A novel type of periodic motion has been observed in a horizontal 50‐Hz ac DyI3/CsI/Hg arc, placed in a magnetic field perpendicular to the arc axis. The arc oscillates in the vertical plane with frequencies of 0.5–5 Hz. A mechanism involving a time‐varying arc constriction due to periodic metal iodide evaporation and condensation is discussed in terms of the balance between Lorentz force and buoyant forces.