Convective cells are excited externally in a fully ionized magnetized plasma and their space-time evolution is investigated by two-dimensional potential measurements. A positive cell is excited externally by control of the end losses in the 'scrape off' layer of a plasma column produced by surface ionization. Its interaction with the main plasma produces a new cell with negative polarity. The two cells propagate close to each other and interact only through a mutual perturbation of orbits. The damping of the cells is due mainly to geometrical elongation, end-losses and turbulent fluctuations.