MagTek InSpec 9000 - Manual (Page 59)

Appendix A. Magnetic Encoding
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The magnetic field is directional like a DC current is directional. The convention for describing
the direction of DC current flow is for electrons to flow from the positive (+) terminal to the
negative (-) terminal. Similarly it is also conventional to describe magnetic field lines as going
from the "South Pole" to the "North Pole".
Now, back to our iron rod and wire coil. Disconnecting the current source will not stop the
magnetic field, since the iron itself has become magnetized. The magnetic field lines will be the
same as shown in Figure A-1.
This ability of iron to retain the magnetic field created by the wire coil and electric current is the
fundamental element in any magnetic media's ability to store information. The key concept here
is the ability of iron to keep or retain the magnetic field after the electric current has been
removed.
We can reverse the magnetic field direction by reconnecting the DC power source so that the
current in the wire coil flows in the opposite direction, as shown in Figure A-2.
Figure A-2. Reverse Polarity
Again, disconnecting the DC current source will not stop the magnetic field since the iron itself
has become magnetized, now in the opposite direction.