Saturday, January 14, 2012

What are some objects that accidentally set off the library book alarm?

I frequently see people who aren't carrying anything set off the security gates at the library where I work. What is causing the problem? I've heard that magnets or watches can sometimes to do this. But why don't all magnets or all watches have this effect?What are some objects that accidentally set off the library book alarm?
It is not easy to answer because it depends on the type of alarm system used.



The most popular system is sending a low power radio frequency and search for "something" that will be a tuned circuit for that freqency.



a Tuned circuit is typically a (small) piece of metal where one part behaves like a coil (it can be a long piece of wire more or less folded)

and a capacitor (i.e. two pieces of metal with an insulator between them)



Theoretically, it is very unprobable to find an object that will exactly have the right shape to trigger the alarm. I suspect this alarm sytem need some tuning to reduce its sensitivity.

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