In 1824, Oersted discovered that current passing though
a coil created a magnetic field capable of shifting a compass needle. Seven years
later, Faraday and Henry discovered just the opposite. They noticed
that a moving magnetic field would induce current in an electrical
conductor. This process of generating electrical current in a
conductor by placing the conductor in a changing magnetic field
is called electromagnetic induction or just induction.
It is called induction because the current is said to be induced
in the conductor by the magnetic field.
Faraday also noticed that the rate at which the magnetic field
changed also had an effect on the amount of current or voltage
that was induced. Faraday's Law for an uncoiled conductor
states that the amount of induced voltage is proportional to the
rate of change of flux lines cutting the conductor. Faraday's
Law for a straight wire is shown below.
VL = the induced voltage in volts
dø/dt = the rate of change of magnetic flux in webers/second
Induction is measured in unit of Henries (H) which reflects
this dependence on the rate of change of the magnetic field. One
henry is the amount of inductance that is required to generate
one volt of induced voltage when the current is changing at the
rate of one ampere per second. Note that current is used in the
definition rather than magnetic field. This is because current
can be used to generate the magnetic field and is easier to measure
and control than magnetic flux.
When induction occurs in an electrical circuit and affects
the flow of electricity it is called inductance, L. Self-inductance, or simply inductance, is the property of a circuit whereby
a change in current causes a change in voltage in the same circuit.
When one circuit induces current flow in a second nearby circuit,
it is known as mutual-inductance. The image to the right
shows an example of mutual-inductance. When an AC current is flowing
through a piece of wire in a circuit, an electromagnetic field
is produced that is constantly growing and shrinking and changing
direction due to the constantly changing current in the wire.
This changing magnetic field will induce electrical current in
another wire or circuit that is brought close to the wire in the
primary circuit. The current in the second wire will also be AC
and in fact will look very similar to the current flowing in the
first wire. An electrical transformer uses inductance to change
the voltage of electricity into a more useful level. In nondestructive
testing, inductance is used to generate eddy currents in the test
It should be noted that since it is the changing magnetic field
that is responsible for inductance, it is only present in AC circuits. High frequency AC will result in greater inductive reactance
since the magnetic field is changing more rapidly.
Self-inductance and mutual-inductance will be
discussed in more detail in the following pages.