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When you walk through an airport-security portal, you’re walking through a rapidly changing magnetic field. The laws of physics dictate that if you put a conducting material in a changing magnetic field, electric currents will arise, and those electric currents will create a secondary magnetic field. This secondary field is often referred to as the induced magnetic field, because it is induced by the primary field of the portal. Within the portal are detectors that can sense when an induced field is present. When they do, the alarm goes off, and you’re whisked over to the special search line.
World War I was the first mechanized war, the Second World War could be considered the first struggle based around communication technologies. Unlike previous conflicts, there was heavy utilization of radio communication among military forces.
The field of cryptography advanced quickly, in order to keep messages secret and hidden from adversaries. Also, for the first time in combat, radar was used to detect and track aircraft, thereby surpassing conventional visual capabilities that ended on the horizon.
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