"William E. Sabin" wrote in message
news:Roybe.17552$WI3.12208@attbi_s71...
In a solenoidal, helically wound coil, most of the magnetic flux is
aligned with the center of the coil and flows along the axis of the coil.
Relatively little flux is outside the coil because of cancellation
effects. For a single turn, current in different parts of that turn tend
to partially cancel flux in regions that are outside that turn.

To see this, draw a circle of wire and show a current at one point flowing
in one direction (out of the page). At the corresponding point
diametrically opposite, that same current flows in the opposite direction
(into the page). Inside the turn the two magnetic fluxes that encircle
the wire add in-phase, but outside the turn these same fluxes cancel in
opposite phase. This effect is more pronounced in a coil of small
diameter.

Coils of that kind are pretty well self-shielding for regions beyond the
*side* of the coil.

The flux at the center of the coil is highly concentrated. This flux then
leaves one end of the coil and returns to the other end via an external
path, just as it is supposed to do, but at a much smaller value of flux
*density* in the external path. This external flux can induce "eddy"
currents in a shield that can decrease coil inductance and Q, usually only a
small amount if the shield is not too "close".

This is in addition to the things mentioned before.

Bill W0IYH