Current and voltage measurements were performed on a short (3 mm) arc burning at atmospheric pressure in air and driven (100–200 ms−1) along two rail electrodes by a transverse magnetic field. The arc current was pulsed from a steady state value of 40 A to 200–1000 A in < 10 μs. Simultaneous measurements revealed the arc voltage to be constant. On the contrary, a stationary arc, subjected to the same current step, showed a simultaneous temporary increase of the arc voltage of 10–40 V. To facilitate the interpretation of the results, the motion of the arc was recorded using a fast streak camera. It was found that, after applying the current step, the velocity of the arc does not change instantaneously, but that the luminous plasma expands in the wake of the arc. An interpretation of the results is proposed; it is based on the assumption that in a moving arc, the distributions of the current and the conductance are not similar.