Electromagnets produce a magnetic field when a current is passed through them and only while a current is applied. As soon as the current is removed, the magnetic field completely disappears. There is an exception, in the case of permanent electromagnets which permanently produce a magnetic field which is then switched off, or neutralised, by an electric current.
An ‘energise-to-hold’ electromagnet, such as those listed above generate a magnetic field as current is passed through them. This occurs as the magnetic domains contained within the electromagnet’s ferritic core align with the magnetic field produced by the electrified coiled wire surrounding the core. As the current is increased, more magnetic domains are aligned and the strength of the magnetic field increases, until such point that all domains are aligned and the magnetic field reaches its maximum strength for the size of the device.
Electromagnets have an advantage over permanent magnets as they provide the ability to manipulate a magnetic field at the flick of switch. In addition to turning on and off the magnetic field, the strength of the field can be controlled by the size of the current passed through the coil.