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