The evolution of plasma in a current-carrying magnetic flux tube of variable cross section is investigated using a one-dimensional numerical simulation. The flux tube is narrow at the two ends and broad in the middle. The middle region of the flux tube is loaded with a hot plasma population, and the two ends have a much more dense gravitationally bound plasma. The result is that the number of particles per unit of flux tube length has a minimum near each end and a maximum in the middle where the flux tube has the largest cross section. A large potential difference, parallel to the magnetic field, is applied across the domain. The general result is that density minimum tends to deepen, primarily in the cathode end, and the entire potential drop becomes concentrated across the region of density minimum. The potential drop is maintained primarily by electron inertia. The evolution of the simulation shows some sensitivity to particle boundary conditions, but the simulations inevitably evolve into a final state with a nearly stationary double layer near the cathode end. The simulation results are at sufficient variance with observations that it appears unlikely the auroral electrons can be explained by a simple process of acceleration through a field-aligned potential drop. ¿ American Geophysical Union 1992