Stop thief! Have you ever set off the security alarm in the doorway of a store - or in somewhere like a public library? It's very easy to do: virtually everything in shops and libraries has a security "tag" stuck to it inside a small paper or plastic label. How do these things work? Hold one up to the light and you can see. The tag is a tiny circuit consisting of an antenna (aerial), wrapped aroun...d in a loop like a race track, and a chip. There's no battery. In the simplest tag, the antenna picks up radio waves from one of the gates in the shop or library doorway. There's enough power in the waves to activate the chip, which sends a different radio wave back again to the gate on the other side of the doorway. BBBBZZZZZZZZZZZ! Whoops. You just set off the alarm!
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