The onset of a substorm is widely understood as an instability of the magnetosphere-ionosphere system. A promising candidate for the instability is the ion-tearing mode (Schindler, 1974). The treatment of Galeev and Zelenyi (1976) shows that during the expansion phase, electric potentials may arise in the plasma sheet if the magnetosphere is regarded as an isolated system. In the present approach we consider the influence of ionospheric conductivity. We find that the occurrence of several neutral lines in the tail, created by the tearing mode, would generate a series of sheets of Birkeland currents. In the present highly simplified picture these sheets are aligned in the east-west direction; adjacent sheets carry currents of opposite signs. The results are shown to be in agreement with the observed multiple-sheet structure. The total current per sheet is estimated to be of the order of 106 A.