This rotation is associated with an azimuthal drift arising from the existence of a radial electric field and a longitudinal magnetic field. The observed oscillation frequency appears to be related to the rotational frequency. The methods of measurement of both the instability and the rotational frequency are discussed. A theory is developed based on the two-fluid equations in slab geometry, in which an electric field parallel to the direction of the density variation and perpendicular to a magnetic field is considered. Collisions of the ions with neutrals are also taken into account. The theoretical predictions are then compared with the experimental observations; relatively good agreement with the measurements is obtained.