  The stars of the middle main sequence often have relatively quiescent outer
layers and spot-like chemical structures may develope in their atmospheres.
Recent observations show that abundance peculiarities can change as stars
evolve on the main sequence and the timescale of these changes lies in a wide
range from million years to months. These observations imply that, perhaps, our
understanding of diffusion processes at work in magnetic stars is incomplete
and a more detailed analysis of these processes is required. In the present
paper, we consider diffusion caused by a combined influence of the electric
current and the Hall effect.Such diffusion has a number of very particular
properties and, generally, can change the surface chemistry of stars in
combination with other diffusion processes. For instance, current-driven
diffusion is accompanied by a propagation of the special type of waves in which
only the impurity number density oscillates. Propagation of such waves changes
the shape and size of spots as well as chemical abundances within them. The
period of compositional waves depends on the parameters of plasma (magnetic
field, electric current, temperature, etc.) and can be different for the waves
of different elements. Compositional waves exist in the regions where the
magnetic pressure is greater than the gas pressure. These waves can be the
reason of variations with different timescales in the abundance peculiarities
of magnetic stars.
