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Re: Displacement Current Revisited
2/28/99
Yesterday I lashed up a horizontal TC. I bolted my 4" coil to a piece of
1/2 plywood with three 1/4" nylon bolts and fastened an old 6" helical primary
to the same bolts. There was only 1" between the primary and secondary.
Without major retuning, I placed it in the horizontal position. The TC was
placed horizontally with the base East and ball termination West. I attached
a cheap plastic compass to a 5' wooden stick with masking tape.
With the coil firing at low power I approached the side of the coil with the
compass. The compass reacted to the TC magnetic field(s). The fields were
nonuniform, probably due to poor tuning. At this point I noticed occasional
racing sparks at the lower 1/3 of the coil arcing to the top of the primary.
I could actually touch the compass to the side of the coil and there was no
arcing to the compass, although if I approached the coil termination there
were small arcs to the compass which did not seem to affect the needle
orientation.
At about the upper 1/3 of the TC, the compass was attracted to a longitudnal
orientation with the long axis of the coil. The attraction was not uniform
and was intermittantly strongest with the larger and longer sparks. The
North seeking needle was attracted to the base of the secondary. If there
is similarity to the drinking straw air core solenoid, this would be the
negative end of the coil.
Strangly enough, I also noticed that the end of the compass needle tended to
deflect and dip or rise sometimes when the side of the coil was approached.
Could there be more than one magnetic field orientation with a TC? I turned
the TC vertically and approached the mid section of the TC from the East.
The North seeking end of the needle swung East away from the coil.
Approaching from the West the North seeking end of the needle again swung
East toward he coil.
At this point the arcing from primary to secondary was almost constant and the
introduction of HV 60 Hz current into the secondary made it too dangerous to
continue with this experimental platform. I stopped. Inspection reveals two
loops of burned out Cu wire on the secondary. I attribute this to poor tuning
and the closeness of the primary to the secondary.
Conclusions: There are definitely magnetic fields associated with a disruptive
TC as measured with a compass. The compass is a crude, but easy, qualitative
way to assess magnetic fields of a TC. There is an overall magnetic field, as
measured by the compass, that that is oriented longitudnally with the long axis
of the TC secondary. There is a net sum bias of the alternating magnetic
fields
due to a DC current in the coil. If the DC current air coil solenoid and
compass experiment is extended to the TC, the base of the coil has a negative
DC bias with respect to the termination. This magnetic field is not continous,
but varies irregurally with the intensity of the discharges from the coil.
(I leave it to others to determine if this DC current and magnetic field which
appears most strongly during discharges is from some type of gaseous
rectification or is intrinsic to the TC) There is also "another" magnetic
field
that appears perpendicular to the first magnetic field and the long axis of
the coil. This magnetic field would appear consistant with the DC current
single wire compass experiment. As improbable as it may seem, the long axis
of the TC secondary can be viewed as a long wire. I leave it to the
transmission ine experts to explain this one.
The compass is quite crude, but does respond to TC magnetic fields. Further
evaluation should be carried out with more sensitive and precise devices.
Perhaps better detectors such as Terry's planar antenna and fiber optics or
Hall devices will better map these fields and give a better explanation of
TC function.
RWW