The gas centrifuges which were used for isotope separation 1,2) have cylindrical rotors. In consequence of the equality of the relaxation times for diffusion and gas motion, and taking into account the pressure-dependence of the diffusion constant, the time-dependent equation for a cylindrical centrifuge is rather complicated 3). Therefore a simple centrifuge-model where gas motion does not need to be considered is of interest. Though the theoretical maximum peripheral velocity for this model is up to √2-times higher than for a cylindrical rotor 1), this interest is only mathematical.