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Carl,
If the mu metal is close to the compass, then the magnet in the compass
could be "attaching" to the mu muetal, i.e., the attraction of the
compass magnet for the mu metal might be stronger than the field from
the current in the wires you're trying to shield.  That would become
obvious when you swing the plane, the compass won't want to move.

A thought would be to try and find a possible source by using a compass
to move around the panel and look for the strongest reaction while the
power is on.  Is it the power wiring -- supply and return, or is it a
piece of equipment, perhaps the power supply in a radio for instance?
If it is the power wiring, one solution would be to try and minimize the
loop area of the supply and return cabling. The larger this loop, the
more likely the field will encompass the compass and cause a reaction.
Another item to look at would be to think of the loop formed by the
supply and return wiring and think of the imaginary plane it lies on in
space.  Twist that plane by adjusting the wire routing for minimum
compass reaction.  I.e., this is hard to explain, but I'm not saying
totally reroute the power cabling, rather by 'twisting' the pair
differently you can adjust the field at the compass to be vertical,
thereby the compass will not react.  Doing it this way, the amount of
current won't matter, as you 'nulled' the field in relation to the
compass.

Charles R. Patton
N360JM

Carl Cadwell wrote:
> In switching on the power, the wet compass swung 40 degrees.  ...
> The only solution that I came up with is a 6" x 6" mu metal sheet that
> was placed under the glare shield and reduced the swing to 2-3 degrees
> when the power is turned on.  ...
> Does anyone know if this is an acceptable solution?
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