Re: How do security strips in library books work?

The strip contains a "plastic magnet" similar to those used to stick 
things to a refrigerator.  When it is magnatized, that will be detected by 
the sensors and the alarm will sound.  It is demagnitized by hitting it 
with an AC magnetic field that scrambles the magnetic atoms and 
demagnitizes the plastic material.  

There is a very slight possible that when the AC field collapses, it 
leaves the plastic in a remagnetized state, so occasionally, a strip that 
has been run through the demagnitizer will still set off the alarm.  Take 
it back and zap it again.