Retrograde motion of mercury arc spots when in a groove in the cathode above the level of mercury was compared with motion of spots at the junction of mercury and the molybdenum cathode. A rapid rise in retrograde velocity occurs at about 10 000 oe for junction motion but at less than 5000 oe for groove motion. Spectra show more multiply charged ions for groove than for junction motion for the same magnetic field strength. The velocity for groove motion always exceeds the velocity for junction motion, and it increases markedly with a reduction in mercury vapor pressure. For groove motion increasing current causes the retrograde velocity to pass through a maximum. Observations are described by the mechanism proposed by St. John and Winans.