Lynn, I'm afraid you'll have to eat this one. You use steel,
iron or "Mu" metal to shield from magnetic fields.
I did two experiments this morning in my minus 3 degrees shop.
First I placed a ring magnet from the magnetron of a microwave oven
tight against a 2' by 2' by 1/16" 2024-T3 sheet. I placed 40 10-32
screws against the plate and adjacent to the magnet and rotated the
plate into the vertical plane. All but three screws stayed on the
plate.
I next repeated the experiment with a 2' by 2' by 1/16" sheet of
4130 steel. All but 4 screws fell off. In addition, the whole plate
became magnetic to the extent that screws were attracted to it but not
enough to support their weight.
I placed a piece of hard board about 1/8" thick to see if a
distance would reduce the power of the magnet. I can pick up my tool
box with that spacer in between.
In my reading last night, I see that the iron or steel shielding
should have an air gap between the source and the shielding. If the
single layer does not contain the flux field, a second or third layer
with air gaps between can be added. The iron shielding can become
saturated by a strong flux field and need the additional layer of
material.
The specialty materials used for this must be expensive, are
thin foils, and are applied to the device that needs protection rather
than the source of the flux field in most cases. In newer buildings
with low ceilings and much overhead wiring above the equipment being
used, can cause problems from radiating flux fields. So some
instrumentation must be protected.
My experiments gathered only vestigial data, Because, 4130 has a
hint of chrome in it.
2024-T3 has a hint of iron in it. The magnet I used has a nearly
unbelievably powerful flux field.
On examining my friends 1960 Bonanza, I note that the battery
cable is not shielded. The glare shield is an upholstered aluminum
sheet. The total wiring package is white with microscopic numbers
printed on them. Both waxed string and real Tye Wraps with the metal
locking tabs are used to bundle every kind of wire bundle. Many changes
have been done over the years to upgrade navaids and modern radios and
everything is in duplicate.
The compass is original, and mounted high on the top of the
windshield frame, about 16" above the glare shield. It seems to
function perfectly in the absence of any kind of shielding anywhere in
the panel. It is mounded with brass screws, and there is no iron based
material in the area of the compass.
I deduce from this that unless your battery cable encircles the
compass, it is likely to work just fine.
My assertions that aluminum shields a (standing) flux field are
baseless.
As John Cleese would say: I apologize unreservedly.
Lynn E. Hanover