Impedance is the name given to that property of any electrical circuit which
limits the current flow in the circuit when a voltage is applied to its terminals, and
is made up of three factors: resistance (already described for DC circuits under Ohm's.
law), inductance, and capacity.
The pure resistance of a circuit to an alternating current is the same as to a direct
Inductance does not affect a direct current but has considerable effect on an
alternating current. When a current flows in a conductor a magnetic field is formed
around the wire in proportion to the strength of the current. If the conductor is in the
form of a coil, a strong magnetic field is produced, and, if the coil is wound on an iron
core, an even stronger magnetic field is set up. When a magnetic field is created the
number of lines of force through the coil producing the field must increase, but if the
number of lines of force through the coil is increased, a voltage will be generated in the
coil. The generated voltage opposes the applied voltage and prevents the current from
increasing rapidly. This property of a circuit which prevents a rapid change of current is
called inductance. Inductance can be considered as an electrical inertia. From this
discussion we see why the rapidly changing alternating current does not reach a
destructive value in a transformer, while the steady direct current increases rapidly to a
value limited only by the pure resistance of the circuit. Coils wound for use in circuits to
offer a high impedance to an alternating current while offering a low resistance to a direct
current are called "inductors" or "reactors." Such devices are used in certain types of
amplifiers, or in the filter circuits of socket power units used to supply the DC voltage for
The third type of impedance is capacity, and the device used in obtaining this type of
impedance is called a condenser. A condenser will not pass direct current but passes
alternating current in proportion to the frequency of the applied voltage. Condensers are
used in the filter circuits of socket power units and, when so used, act as reservoirs to
absorb ripples in the direct current and thereby smooth out its flow. Condensers are also
used to prevent the flow of direct current while passing the desired alternating current,
and when used in this manner they are called bypass condensers.
Copyright 1930 RCA Photophone, Inc.