Magneto motive force (mmf) is the force which establishes the magnetic flux in a magnetic circuit. When a current is passed through a coil, a magnetic flux is set up around it. The product of current (I) and number of turns (N) of the coil gives MMF.

MMF F is given by the expression,

F = NI

Its unit is Ampere-turns (AT)

Magnetic field intensity

Magnetic field intensity is the magnetic motive force per unit length of the magnetic circuit. It is denoted by H and its unit is Ampere-turns/ meter. Magnetic field intensity is also called magnetic field strength or magnetizing force.

Permeability

The ability of a material to conduct magnetic flux through it is called permeability of that material. It is represented by the letter µ and the unit Henry/meter. The flux and hence the flux density B are directly proportional to the magnetizing force H.

i.e.,

Thus coefficient of coupling may be defined as the ratio of actual mutual inductance present between the two coils to the maximum possible value of mutual inductance.