Plasma current convection of a positive gas-discharge column has hitherto been studied under diffusion conditions, when the free path lengths of the electrons and ions λe and λp were smaller than the radius α of the tube. This paper gives the results of experiments with a positive mercury column at pressure of 2 × 10−3—4 × 10−3 torr, for which λe and λp≫α. In addition for a solenoid of length L = 20 cm, in our experiments λe > L for low pressures. Under these conditions the hydrodynamic theory that does not consider ion inertia is inapplicable, and new mechanisms of inhomogeneous plasma instability are possible, among them mechanisms that are not connected with the longitudinal current.