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From: Brent Regan <Brent@regandesigns.com>
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Subject: Re: 40 degree compass error
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Charles writes:
<<I would have bad feelings about this solution if, as likely, it is due 
to field excitation current effecting the compass.>>

I have an older B&C standby alternator and it uses permanent magnets for 
the field. If the iron path in the magnetic circuit is saturating then 
you will have a stray field that can effect the compass. When the 
alternator (generator?) is turning the field is alternating and the 
average field goes to zero, as does the compass error. To track down the 
source of the (DC) magnetic interference, use a hand compass and 
triangulate to the source.

You can tell if your SB alternator uses permanent magnets by turning it 
by hand and seeing if it has significant cogging torque. No cogging, no 
magnets.

BTW, the same may be true for an electromagnetic field if the field coil 
has magnetized the pole pieces.

Regards
Brent Regan