  Equilibria of polygonal linkage with respect to Coulomb potential of point
charges placed at the vertices of linkage are considered. It is proved that any
convex configuration of a quadrilateral linkage is the point of global minimum
of Coulomb potential for appropriate values of charges of vertices. Similar
problems are treated for the equilateral pentagonal linkage. Some corollaries
and applications in the spirit of control theory are also presented.
