  A forgotten experiment by Andr\'e Blondel (1914) proves, as held on the basis
of theoretical arguments in a previous paper, that the time variation of the
magnetic flux is not the cause of the induced $emf$: the physical agent is
instead the vector potential through the term $-\partial\vec A/\partial t$
(when the induced circuit is at rest). The "good electromagnetic potentials"
are determined by the Lorenz condition and retarded formulas. Other pairs of
potentials derived through appropriate gauge functions are only mathematical
devices for calculating the fields: they are not physically related to the
sources. The physical meaning of a theoretical term relies, primarily, on
theoretical grounds: a theoretical term has physical meaning if it cannot be
withdrawn without reducing the predictive power of a theory or, in a weaker
sense, if it cannot be withdrawn without reducing the descriptive proficiency
of a theory.
