The diagram shows how large and small magnets are used in anti-theft security devices in stores and libraries.

• Activated tag: An activated tag is slightly demagnetized. When it passes through the pedestal’s EM field, the tag’s magnetic domains line up with the field. This change in magnetic domain emits a signal that is picked up by the receiver, which sets off the alarm.
• Deactivated tag: A deactivated tag is fully magnetized. When it passes through the exit, the tag’s domains do not change. Because no signal is emitted, the alarm is not set off.
• The pedestals: The transmitter pedestal contains a wire loop that produces a changing EM field in the region between the pedestals. The receiver pedestal picks up any signal produced by the tag.

The image of the activated tag and that of a woman carrying a handbag shows the following labels:

• Activated tag (demagnetized)
• Magnetic domain
• Changing electromagnetic field
• Transmitter pedestal
• Receiver pedestal
• Flashing alarm light