  Geodesic equations of timelike and null charged particles in the Ernst metric
are studied. We consider two distinct forms of the Ernst solution where the
Maxwell potential represents either a uniform electric or magnetic field.
Circular orbits in various configurations are considered, as well as their
perturbations and stability. We find that the electric field strength must be
below a certain charge-dependent critical value for these orbits to be stable.
The case of the magnetic Ernst metric contains a limit which reduces to the
Melvin magnetic universe. In this case the equations of motion are solved to
reveal cycloidlike or trochoidlike motion, similar to those found by Frolov and
Shoom around black holes immersed in test magnetic fields.
