The mechanism of convective losses in the phase preceding the formation of rotational transform is studied on the CASTOR tokamak. It is shown that convection is caused by × drift associated with a strong perpendicular electrostatic field. This field is generated by charge separation as a result of a directed flow of electrons along non-toroidal magnetic field lines intersecting the wall. Convection exceeds Bohm diffusion by a factor of 700 for a perpendicular magnetic field of 10−3 T.