Displacement Current Revisited

To: tesla-at-pupman-dot-com
Subject: Displacement Current Revisited
From: "John H. Couture" <COUTUREJH-at-worldnet.att-dot-net> (by way of Terry Fritz <twf-at-verinet-dot-com>)
Date: Sat, 13 Mar 1999 05:58:09 -0700
Approved: twf-at-verinet-dot-com

  To All -

  Now that I have told the List that displacement currents don't exist I
find after more research that these currents may exist in a different form
compared to the regular currents we are accustomed to using when working
with Tesla coils. If this is correct I owe apologies to the coilers who
believed that displacement currents existed even though they were not clear
in their understanding of these currents that are different.

  Displacement currents around a Tesla coil may be real. However, coilers
will probably never be able to measure them or use them in their
calculations.  Currents are moving charges and there are moving charges
surrounding all energized Tesla coils so coilers would be interested in
knowing more about them.

  Maxwell's equations are of a very general nature and can be used with
several concepts of the electromagnetic field. One of these concepts appears
to be in  the use of displacement currents. It is my understanding that
certain equations derived from Maxwell's theories are based on moving charge
DENSITIES. These densities are in the form of charges per unit volume.
Displacement currents, therefore, would be current densities in a
dielectric. In other words the moving charges around operating Tesla coils
can be considered as displacement currents in the air (dielectric) around
the coil.

  The standard ammeter measures current intensities (quantities) not current
densities (quantities per unit volume) so the displacement currents around
Tesla coils cannot be be measured with the standard ammeter. This means that
the equations using displacement currents could not be solved unless certain
assumptions are made about the displacement currents.

  Unlike currents in a conductor the density of displacement currents in a
dielectric reduces as the distance increases. These currents (moving
charges) must be measured by a different method. One method would be to
place a special probe in the path of the moving currents which would induce
an electrical effect in the probe to operate a meter, lamp, etc. The meter
indication would reduce as the probe is moved away from the coil unlike the
ammeter in a conductor circuit.

 The moving charges or displacement currents around the Tesla coil are
called the electromagnetic field radiating from the Tesla coil. Note that
the electromagnetic field is normally measured by radio engineers (or
coilers) with a field strength meter measuring the "E" electric field not
the current created by the "B" magnetic field!!?

  The bottom line is that there appears to be a different type of current
called displacement currents surrounding the Tesla coil. Coilers will not be
able to measure these displacement currents with ammeters but they can
possibly be measured with meters sensitive to other electrical effects.
Coilers will also not be able to calculate these currents unless they are
capable of solving the difficult theoretical equations.

 If we are to understand how a Tesla coil operates we wiil need to know more
about the electromagnetic field surrounding the Tesla coil. I would be
interested in hearing what other coilers think about the above. It should
also be noted that more testing of these fields is required by the coilers
with the coils and necessary measuring equipment.

  There is much more to this story. Some of it came from "The Theory of the
Electromagnetic Field" by David Cook and published by Prentice-Hall Inc. The
rest is pure speculation by an electrical power engineer not a radio engineer.

  John Couture

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