= Electromagnet
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<Electromagnets> allow us to create controllable <magnetic fields>, i.e.: they act as <magnets> that we can turn on and off as we please but controlling an input <voltage>.
Compare them to <permanent magnet>: on a magnet, you always have a fixed generated magnetic field. But with an <electromagnet> you can control the field, and even turn it off entirely.
This type of "useful looking thing that can be controlled by a voltage" tends to be of huge importance in <electrical engineering>, the <transistor> being another example.