Mutual Induction:

        A volage can be induced in a wire or coil by a change of
current in an adjacent wire or coil. This electromagnetic
induction, due to the moving flux caused by one conductor
cutting the other conductors, is calledMutual Induction.

        If you wind on a single iron core two coils of wire, the
magnetic field set up by one will cut the other. Voltage and current 
will be induced in the secondary winding, this is mutual induction.
If you apply power to one, call it the Primary, then a voltage will 
be induced in the other, call it the Secondary. The amount of voltage 
induced in the secondary is determined by many things, The ratio of 
turns in the primary verses the secondary, the amount of power applied
to the primary, the cycles (hertz), or number of times per second 
the voltage changes and so on. An important thing to remember is that
there must be motion between the magnetic lines and the secondary.
A varying primary current causes a changing magnetic field, and a 
secondary placed in the field will be cut by the magnetic lines as
they move away from and return toward the primary carrying this 
current. The effect of mutual induction between two wires is the
same as that between two coils see inductors.

        Mutual induction is the ACT of inducing a voltage in one coil or
conductor due to a changing current in another. 
        Mutual Inductance(M) is the ABILITY to induce the voltage and
this ability exists regardless of the current level in the circuit.

Linkage:

        When two coils are placed so that all or part of the magnetic
field of one passes or cuts through the conductors of the other, electric
energy is transferred from one coil to the other by the mutual induction
between them. Because the two circuits are coupled or linked by the
changing magnetic lines of force, mutual induction is often referred
to as Inductive Coupling. The closer the coils, the greater the number
of lines of force, due to the primary current, that link with the turns
of the secondary, and the closer or tighter the coupling is said to be.
        The productof the magnetic lines of force and the number
of turns in the coil through which they pass is calledFlux Linkage.
In most cases, the position of the coil with respect to a magnetic field
determines the actual flux linkage. If the coil is close to a magnetic
field so that most of the flux lines thread through the turns, the flus
linkage is high, but if the coil is at a distance from the magnetic field,
very few lines link with the coil, and the flux linkage is low.

Series Inductors:

        In some circuits it becomes necessary to use two or more inductors
connected in series. To find the total inductance of such a circuit, you
must first determine if magnetic coupling (Mutual Inductance) occurs
between the inductors. With no magnetic coupling between the inductors
connected in series, you can determine the total inductance in the same
way that you determine the total resistance of a circuit containing
resistors connected in series: Ltotal=L1 + L2 + L3 + .......
        If two inductors connected in series are arranged so that the 
magnetic fiel of one inductor is coupled to the other inductor, the
total inductance would be:

                Ltotal=L1 + L2+or-2M
                L1 and L2 are self-inductances and M is the
                mutual inductance between L1 and L2 
                The plus is used when the magnetomotive forces
                of the two inductors are aiding each other
                and the minus the forces are opposing each other
                The factor 2 accounts for the influence of
                L1 on L2 and L2 on L1.

        For example, suppose we have a circuit containing two magnetic
        coupled inductors connected in series. If L1 is 4 henries,
        L2 is 5 henries, and M is .5 henry, and the inductors are
        connected so that the magnetomotive forces are aiding each
        other, the total inductance of this circuit would be:
                
                Ltotal= L1 = L2 + 2M = 4 henries + 5 henries
                 + 2 x .5 henry = 4 + 5 + 1 = 10 henries

Parallel Inductors:
        When inductors are connected in parallel, you determine the
        total inductance the same as you determine the total resistance
        of resistors in parallel:

        1     1       1     1
       --- = ---  +  --- + --- +  any number of inductors
       Lt    L1       L2   L3

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