  A version of Noether's second theorem using Lagrange multipliers is used to
investigate fluid relabelling symmetries conservation laws in
magnetohydrodynamics (MHD). We obtain a new generalized potential vorticity
type conservation equation for MHD which takes into account entropy gradients
and the ${\bf J}\times{\bf B}$ force on the plasma due to the current ${\bf J}$
and magnetic induction ${\bf B}$. This new conservation law for MHD is derived
by using Noether's second theorem in conjunction with a class of fluid
relabelling symmetries in which the symmetry generator for the Lagrange label
transformations is non-parallel to the magnetic field induction in Lagrange
label space. This is associated with an Abelian Lie pseudo algebra and a
foliated phase space in Lagrange label space. It contains as a special case
Ertel's theorem in ideal fluid mechanics.
  An independent derivation shows that the new conservation law is also valid
for more general physical situations.
