A magnetic circuit is made up of one or more permanent magnets or/and electric current carrying conductor, and generally some high permeability materials (soft magnetic material), and generally one or more air gaps.

A closed circuit condition exists wherein the external flux path of a permanent magnet is confined within high permeability material. In order to do useful work, a so-called closed circuit always involves at least one air gap where the useful performance can be carried out. In other words, a closed circuit is defined as the magnetic configuration wherein air gap with limited length exists.

An open circuit condition exists wherein a magnetized permanent magnet is working by itself with no high permeability material. This means the air gap for the magnetic circuit is limitlessly large.

High permeability material is usually pure iron or low carbon steel, and is known as soft iron. It is normally placed on or between the pole faces of a permanent magnet to decrease the reluctance of the circuit and thereby reduce the flux leakage from the magnet. It also makes the magnet less susceptible to demagnetizing influences. Moreover, it acts as frame support for the total circuit configuration; so, it is also known as magnetic keeper.