CoachAn electromagnet is a coil of insulated wire — often wound on an iron core — that becomes a magnet while current flows through it: north pole, south pole, iron-grabbing power and all. Unlike any stone magnet, it obeys a switch: current on, magnet on; current off, magnet gone; current reversed, poles exchanged.
示例
Example 54.5 (Switchable magnetism runs the world)
The scrapyard crane grips a car with an electromagnet and drops it by opening a switch — try that with a stone magnet. Electric door-locks, the clack of a relay, the doorbell’s hammer (an electromagnet snatching it against the bell, cutting its own current, springing back: rattle made law), the reading head of old telephones, and — supremely — every electric motor: current-made magnetism pushing against magnets, thousands of times a second. The magnetic effect is the muscle of the electrical age.