  We study self-consistency for the pulsar polar cap model. In general, there
will remain an unscreened electric field at the pair production front (PPF). We
have derived the condition of electric field screening by pairs in the presence
of returning particles. A previous belief that pairs with a density higher than
the Goldreich-Julian density immediately screen out the electric field beyond
PPF is unjustified. Pairs have little contribution on screening the electric
field, and the geometrical screening is the only known way of getting a finite
potential drop.
