NavList:

i know for a fact that DC flows in almost ANY conductor, isolating sensitive meters from stray fields is a daunting task.
that task is made worce by AC fields having a tendency to couple in 3 ways, resistive, inductive and capacitive.
you may ask how this relates to compass, any oxide, dirt... can act as rectifier, even a few microvolts in a powerline that is few miles long wil create a measurable deviation.
just think how crystal radio with mineral detector works, they use lead, zinc, iron, tin, copper...oxide or sulfide as rectifier.
one can even make one with a piece of polished iron or stel that is heated until it start to show blue, purple... colours, then one let a pointed copper wire touch surface, this is now a "point contact diode" not very efficient but enough to detect local AM radio if antenna and ground is connected (antenna to point then sensitive phones between ground and iron, modern phones likely need impedace matching)
----------------------------------------------------------------
NavList message boards and member settings: www.fer3.com/NavList
Members may optionally receive posts by email.
To cancel email delivery, send a message to NoMail[at]fer3.com
----------------------------------------------------------------