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Re: Displacement current

to: Rich Hull

Displacement currents do exist and they do have a magnetic field.  A common
example of displacement current is a EM wave, ie, radio.  It has both a
magnetic and electric component.  At UW (Wis) and Sandia they measure
displacement currents and magnetic fields in plasmas almost on a daily
basis.

A SQUID is not the proper type of probe for this type of experiment.  The
problem is that the magnetic field is very small even though the current is
very high --- the current is spread out through the entire plate area and
usually only amounts to 1/2 to 1 ampere per cm squared even in caps firing
thousands of amperes in discharge mode.  The current density is very small
and the SQUID would be a very poor choice as a measuring instrument for this
magnetic field.  In this experiment you described the SQUID would only
measure 1/2% (or less) of the total magnetic field in the area and may not
produce any results at all --- as you described.

The second problem is a vacuum has a very low dielectric constant.  To
properly conduct this type of experiment a very high dielectric constant
material such as barium titanate would be used and microwaves would also be
employed.  It can and is daily measured in a vacuum but with microwaves not
low frequency waves --- the magnetic field value is so low at low
frequencies it is nearly impossible to measure -- at least extremely
difficult.

Maxwell's frequencies were relatively low even after Hertz proved their
existence.  Only during the "big bomb" research during WWII did accurate
methods of measuring displacement currents develop using magnetrons and
better dielectrics.

This data from my old group at UW-Wisconsin.

Also, if you happen to have the reference on Dr. Bartlett's work one of them
would like to look it up for reference.  Please advise.

Regards,

Dr.Resonance-at-next-wave-dot-net



-----Original Message-----
From: Tesla List <tesla-at-pupman-dot-com>
To: tesla-at-pupman-dot-com <tesla-at-pupman-dot-com>
Date: Monday, March 01, 1999 2:26 PM
Subject: Re: Displacement current


>Original Poster: "Dr. Resonance" <Dr.Resonance-at-next-wave-dot-net>
>
>to: Richard
>
>An interesting point.  I'm going to run this thru a few really great
>research physicists and get their reaction to this magnetic field paradox.
>I will report back in a week or so.  One works at Sandia on HV cap projects
>and the other works for Maxwell.
>
>Regards,
>
>Dr.Resonance-at-next-wave-dot-net
>
>
>-----Original Message-----
>From: Tesla List <tesla-at-pupman-dot-com>
>To: tesla-at-pupman-dot-com <tesla-at-pupman-dot-com>
>Date: Sunday, February 28, 1999 2:13 AM
>Subject: Displacement current
>
>
>>Original Poster: Richard Hull <rhull-at-richmond.infi-dot-net>
>>
>>Dr Resonance wrote:
>>
>>"It's not magic -- it's physics.  A Tesla coil produces "displacement
>>currents" in air because the air represents a dielectric while the
>>terminal
>>represents a plate of a capacitor.  The second plate is represented by
>>isotropic capacitance and plate to ground capacitance of nearby room
>>walls,
>>etc.  A displacement current has unique and different properties than
>>normal
>>air to ground currents which produce the wandering spark phenomonea of
>>Tesla
>>oscillators.
>>
>>You might read some early Maxwell for good descriptions of displacement
>>currents."
>>
>>*********************************
>>
>>All,
>>
>>It is interesting to note that in Maxwell's day he invented the
>>displacement current on a whim as a logical and common sense item, even
>>though there is absolutely no conductive item in a capacitor's
>>dielectric to support the classic current seen in wires and metallic
>>conductors.  It also made his equations a full featured set.  This
>>current is found in all capacitor dielectrics, according to theory.
>>However, in a vacuum dielectric capacitor, it appears to generate no
>>magnetic field between the plates!!!!
>>
>>This was most troubling to Maxwell and he wrote to several scientifc
>>friends that he hoped some proof of exisitance of the vacuum
>>displacement current's demanded magnetic field might be found in his
>>lifetime.  It was not to be done however.  As recently as 1994 Dr. D.F.
>>Bartlett of the Univ of Colorado made the attempt using an ultra
>>sensitive SQUID between two plates of a flat plate capacitor in vacuo.
>>.
>> He found 'zippo' mag field during charge or discharge.  (this is a
>>though experiment as the wires to the cap and metallic plate elements
>>will create a field as we all know.)  Being a lock step physicist he
>>struggled for an explanation in his paper.  Ortherwise all sorts of
>>skeletons come out of the closet...(action at a distance, etc)   He went
>>into an arcane mathematical diatribe using the now, for the most part,
>>disgarded Amperian current force laws to say that the circuit currents
>>in their flow and ebb create canceling mag fields via the plates.  (most
>>folks cleave only unto the lorentz law which is in direct conflict with
>>ampere's old law.)  The Lorentz laws somehow don't generate canceling
>>fields....Gee, sure wish they did.
>>
>>Maxwell could see logical delema this in his own life time and was
>>adverse to using the Amperian current laws as they produce forces not in
>>line with his theory.  It vexed him mightily and to his dying day he was
>>most concerned about the demanded, but seemingly missing magnetic field
>>in the interplate vacuum dielectric.
>>
>>So by all means, read up on the displacement current.  It is dished out
>>in huge helpings to all us engineers.  Fortunately we don't need to fret
>>about mag fields inside our dielectrics so we never "auto-generate" the
>>question.  Physicsists, however, are not so lucky.
>>
>>While reading up on the THEORY it might be wise to heed the words of
>>Sir Frances Bacon from his essay on studies.................
>>."Read not to contradict or confute, nor to believe and take for
>>granted,  nor to find talk and discourse, but to weigh and consider".
>>
>>Richard Hull, TCBOR
>>
>>
>
>