  A modification of the Maxwell equations due to the presence of a
gravitational field was formerly proposed for a scalar theory with a preferred
reference frame. With this modification, the electric charge is not conserved.
The aim of the present work was to numerically assess the amount of charge
production or destruction. We propose an asymptotic scheme for the
electromagnetic field in a weak and slowly varying gravitational field. This
scheme is valid independently of the theory and the "gravitationally-modified"
Maxwell equations. Then we apply this scheme to plane waves and to a group of
Hertzian dipoles in the scalar ether theory. The predicted amounts of charge
production/destruction discard the formerly proposed gravitationally-modified
Maxwell equations. The theoretical reason for that is the assumption that the
total energy tensor is the sum of the energy tensor of the medium producing the
electromagnetic (e.m.) field and the e.m. energy tensor. This means that an
additional, "interaction" tensor has to be present. With this assumption, the
standard Maxwell equations in a curved spacetime, which predict charge
conservation, are compatible with the investigated theory. We find that the
interaction energy might contribute to the dark matter.
