Inductors:

        Regardless of it's size or use, practically every type of
electrical equipment uses inductors/or transformers. Many types of
equipment have a transformer in the power supply, although this isn't
the only place where transformers are used.

        Whenever one circuit supplies voltage and current to another,
if this combination isn't correct for best operation, a transformer 
permits the voltage and the current values to be changed to the correct
values for proper circuit operation. That is, when the voltage is too 
low, the transformer increases the voltage and reduces the current, or
where a large current is needed, the transformer increases the current
and reduces the voltage.

        For this action, the transformer makes use of several important
factors. One of these is that a voltage can be induced into a wire or
coil whenever the wire or coil either cuts or is cut by a magnetic field.
In a transformer, however, the coil remains stationary while the magnetic
field "moves" (expands and contracts). The second important factor used
in transformers is that an alternating current produces a moving magnetic
field.

Self-Induction:
        Electron flow through a wire produces a magnetic field. Let's
consider a direct current passing through a wire, as long as there is no
complete circuit, there is no current flow and therefore no magnetic field
around the wire, but when the circuit is complete, the current flow through
the wire causes a magnetic field to build up around the wire. If there is
another wire close to the first, a current flow will be caused in that
wire as well. The current and voltage in the second wire is called induced
voltage and current. The electron flow in the second wire will be of the
opposite polarity of the first, because the magnetic lines of force induced
in the second wire are flowing in the opposite direction. A similar opposite
current is generated in the first wire. Since the voltages in each strand
oppose each other, the effective voltage in each of them is the difference
between the original supply voltage and the induced voltage. Now, by recom-
bining the two as one wire, the voltage induced in the wire by the expanding
field opposes the applied voltage so that the resulting voltage is lower. 
There is also less current. This induced voltage is called a Counter
Electromotive Force (cemf).

        Inducing a voltage into a conductor is called Self-Induction
and like all forms of induction, it occurs only while the magnetic field
is changing. Although this action takes place very rapidly, this self-
induction does prevent the current from rising instantaneously to its full
value.

        Self-induction in a coil of wire is much greater than in a straight
piece of wire of the same length, because the magnetic field around each 
turn cuts not only the turn that sets it up, but also the turns close to it. As
this overlapping of magnetic lines occurs between all of the turns, the 
induced CEMF will be much greater than in a single pair of wires. The
actual amount of self-induction in a coil depends on the ampere-turns and
the reluctance of the magnetic circuit. That is, the stronger the magnetic
field, the greater the self-induction and the greater the CEMF.

        The effects of self-induction are as follows:

        

        
Whenever a circuit current increases, the induced counter
emf opposes the change of current and prevents it from rising
instantly to it's steady value.
        
Whenever a circuit current decreases, the induced emf is
in a direction to oppose the change and tends to maintain the
current.
	
Notice particularly that the induction occurs only while the current is
changing, and the counter emf is always in a direction to oppose the change
of current level. With a uniform or steady current, there is no induction, 
and therefore, no counter emf.

Inductance:
        When there is a variation of current in a circuit, the magnetic
flux also varies, expanding as the current increases and contracting as
the current decreases. In moving, the magnetic lines of force cut any 
conductor which is within range and induce a voltage that is always in
such a direction as to oppose the current change. The ability to produce
a voltage by electromagnetic induction when the current changes is a 
property possessed by a circuit because of its physical arrangement and
is known as Inductance. A conductor has inductance whether or not
there is current in it.

        The inductance of a circuit depends on the number of magnetic lines
that cut the conductor for each ampere change in current. Anything that
increase the number of flux lines cutting the conductor for each ampere
change in current increases the inductance. A conductor wound into a coil
has a greater inductance than a straight wire. This is because the flux
developed by one turn of wire in the coil also cuts practically every other
turn as it expands or contracts. As a result of this additive action, the
inductance of a coil is proportional to the number of turns squared. For
the same reason, an iron core placed within the coil increases the
inductance because the lower reluctance of the magnetic circuit permits a
greater number of flux lines, and therefore, more lines cut the conductor
as the current changes. Since a coil of wire possesses the property of
inductance it is called an Inductor(L).

        Induction is the result of electromagnetic action in a circuit
        containing inductance, and as a general definition:

     Self-inductance is the ability of a circuit to produce a
     voltage within itself by induction when the current in it
     changes.

Do not confuse self-induction with self-inductance. Self-induction is
the ACT or process of inducing a voltage in the wire or coil, While 
self-inductance is the ABILITY to induce the voltage.

        The unit of measurement for inductance is the Henry(H),
named for the American Physicist Joseph Henry (1797 - 1878), who in 
1831, discovered the voltage caused by self-induction.

        The Definition:
     The Henry is the amount of inductance which induces a counter
     emf of one volt when the current is changing at the rate of 
     one ampere per second.

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