Besides the natural static electric and magnetic fields (Natural static fields), there are those artificially generated by technological applications. Static fields don’t change their polarity or only very slowly, thus, the frequency is 0 Hz or almost 0 Hz (< 0,1 Hz) (General). Technically generated static fields are used e.g. in public transport (Public rail passenger transportation systems), in high voltage power transmission lines (High-voltage direct current (HVDC)) or in medicine (MRI; MRI). They are generated either by permanent magnets or electromagnets.
With the aid of electromagnets the magnetic field can be switched on and off (in contrast to permanent magnets). They consist of an iron core and an outer electric coil winded around the iron core. Electromagnets are used e.g. in magnetic levitation trains or door lock switch systems. Superconducting magnets are a special form of electromagnets with extremely high magnetic flux densities used e.g. in MRI (MRI).