An electromagnet is a device which creates a magnetic field on demand when excited with electrical current. Opposed to conventional permanent magnets, electromagnets are generally weaker and less reliable. However, the ability to control the timing and intensity of the magnetic field had led to the incorporation of electromagnets in almost all aspects of daily life. Electric motors, relays, speakers, disk drives and even automotive fuel injections systems all depend upon the accuracy and consistency provided by modern electromagnets.
Most electromagnets are based upon a common, simple design. As the flow of electrical current through a conductor produces a magnetic field perpendicular to the direction of flow, wrapping a thin conductor of electricity (such as copper wire) in a tight spiral causes the magnetic fields of the individual lengths of the wire to accumulate and reinforce each other. Tighter spirals, and more current passing through the wire, create stronger the magnetic fields. Additionally, if the field generated is not required to reach a value of zero then a small conventional magnet can be placed inside the spiral. This increases an electromagnet's effectiveness by making the magnetic field stronger while requiring less electric current.
A common device that demonstrates several electromagnetic properties is the Electronic Article Surveillance system used to prevent shoplifting in many retail stores. While RF and acousto-magnetic EAS systems are also used, the cost-effective electromagnetic EAS remains popular in libraries because of its low cost and reactivation ability. In these systems, a thin glassy magnet is attached to the product. A transmitter that charges magnets is placed at one side of a doorway, and a receiver that detects magnetic objects is placed on the other. The low magnetic saturation value of the glassy magnet means that the magnet can become 'saturated' with electrical energy very quickly, thus no longer functioning as a magnet. As the glassy magnet approaches the receiver it is detected. However, due to the physical design of the system, at this time it is also being charged by the electromagnetic radiation of the transmitter. When it becomes fully charged, it no longer functions as a magnet and the receiver detects the abrupt change. In this case, an alarm sounds. To deactivate the magnet, so that it can pass between the transmitter and receiver without sounding an alarm, the librarian charges the glassy magnet with a strong electromagnetic field. To reactivate the magnet, it is simply demagnetized.
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