By calculating the Boltzmann collision integral for collisions between ions and neutral particles, two different formulae for the rotation velocity υΘ of a cylindrical plasma column driven by Ir Bz-iorces have been derived. For υΘ<υth the rotation velocity is proportional to IrBz, whereas for υΘ>υth one gets a dependency υΘ ~ (Ir Bz)1/2. υth denotes the mean thermal velocity of the ions