Electromagnets are fundamentally useful devices, producing controllable amounts of magnetic force from an electric current. The strongest magnets are cool, have many turns of wire in their coils and use large amounts of current.
An electromagnet is usually a coil of insulated wire wound around an iron core. It becomes magnetized when you run an electric current through it and loses magnetism when the current stops.
To make an electromagnet stronger, you can wind a coil with more turns of wire. The number of turns multiplied by current in amperes gives ampere-turns, a factor that determines magnet strength.
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Generally, a magnet’s strength increases with greater electric current. At a certain point, the magnet will saturate, reaching a maximum strength.
An electromagnet will be stronger if you wind the wire turns around a metal core. The best metals are typically ferrous, or iron-bearing.
The current-carrying ability of wire improves with very low temperatures. The strongest magnets are made of superconductors cooled with liquefied nitrogen or helium.