Oxford Learner’s Dictionaries defines electromagnet as “a piece of metal that becomes magnetic when electricity is passed through it.” Electromagnets are the embodiment of the physics fact that charged particles in motion (electric currents) produce and react to magnetic forces. Electromagnets usually consist of wire wound into a coil. Electromagnets are integral parts of common electrical devices, such as transformers, generators, motors, relays, etc., and are widely used in advanced technologies including particle accelerators, magnetic resonance imaging, proton therapy, mass spectrometers, magnetic levitation, e-mobility, and more. 
There are many ways to categorize electromagnets. One way is by the type of power source: there are DC, AC, and pulsed electromagnets. Another way is by the type of coil wires; there are traditional resistive and superconducting electromagnets. This article excludes superconducting electromagnets but focuses on traditional resistive electromagnets. Aspects of electromagnet design considerations will be presented in the below sections for a design engineers’ reference. The article begins by a discussion on electromagnet core material’s permeability and nonlinearity, as well as the residual field and its mitigation. The focus of the article then shifts to design techniques in creating highly uniform field or high strength field. Efficiency of magnetic designs is also discussed. The last aspect of the article covers safety considerations in electromagnet designs.
Author: Bo Zhang, Lead Magnetics Engineer Electromagnetic, Electromagnetic – Wikipedia