla’s belief that current would flow long
distances through the stratosphere from a single high potential terminal
was wrong.
In the open circuit system (see enclosed diagram), a generator in
the transmitter applied a high voltage between an elevated conducting
body (called a “terminal”) and the ground via a step-up transformer. For
high frequencies, the generator or oscillator was some sort of circuitry
involving high speed circuit breakers plus resonant circuits. The AC
voltage from the secondary coil of the transformer would cause charge to
oscillate back and forth between the elevated terminal and the ground,
and the current at the ground terminal would flow into the surrounding
earth.
Some authors confuse the elevated terminal with a radio
transmitting antenna, but that was not its intended purpose. The
terminal was simply intended to be a source and sink for the electric
charge that oscillated between the terminal and the ground. The larger
the terminal, the larger the amount of charge that the applied voltage
could set in motion. The ability of an isolated conductor to store
charge is sometimes referred to as its “self-capacitance”, or just
capacitance. The use of the word “terminal” to describe the elevated
conductor probably was a leftover from when Tesla intended to use it to
conduct electricity into the stratosphere. (The self-capacitance of the
human body causes detuning of a RF resonant circuit when you touch the
ungrounded side of the circuit, even when you are well isolated from the
ground. If you touch the hot side of a transmitter tank circuit, the
“charging current” due to your self-capacitance can give you a burn, as
you probably well know. The self-capacitance of a spherical conductor is
proportional to its radius, and is 111 pF or micro-micro farads per
metre of radius.)
The current between the terminal and ground in Tesla’s proposed
transmitter is the same phenomenon that occurs in a conventional LF or
VLF radio transmitter connected to a vertical antenna and ground. In a
radio transmitter, the output transformer applies a RF voltage between
the antenna and ground, and a current flows in this open circuit by
virtue of the self-capacitance of the antenna. (Depending on the
dimensions and elevation of the antenna, some of its capacitance may
also be capacitance to ground.) In radio work, no one pays any attention
to where the ground current goes because the main interest is in the
radiation of radio waves from the antenna. In the Tesla system, it is
the ground current that is of interest. It presumably spreads out around
the Earth, and the terminal and ground of a receiver tap this to either
receive a communications signal or deliver electric power to a load.
This raises the legitimate question of whether the signals received
from conventional LF or VLF radio stations are due mainly to the radio
wave, or to current from the ground terminal of the transmitter. This
question continued to be debated for the first couple of decades of the
twentieth century. (See the enclosed copy of questions and answers from
the April 1919 Electrical Experimenter.) Variations in received signal
strength attributable to the ionosphere indicate that the signal is
mainly due to the radio wave, but more about this later. Also some or
most LF and VLF stations utilize a network of wires for the ground
(ground screen or counterpoise) instead of an earth connection, so they
don’t necessarily emit a ground current.
Tesla said very little that I have seen about the principles of his
receiver. The description in the enclosed patent applications indicate
that it worked like the transmitter in reverse. Presumably, the
transmitted earth current would be accompanied by an AC variation in the
ground potential and a vertical AC electric field above the surface of
the earth. The electric field would induce a voltage between the
receiver terminal and ground, and produce a current in primary of the
coupling transformer as in a conventional radio receiver. The
transformer applies the signal to the detector for communications, or to
the load to deliver power. The transformer acts as a voltage step-down
and current step-up transformer.
Tesla appears to have been aware that energy losses in the earth
would be greater at the higher frequencies used for communications than
at the very low ones that he presumably intended to use for power
transmission, and therefore the ranges would be shorter. I.e., the high
frequencies used for communications would not set the whole Earth into
oscillation. However, he still thought that the ranges would be far
longer than those provided by radio because he believed that radio waves
spread out in inverse square fashion, and continue straight out into
space. (See his diagram in the Feb 1919 Radio Experimenter article.)
Early skeptics of Marconi’s first transatlantic radio transmission
thought the same thing. This of course is true for VHF and higher
frequencies, and limits their range to roughly line-of-sight. However,
Tesla was unaware that at HF (short wave) and lower frequencies, the
ionosphere reflects radio waves back to earth, making long distance
radio communication possible. When Marconi and others established
transatlantic radio communications in the early 1900’s, he claimed that
the signals actually were being conducted through the ground and/or
ocean, just as he planned to do. He clung to this explanation long after
everyone else had agreed that the transatlantic signals were via radio
waves.
I had not paid much attention to Tesla’s concept of wireless
transmission of power before, but your query inspired me to do a few
calculations. I assumed that he intended to use the arrangements of
elevated spheres plus ground connections described in his patent
applications, plus conduction through the earth, because I have not read
of any other proposals by him. I assumed a frequency of 7.5 Hz, the
fundamental Schumann resonance frequency, because Tesla talked of whole
Earth oscillations in connection with wireless power transmission. I
also gave him the benefit of the doubt and neglected the overall
resistance of the Earth. According to simple electrostatic theory,
applying a voltage between an elevated conductor (I assumed a sphere)
and the ground would cause the potential of the whole Earth to change a
small amount. This is consistent with Tesla’s analogy of pumping up the
pressure in a ball (Electrical Experimenter, Feb 1919, enclosed).
For the purposes of calculation, I assumed that the radius of the
spherical transmitter terminal was 10 metres (about the same as at
Wardenclyffe) and the maximum voltage applied between it and the ground
was 10 million volts (close to corona breakdown in dry air for a sphere
of this size). I assumed that the receiver dimensions were the same as
the transmitter ones, and that the spherical terminal was 60 metres
above the ground. According to my calculations, 10 million volts applied
between the transmitter terminal and ground would change the potential
of the whole Earth by about 16 volts. This would produce a vertical AC
(7.5 Hz) electric field of about 2.5 microvolts per metre above the
surface of the Earth anywhere fairly far from the transmitter terminal.
This field would induce a voltage between the receiver terminal and
ground of about 150 microvolts. The equivalent circuit for the receiver
(enclosed) would be a 7.5 Hz generator producing 150 microvolts in
series with a capacitance of 1110 picofarads (micro-micro farads) and
the load. The capacitance is the self-capacitance of the spherical
receiver terminal. This would deliver about 10 to the power -15 watts to
an optimum load! That is 1/1000,000,000,000,000 of a watt. The power
could be increased by tuning out the reactance of the capacitance, but
this would require a coil of impossibly high inductance and Q because
the frequency is so low. So Tesla’s concept of drawing power from a
whole Earth oscillation anywhere on Earth by means of a resonant
receiver appears to me to be correct in principle, but unrealizable in
practice at Earth resonance frequencies.
My electrostatic calculations underestimate the received signal
because they do not take into account the reinforcement of the electric
field due to successive reflections of the transmitted signal from the
opposite end of the Earth. These reflections would generate a standing
current wave embracing the whole Earth. Tesla probably thought that the
standing wave would be very strong because he thought the resistance of
the whole Earth was negligible. However, I have never seen any of his
numbers, or as I said, even a clear explanation of the principles of his
proposed receiver. According to modern electromagnetic noise
measurements, the Q of the Schumann resonance, or that of the spherical
Earth-ionosphere cavity that produces it, is around 5 to 10. I would
therefore expect the received power to be about 5 or 10 times greater
than the amount I calculated using electrostatic theory. This still
would mean that the received power delivered to the load would only be
around 10 to the power -14 watts. Not very promising, to say the least!
However, the electric field strength of a few microvolts per metre might
be strong enough relative to natural background noise for ULF
communications at a very slow information rate, such as one-way
communication with submarines.
(I have used the term”Q” in three diffent senses above, but they
are all equivalent. For a coil, it is the reactance divided by the
resistance, for Schumann resonance observations it is the centre
frequency of the observed noise peak divided by its bandwidth, and for
the whole Earth standing wave it is the energy stored per cycle divided
by the energy provided by the source per cycle.)
Tesla’s system applied to wireless communications at LF or VLF
frequencies is much more promising. Most of his biographers confuse this
with radio, and some of his more ardent fans want to credit him with the
invention of radio, even though his system was intended to produce
ground currents rather than radio waves, and even though he never
demonstrated that it would work. His proposed transmitter definitely
could inject large LF or VLF currents into the earth, but how effective
these currents would be for communications purposes has never been
explored to my knowledge. The currents in the earth would generate RF
electromagnetic fields in the air above the surface of the earth.
Conversely, a radio wave in the air induces currents in the earth over
which it passes. In both cases the currents are confined to a layer
near the surface by the “skin effect”. (The thickness of this layer is
called the “skin depth”, and increases as the frequency decreases.)
In my opinion, although the signal produced by a Tesla transmitter
would be mostly ground current and very little radio wave near the
transmitter, as Tesla intended, it would gradually convert into a
conventional combination of radio wave plus associated ground current at
a long distance from the transmitter. If so, the Tesla transmitting
system basically would be an alternate method of generating a radio
wave, although that probably is not what Tesla had in mind. His method
would be different from conventional LF or VLF radio because extremely
high voltages and an elevated terminal plus ground would be used to
generate the signal, rather than moderate voltages and a radio antenna
plus ground. Tesla’s elevated terminal would not radiate much because
its height would be very small compared to a quarter wavelength. This
was as Tesla intended, because he felt that such radiation was wasted
energy.
Since both the Tesla ground wave and the conventional LF or VLF
radio wave lose energy mostly by absorption in the ground, I suspect
that the amplitude of the Tesla ground wave would decrease with distance
at a rate similar to a radio wave of the same frequency. However, the
Tesla transmitter might be able to generate a stronger wave than a
conventional LF or VLF transmitter for a given output current or power
because LF and VLF radio transmitting antennas of practical size
typically are very inefficient. Whether this advantage would outweigh
the practical disadvantage of dealing with the extremely high voltages
of the Tesla system is hard to say.
Incidentally, at communication frequencies, the transformer in the
diagram of Tesla’s transmitter was a special design called a “Tesla
coil”. The length of wire on the secondary coil of this transformer was
about 1/4 wavelength. Consequently there were current and potential
standing waves on the coil. The potential was a maximum at the
connection to the elevated terminal, and the current was a maximum at
the ground connection. The boost to both the current and potential
produced by the standing waves presumably would result in a larger
ground current than would be obtained with a conventional RF
transformer.
MY CONCLUSIONS
In summary, the information about what Tesla was trying to do and
how he intended to do it is so sketchy that I can only make educated
guesses about it. On that basis, I would say that all of Tesla’s ideas
except electrical transmission through the stratosphere make sense
according to conventional electromagnetic theory. However, to determine
whether his proposed system would be useful for wireless communications
or power transmission would require quantitative calculations and
measurements. To my knowledge this has never been done. Because of
regulatory restrictions on radio transmissions, and the difficulty in
distinguishing radio wave signals from ground current signals, I believe
that measurements would be difficult to do today, and a theoretical
analysis would be more reliable. Since the mathematics would be very
difficult, I believe that the best approach would be a numerical
computer analysis, tracing the signal from the transmitter terminal and
ground to the opposite end of the Earth, and done from a few Hertz to
about a megahertz. Since a good mathematical model of the ionosphere and
upper layers of the earth or ocean would be required, the same analysis
could be done for conventional LF and VLF radio transmitters for
comparison. I have suggested to some people that this would make a good
university research project, but so far no one has taken the bait.
My own rough analysis above leads me to believe that the Tesla
system would be hopelessly ineffective for wireless transmission of
power at the ultra-low Schumann resonance frequencies. If my numbers are
correct, I cannot imagine what Tesla was thinking of. Tesla was no fool,
but he may have been wrong about harnessing whole Earth resonance, if in
fact that was what he planned to do. Unfortunately Tesla left us too
little information to properly assess his theories. He also did not
demonstrate transmission of power or communication signals with his
system over useful distances, or market any apparatus utilizing his
principles. Instead he built a laboratory at Colorado Springs in 1899
where he experimented with the parameters of his transmitter, and
presumably satisfied himself that he understood its operation. He then
made the great leap of building a large transmitter at his Wardenclyffe
station on Long Island which was intended for transmitting communication
signals and perhaps also power over long distances, perhaps worldwide.
Unfortunately, he ran out of money before completing the station, so the
world was left with no evidence that his concepts of wireless conduction
“through the natural media” had practical value.
At higher frequencies, the Tesla ground current probably would
suffer from spreading and absorption much like a radio wave.
Consequently whole Earth resonance would not be achievable, and most of
the transmitted power would be dissipated in “the natural media”, as it
is in a radio wave. Therefore I believe that wireless transmission of
power is not practical at any frequency from HF (short wave) on down.
It has been suggested that power could be beamed to Earth from space at
microwave frequencies, but I cannot imagine a useful terrestial
application. In any case, this would be an application of conventional
radio technology, not Tesla’s system of electrical conduction “through
the natural media”.
As for wireless communications at LF or VLF, I believe that the
earth current signal from a Tesla transmitter would evolve into a
conventional radio wave plus associated earth current at large distances
from the transmitter. I believe that radio waves and Tesla earth
currents would diminish with distance from their respective transmitters
at roughly similar rates. However, for a given transmitter output
current or power, the Tesla system might produce a stronger radio wave
at large distances from the transmitter because it would not suffer from
the typical low efficiency of conventional LF and VLF radio transmitting
antennas. Whether this advantage, if it exists, would outweight the
disadvantage of dealing with extremely high voltages is a practical
question requiring more detailed analysis. If Tesla had completed his
Wardenclyffe station, the world might have been able to make a practical
comparison. After about 1920, when vacuum tube receivers came into use,
an ordinary radio receiver with a vertical aerial and ground, or a loop
antenna, likely would have have been adequate to receive a Tesla ground
wave signal, and the elevated receiving terminal could have been
dispensed with.
I hope that this long winded reply throws a little light on the
question.
Henry Bradford.