Referring to a Cathode Ray Tube (CRT) monitor "damaged" by a magnet:
Most monitors have an automatic degaussing coil around the screen.
When you switch it on, a heavy alternating current flows through a cold
thermistor in series with the coil.  The thermistor heats up, increases
in resistance, reducing the current to essentially zero over a few seconds.
This alternating magnetic field should swamp any residual magnetic field
and gradually fade away to nothing.  (This is why you should keep your
floppies away from the screen.)

Sometimes the thermistor fails (open).  If you are DEAD CERTAIN you
know how to avoid the potentially LETHAL 30,000+ Volts stored inside
your monitor - EVEN AFTER THE PLUG IS PULLED - and the implosion hazard
posed by a large vacuum tube which can also MAIM, BLIND, and KILL - you
can replace the thermistor.  Otherwise, get someone who knows how (like
a TV repairman) to replace it.

Some monitors have a manual degaussing switch.  In that case, press it
for a second or so after the monitor is switched on.  Either way, the
distortion caused by the residual magnetism should fade away after one or
two degaussings.

Boyd Ramsay

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