  A Poisson-Boltzmann approach is used to determine the double-layer integral
and differential capacitances in a finite-length situation for an electrolytic
cell. By means of simple analytical calculations, it is shown how these
quantities exhibit the bell-like and the camel-like shapes as a function of the
applied voltage, when the thickness of the sample or the surface charge are
varied. The problem is formulated treating the bulk liquid as a member of a
grand canonical ensemble in contact with blocking electrodes. As a consequence,
an applied voltage dependent Debye's screening length arises as a fundamental
length governing the electrical behavior of the system.
