For all practical purposes, magnetic saturation may be defined as: That point of magnetization where a very large increase in the magnetizing force does not produce any perceptible increase in the magnetization.
From tests it has been shown that permeability increases with the flux density up to a certain point and then decreases, indicating that the iron is approaching a state of saturation.
Magnetomotive Force.—This is a force similar to electromotive force, that is, magnetic pressure. When a coil passes around a core several times, its magnetizing power, or magnetomotive force, (m.m.f.) is proportional both to the strength of the current and to the number of turns in the coil. The product of the current passing through the coil multiplied by the number of turns composing the coil is called the ampere turns.
It is known by experiment that one ampere turn produces 1.2566 units of magnetic pressure, hence:
magnetic pressure = 1.2566 × turns × amperes
that is,
magnetomotive force (m.m.f.) = 1.2566 × n × I.
The unit of magnetic pressure is the gilbert (named after William Gilbert, the English physicist) and is equal to
1 ÷ 1.2566 ampere turn = .7958 ampere turn.
Reluctance.—The magnetic pressure (magnetomotive force) acting in a magnetic circuit encounters a certain opposition to the production of a magnetic field, just as electromotive force in an electric circuit encounters opposition to the production of a current. In the magnetic circuit this opposition is called the reluctance; it is simply magnetic resistance and may be defined as: the resistance offered to the magnetic flux by the substance magnetized, being the ratio of the magnetomotive force to the magnetic flux.