"Richard Harrison" wrote in message
...

As electrons move along a conductor a magnetic field expands from some
depth inside the conductor itself. The magnetic lines of force sweep
outward from the conductor while inducing an emf in the conductor
itself. The self induced emf opposes instantaneous change of current in
the inductance of the conductor. This is the basis of Lenz`s law:
"In all cases of electromagnetic induction, induced electromotive force
and resultant current are in such a direction as to oppose the effect
producing them."

Skin effect prevents penetration of RF very deep into a good conductor.
Skin effect makes RF coil shields impenetrable. Electric hields are
shorted to ground by the conductive shield. Magnetic fields induce
counter fields from the currents they induce on the surface of the
shield.

A Faraday screen breaks the current path on the shield preventing the
counter fields from being magneticly induced. Result is a shield that is
penetrable by the magnetic field but impenetrable by the electric field.
The electric field is still shorted to ground by its conductive path.
Faraday screens are used because they work.

Best regards, Richard Harrison, KB5WZI


I would agree, Richard, but at HF frequencies the current path
around the shield isn't really broken by the gap. Due to the skin
effect, the RF current flowing on the inside of the loop shield is free
to flow around the edge of the shield conductor and onto the outside
of the shield at the gap. At very low frequencies, where the skin
depth is large, this wouldn't necessarily be true, but at HF as long
as there are a few skin depths between the outside and the inside
surface of the conductor, then the inside surface of the shield and
the outside surface of the shield can be treated as independent
conductors.

73, Mike W4EF........................