The dynamics of a liquid dielectric attracted by a vertical cylindrical capacitor are studied. Contrary to what might be expected from the standard calculation of the force exerted by the capacitor, the motion of the dielectric is different depending on whether the charge or the voltage of the capacitor is held constant. The problem turns out to be an unconventional example of the dynamics of a system with variable mass, whose velocity can, in certain circumstances, suffer abrupt changes. Under the hypothesis that the voltage remains constant the motion is described in qualitative and quantitative details, and a very brief qualitative discussion is presented of the constant charge case. (Contains 3 figures.)