In order to study the effect of the variation in load due to the motion of the current sheet on the driving current of a magnetically driven shock tube, we first demonstrate that the usual `snow-plough' model is adequate for the purpose.On the basis of this model it is shown theoretically that a shock moving at a constant speed can be obtained by using a constant voltage source.The result of the analysis shows that the value of the steady-state current Î depends on a single parameter ε related to the voltage and resistance of the battery, the geometry of the electrodes and the initial gas pressure p₁; the steady-state speed of the current sheet û varies as Î/p^1/2 and does not depend explicitly on the resistance of the battery.