An electro-magnet is a piece of iron temporarily converted into a magnet by means of a current of electricity sent through a wire which is coiled around it. The wire is usually covered with silk, cotton, gutta percha or some other insulator, to prevent the current from leaping across, and compel it to travel through the whole length of the wire.

The more pure and soft the iron is, the stronger will its magnetism be while it lasts, and the more completely will it disappear when the current stops. Steel is less affected than soft iron for the time, but remains permanently magnetized after the current ceases. Electro-magnets are usually much more powerful than other magnets of the same size.

The iron which is magnetized by the current passing around it is called the core. It is frequently straight, the wire being wound upon it like thread upon a reel; but very frequently it has the shape of a U or horseshoe, the wire being coiled round the two ends and the bend of the U left uncovered.