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_. The unit of reluctance or magnetic resistance is the _oersted_ (named after Hans Christian Oersted, the Danish physicist) and is defined as: _the reluctance offered by a cubic centimetre of vacuum_. [Illustration: FIG. 123.--Mutual action of solenoids. When two solenoids traversed by a current are allowed to act on each other, one of them being held in the hand and the other being movable about a vertical axis, as shown in the figure, attraction and repulsion will take place just as in the case of two magnets (see figs. 110 and 111).] =Analogy Between Electric and Magnetic Circuits.=--The total number of magnetic lines of force, or magnetic flux, produced in any magnetic circuit will depend on the magnetic pressure (m.m.f.) acting on the circuit and the total reluctance of the circuit, just as the current in the electrical circuit depends upon the electrical pressure and the resistance of the circuit. To make this plain, Ohm’s law states that electric current = electromotive force / resistance or I = E/R expressed in units amperes = volts / ohms The resistance, as already explained, depends on the materials of which the circuit is composed, and their geometrical shape and size. Similarly, in the magnetic circuit, the total number of magnetic lines produced by a given magnetizing solenoid depends on the magnetic pressure, the material composing the circuit, and its shape and size. That is, magnetic flux = magnetomotive force / reluctance