Rates of heat transfer to the inner surface of a surrounding cylindrical jacket are measured as a function of height for inert, dissociating and reacting gases, in order to resolve fluid mechanical effects from those due to bond rearrangements. Variation of mass flow rate, arc current and magnetic field applied by a circumferential coil allows the specific power input, electromagnetic torque and rate of gas rotation to be varied. The flow pattern is examined by photographing particle tracks in a transparent jacket and by various auxiliary methods, including a torque meter and stroboscopic schlieren apparatus. It is concluded that, to a first approximation, the system may be treated in terms of plug flow rotating about its axis with a gradually decaying angular velocity, and a method of reducing experimental data on a computer to yield heat transfer coefficients is developed on this model. The results show that, as regards fluid mechanical effects, swirl is dominant, with temperature stratification and turbulence playing a relatively negligible part--at least outside the immediate vicinity of the arc. The heat transfer coefficient is indeed proportional to the ratio of swirl to axial velocity, the constant of proportionality depending almost entirely on the molecular nature of the gas. This constant varies by a factor of about 13 in going from pure argon to argon + ethylene + air mixtures and appears to vary approximately as the molar total heat content of the gas.