Sure you can. You must spin the magnet with a motor to generate an alternating
magnetic field, then bring this field towards and away from the tube very
slowly to "erase" the permanent magnetization that caused the green spot.

An Alnico horseshoe magnet with mounting hole at the bride, a bolt, and an
electric drill might do, but this reply is intended to generate some
controversy so I will include the fact that rotating a permanent magnet on any
conceivable axis with any conceivable equipment generates an alternating field
of much lower quality than that produced by house current through the usual
degaussing coil.

Don't mean to stir up the water, but you can tinker with this to learn
something, then degauss properly.

You see, the alternating field must completely and symmetrically reverse to
average out to null. A rotating permanent magnet can have a bias on some axis.


Yours,

Doug Goncz ( ftp://users.aol.com/DGoncz/ )

My physics project at NVCC:
http://groups.google.com/groups?q=dgoncz&scoring=d plus
"bicycle", "fluorescent", "inverter", "flywheel", "ultracapacitor", etc.
in the search box