Convection speeds in commercial high pressure Hg vapor lamps are measured by photographing on a moving film, the tracks of incandescent particles of CaO and MgO. A deep red filter and panchromatic film are used. The convection system is found to consist of a laminar flow of hot gas upward in the center and of cold gas downward near the walls. For the 400 watt lamp (p≅1 atmosphere,m= mass of Hg per cm of tube length = 11.5 mg cm−1) an upward speed of about 40 cm sec.−1is found at the axis in the mid‐section of the lamp. The speed is somewhat greater near the bottom, this effect increasing withm. Speeds are found to increase somewhat less rapidly than in proportion tomas predicted. An approximate theory of these currents is presented, based largely on the work of Elenbaas, which accounts sufficiently well for the order of magnitude of the observed speeds. Forsimilardischarges, speeds do not depend on the tube diameter but simply onm; for such discharges the watts lost by convection are proportional tom2and independent of diameter and length. Convection losses for the 400‐w, 250‐w and 85‐watt lamps are calculated to be 12‐w and roughly 0.9‐w and 0.8‐w, respectively. When a lamp is operated on half‐wave a dark space develops at the bottom of the arc in the off half‐cycle, due to uprush of cold gas from below. Convection speeds estimated from the length of this dark space and the period of zero current are about twice too high; probable reasons for this are given. When the lamp is operated in the horizontal position, the arc bows up against the wall of the tube and softens it. The magnetic force necessary to keep the arc in the axial position is measured and found to be proportional tom(F=0.126mdyne cm−1). This force will be independent of tube diameter forsimilardischarges. An approximate theory is developed which accounts sufficiently well for the order of magnitude of this force. Rotation is also used to center the arc in the horizontal position; for the 400‐watt lamp a speed of at least 7 r.p.s. is necessary for this purpose. No appreciable changes in arc voltage or efficiency are found for horizontal operation with magnetic or rotational control as compared with vertical operation. Photographs of particle tracks show in a striking manner the migration of ions in the electric field; Ca and Mg vapors from the evaporating particles are strongly ionized and excited at the high temperatures and progress rapidly in the field.