  All magnetic field vector measurements lead to ambiguous results. We show
that observations in two different lines belonging to the same multiplet but
having different absorption coefficients so that they are formed at two
different depths like Fe I 6302.5 A and 6301.5 A, enable the resolution of the
azimuth ambiguity remaining from the Zeeman signal interpretation. What is
measured by interpretation of the Zeeman effect is the magnetic field H, and
not the divergence-free magnetic induction B. We analyze how the anisotropy of
the photosphere, which is strongly stratified due to gravity and density at the
star surface, affects divH and how the ambiguity resolution has to be performed
in these conditions. As a consequence, two ambiguity-resolved field vector maps
are obtained at two different but close altitudes, which enable the derivation
of the current density full vector via curlH=J. This reveals the horizontal
component of the current density, which is generally found markedly larger than
the better known vertical one. We observe some systematical trends, of which we
present examples in the paper, like circular currents wrapping spots clockwise
about a positive polarity spot and anticlockwise about a negative polarity spot
and strong horizontal current components crossing active region neutral lines.
We finally remark that the Na I D1 and D2 lines form another such line pair. We
propose them as an access to the low chromosphere where they are formed.
However, a better spatial resolution for both observations and analysis would
probably be necessary in such a medium.
