opposite polarity, switch direction manually. Use less
current (amps) with each cycle. There are machines that do
this automatically, but you can rig up something to do it
from odd bits and pieces.
There are machines that do
this automatically,

Isn't this the 60 Hz line? Just withdrawing the coil will fade the
cyclic magnetic flux.

BTW I had in mind a several ft diameter for the #14 wire concept, so
that a steel tube fuselage could be degaussed.