In the general sense, a
magnetic circuit is any path taken by magnetic flux. More specifically, it is associated with the magnetic flux within (usually) silicon steel 'cores' such as those found in transformer, generators, motors, relays, etc. They can be 'homogenous', where the flux path is completely contained with the same material (e.g. a transformer core), or 'compound', where the flux path incorporates, say, an air gap (e.g. motor/generator fields).
A magnetic circuit can be compared with an electric circuit, where
-magnetomotive force (mmf) is equivalent to
electromotive force-flux is equivalent to e
lectric current-reluctance is equivalent to
resistance The source of a magnetic circuit's
magnetomotive force is a current-carrying coil. The magnitude of this mmf is the product of the current flowing through the coil, and the number of turns (I x N). Since the number of turns is dimensionless, the SI unit of measurement of mmf is the
ampere (A), although it is frequently 'spoken' of as 'ampere turns', to avoid confusion with the unit for electric current.
Magnetic
flux is measured in
webers (Wb), pronounced 'vay-bers'.
Reluctance is measured in
amperes per weber (A/Wb) although, again, it is frequently spoken as 'ampere-turns per weber'.
Another similarity with electric circuits, is that the equivalent of 'Ohm's Law' also applies to magnetic circuits: i.e.
flux = mmf / reluctance.
Finally, magnetic circuits can also be compared with series, parallel, or series-parallel circuits, but this is beyond the scope of this answer!