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The dynamic behaviour of a d.c. arc in a metalplate quenching chamber is discussed. The arc, burning in air at 1 atm, is propelled laterally by an applied external magnetic field. The dynamic-arc equation describing the time development of the current and voltage is derived from the heat-transfer equation formulated semiempirically. The assumption of forced convection, based on a fraction of the mainstream flowing through the arc body, leads to characteristics consistent with experiment. The computed voltage characteristics under conditions of unstable arc operation predict, in agreement with experiment, an almost steady behaviour of arc voltage throughout the period when the currents falls to zero. These characteristics are favourable for application to d.c.-circuit interruption. Measurements of the current/voltage characteristics of the arcs have been performed for currents between 1 and 6 kA. The arc length has been varied from 1.5 to 17.4 mm, giving information on the electric field. Typical values were 70 V/cm for a wide range of initial currents at B = 2.0 T and for an arc length l of 3.2 mm. It was also shown how the `switching offÂ¿ time depends on the time constant of the external circuit, the magnetic field and the initial current.