In this paper, we show that the far field electrostatic potential created by a highly charged finite size cylinder within the nonlinear Poisson-Boltzmann (PB) theory. is remarkably close to the potential created within the linearized PB approximation by the same object at a well-chosen fixed potential. Comparing the nonlinear electrostatic potential with its linear counterpart associated to a fixed potential boundary condition (called the effective surface potential), we deduce the effective chai-e of the highly charged cylinder. Values of the effective Surface potential are provided as a function of the bare surface charge and Debye length of the ionic Solution. This allows to compute the anisotropic electrostatic interaction energy of two distant finite rods. (c) 2005 Published by Elsevier Inc.