Norton's theorem is similar to Thevenin's theorem. The difference is
that the Norton-equivalent circuit has a constant-current source and
the impedance is in parallel with this source, rather than in series.
The above circuit diagram is the Norton equivalent of the same circuit
used to illustrate Thevenin's theorem above. It is important to have a
cursory knowledge of Thevenin's theorem because it relates to
understanding the output impedance of a circuit. Norton's theorem is
generally useful only from a theoretical viewpoint and will not be
discussed further here.