  We analyze the transformation properties of Faraday law in an empty space and
its relationship with Maxwell equations. In our analysis we express the Faraday
law via the four-potential of electromagnetic field and the field of
four-velocity, defined on a circuit under its deforming motion. The obtained
equations show one more facet of the physical meaning of electromagnetic
potentials, where the motional and transformer parts of the flux rule are
incorporated into a common phenomenon, reflecting the dependence of
four-potential on spatial and time coordinates, correspondingly. It has been
explicitly shown that for filamentary closed deforming circuit the flux rule is
Lorentz-invariant. At the same time, analyzing a transformation of e.m.f., we
revealed a controversy: due to causal requirements, the e.m.f. should be the
value of fixed sign, whereas the Lorentz invariance of flux rule admits the
cases, where the e.m.f. might change its sign for different inertial observers.
Possible resolutions of this controversy are discussed.
