A *magnetic circuit *is defined as *the route or path which is followed by a magnetic flux. *Some important terms connected with magnetic circuits are discussed below:

** 1. Permeability ****(µ)**. Permeability of any material is a measure of ease with which the atoms can be arranged. The S.I. unit for permeability is henry/metre *(H/m).*

Mathematically, µ = µ_{0}µ_{r} = H/m … (10)

=4π × 10^{-7} H/m (in S.I. units)

µ_{0} = permeability of free space

= 4π × 10-^{7} H/m (in S.I. units)

u_{r} = relative permeability (= 1 for air and non-magnetic materials)

Relative permeability (µ_{r}) is simply a numeric which expresses the degree to which the material is a better conductor of magnetic flux as compared to free space.

**Note: **µ_{r} for air (and non-magnetic materials) =1

µ_{r}, for diamagnetic materials e slightly less than one

µ_{r}, for paramagnetic materials e slightly higher than one

µ_{r}, for ferromagnetic materials e in the hundreds or thousands

(µ_{r} for ferromagnetic materials changes with the magnetic field strength)

** 2. Magnetomotive force (m.m.f.).** Magnetomotive force *drives or tends to drive flux through *a magnetic *circuit *and corresponds to electromotive force (e.m.f.) in a circuit. M.m.f. is equal to the work done in joules in carrying a unit magnetic pole once through the entire magnetic circuit.

M.m.f. is measured in ampere-turns (AT). Ampere-turns are found by *multiplying number *of turns*(N) **of a magnetic circuit and the current in amperes (I) *in those turns.

Mathematically, *AT=NI *

** 3. Reluctance**. Just as the current in an electric circuit is limited by resistance, the flux in the magnetic circuit is limited by the *reluctance *(symbol S) of the magnetic circuit. Thus *reluctance *is a measure *of opposition offered by a magnetic circuit to the setting up of the flux.*

The reluctance of a magnetic circuit is given by,

Where l = length of the magnetic circuit,

A = Cross-sectional area of the magnetic circuit,

= absolute permeability, and

= relative permeability.

Reluctance of a magnetic circuit is the **ratio of m.m.f. and flux.**

In other words the reluctance of a magnetic circuit is the number of ampere-turns required

per weber of magnetic flux in the circuit. The *unit of reluctance is AT/Wb. *Since 1 AT/Wb = l/henry, the unit of reluctance is *“r**eciprocal henry”.*

** 4. Permeance. **The reciprocal of reluctance is known as permeance.

It is measured in Wb/AT on henry.

** 5. Reluctivity**. It is the *specific reluctance *and corresponds to resistivity which is specific resistance

* Relation between flux density (B) and magnetic field strength (H):*

Comparing eqns. (12) and (13), we get

[where µ (permeability) =Thus, *permeability is the ratio afflux density to magnetic field strength. *

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