  Using a fact that the effective conductivity sigma_{e} of 2D random
heterophase systems in the orthogonal magnetic field is transformed under some
subgroup of the linear fractional group, connected with a group of linear
transformations of two conserved currents, the exact values for sigma_{e} of
isotropic heterophase systems are found. As known, for binary (N=2) systems a
determination of exact values of both conductivities (diagonal sigma_{ed} and
transverse Hall sigma_{et}) is possible only at equal phase concentrations and
arbitrary values of partial conductivities. For heterophase (N > 2) systems
this method gives exact values of effective conductivities, when their partial
conductivities belong to some hypersurfaces in the space of these partial
conductivities and the phase concentrations are pairwise equal. In all these
cases sigma_e does not depend on phase concentrations. The complete,
3-parametric, explicit transformation, connecting sigma_e in binary systems
with a magnetic field and without it, is constructed
