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Re: [TCML] Does AC become DC in a Quarter Wavelength Tesla Coil?
Hello Anthony,
Anthony Etersque wrote:
Hi Everyone,
Somebody told me that at 1/4 wavelength, a Tesla Coil will produce
pulsed DC instead of AC. In other words, at 1/4 wavelength, the AC
will become rectified.
This interpretation is simply incorrect.
They gave this quote from the book "The
Electromagnetic Interaction" by Robin L. Armstrong (1973):
"Since electromagnetic effects are not transmitted instantly from
point to point in space...there is a time lag between changes in
charge and current distribution on the dipole," which "...allows some
of the energy to continue flowing outward even though conditions at
the dipole may have changed to indicate an inward flow of energy...as
if some of the electric and magnetic field has become detached from
the dipole or 'shaken off' by the oscillation."
In the above quote (page 155), Armstrong was discussing electromagnetic
(EM) radiation and beginning to introduce the concept of "retarded time"
later in the section.
Once launched, an EM wave take a finite time to propagate from the
source to a detector. A specific electromagnetic disturbance, once
"detached" (radiated) from the source, propagates outward at the speed
of light and is no longer affected by anything the radiating source does
afterwards. The radiating source could even disappear, but the wave
created by the initial disturbance will still propagate outward at the
speed of light. This independence of source behavior was what Armstrong
was referring to for a launched EM wave.
I don't know whether or not they're interpreting this quote wrong. I
would like to know what those who are experienced and knowledgeable
on this subject think. Does a coil at 1/4 wavelength really produce
DC instead of AC, or are they misinterpreting this quote? If they are
misnterpreting it, could you please explain to me its true meaning?
A Tesla coil resonating at 1/4 wave is simply oscillating at its natural
frequency. Electric (E) and magnetic (H) field detectors will "see" AC
signals that are related to the output voltages and currents in the coil
and the distance from the coil. If the detectors are physically near the
source (significantly less than one wavelength away, in the "near field"
or "reactive zone"), the E and H fields will be about 90 degrees out of
phase. As you move the detectors away from the coil, both E and H fields
weaken, and also begin to get closer in phase to one another. At a
distance of several wavelengths away, the E and H fields are in phase
with one another (the E field peak occurs at the same time as H field
peak). This is true for any propagating EM waves that are distant from
the radiating source.
Nowhere are DC potentials involved.
However, there may be other effects (such as corona rectification,
streamer space charge deposition, or strongly asymmetric operation in
lossy systems) that CAN cause net DC fields to appear near operating
coils. But this was definitely NOT what Armstrong was describing.
If you are interested in learning more about time dependent Maxwell's
equations and retardation, a good discussion can be found here:
http://farside.ph.utexas.edu/teaching/em/lectures/node41.html
They also gave a quote from Tesla's lecture to support what they were
saying:
"This apparent continuity might be ascribed to the deficiency of the
sense of touch, but there is evidence that in currents of such high
frequencies one of the impulses preponderates over the other."
The above quote was extracted from Chapter 25 of Thomas Commerford
Martin's book, "The Inventions, Researches and Writings of Nikola Tesla,
with Special Reference to his Work in Polyphase Currents and High
Potential Lighting". In this chapter, Tesla was describing the behavior
of high frequency versions of induction coils driven by capacitors and
spark gaps during lectures in 1892 before either the IEE or the Royal
Institution.
Induction coils often exhibit asymmetric voltage output when powered
from a pulsed DC sources. When the interrupter opens (for a simple DC
induction coil) or the spark gap fires (for a capacitor-discharge
powered coil), an oscillatory high voltage output pulse is created on
the secondary. The polarity of the RF voltage peak is directly related
to the polarity of the primary circuit. Many laboratory induction coils
have "polarity switches to permit reversing the primary polarity and
hence the polarity of output peaks from the secondary.
In the above-mentioned experiments, Tesla discharged capacitors through
the primary of small induction coils, generating damped RF pulses. For a
given primary polarity and capacitance, the amplitude and polarity of
the RF voltage peak will be consistently positive or consistently
negative depending on the circuit parameters. The lossier the coil, the
larger the difference between the first peak and subsequent peak. The
polarity asymmetry can cause differences in the appearance of the corona
and streamer discharges between output terminals of the induction coil.
However, this does NOT mean that DC rectification is occurring, only
that the system is lossy, and voltage peaks can consistently be higher
for one polarity than the other for a given primary circuit
configuration.
What do you think Tesla was saying here? Was he confirming that at
high frequencies, AC becomes DC, or is this quote also
misinterpreted?
The compete quote follows:
"The study of the properties of the high frequency currents
obtained from these machines is very interesting, as nearly every
experiment discloses something new. Two coils traversed by
such a current attract or repel each other with a force which,
owing to the imperfection of our sense of touch, seems continuous.
An interesting observation, already noted under another
form, is that a piece of iron, surrounded by a coil through which
the current is passing appears to be continuously magnetized.
This apparent continuity might be ascribed to the deficiency of
the sense of touch, but there is evidence that in currents of such
high frequencies one of the impulses preponderates over the
other."
The context of the above was high frequency effects of coils driven by
high frequency alternators, not disruptive capacitor discharges. A
similar, and more relevant quote (for Tesla Coils) is on the following page:
"In connection with these machines the condenser affords a particularly
interesting study. Striking effects are produced by proper adjustments
of capacity and self-induction. It is easy to raise the electromotive
force of the machine to many times the original value by simply
adjusting the capacity of a condenser connected in the induced circuit.
If the condenser be at some distance from the machine, the difference of
potential on the terminals of the latter may be only a small fraction of
that on the condenser.
But the most interesting experiences are gained when the tension of the
currents from the machine is raised by means of an induction coil. In
consequence of the enormous rate of change obtainable in the primary
current, much higher potential differences are obtained than with coils
operated in the usual ways, and, owing to the high frequency, the
secondary discharge possesses many striking peculiarities. Both the
electrodes behave generally alike, though it appears from some
observations that one current impulse preponderates over the other, as
before mentioned."
Tesla was saying that high voltage pulses of one polarity exceeds
("preponderates") the other.
In any event, it looks like the quote is being misinterpreted.
Thank you for your help.
Sincerely,
Anthony Etersque _______________________________________________
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Best wishes,
Bert
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