Metal detectors.

[spiffo]

How do those big metal detectors you walk through work? Also, do they detect every single type of metal, or only certain ones, like a magnet? Also, why is it that sometimes I go through a metal detector with a few coins in my pocket and it goes off, yet other times I go through the same metal detector forgetting to empty my pockets, and it doesn't notice coins, keys, etc?

[rew198]

The detector make you walk through a weak magnetic field, generated by electricity. It's easier for magnetic fields to pass through certain types of metal than through you, so when metal goes through the field, there is a small change in the electricity required to maintain the magnetic field - the change is detected by the machine.