  This paper is a critical study of non-standard Maxwellian electrodynamics. It
explores two important topics: the inclusion of both magnetic and electric
charge to produce what it calls Extended Electrodynamics, and the existence of
a symmetry called Dyality Invariance that exchanges electric and magnetic
quantities. First, the paper summarizes Extended Electrodynamics, including
potentials, gauge transformations, and a new proof of the extended
electrodynamic Poynting theorem. A formal Lagrangian derivation of the extended
Maxwell equations is also given, but its value in fundamental studies is
questioned. The paper then defines Dyality Invariance (form invariance under
the so-called Dyality Transformation that exchanges electric and magnetic
quantities) and shows it to be a valid symmetry if and only if electrodynamics
is given the extended form. The paper suggests that the complete Maxwellian
electrodynamics is extended electrodynamics with its dyality invariance. But
dyality can be interpreted either actively or passively. Since magnetic charge
has not been observed experimentally, the active interpretation is ruled out.
But a passive interpretation can be used to avoid writing magnetic source and
potential terms explicitly. The paper also refutes the idea that dyality
invariance would permit a magnetic charge to be transformed away even if one
existed. If nonzero magnetic charge exists, then experimental evidence for its
existence cannot be hidden by a dyality transformation.
